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5V & 12V REGULATED POWER SUPPLY

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This regulated power supply can be used to power up TTL and CMOS based projects, it provides 5 VDC & 12 VDC outputs with an onboard mains transformer.  The project is based on the industry popular 7800 series voltage regulator in TO220 packages.</div>
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<b><span style="color: #3d85c6;">Features:</span></b></div>
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<ol>
<li>Input: 240 VAC</li>
<li>Output: 5 V, 12 V @ 600 mA regulated low ripple DC voltage</li>
<li>Thermal overload/short circuit protection (provided by IC feature)</li>
<li>Power Battery Terminal (PBT) for easy input and output connection</li>
<li>External On/Off switch connection possible</li>
<li>LED indication for outputs</li>
<li>Four mounting holes of 3.2 mm each</li>
<li>PCB dimensions 87 mm x 49 mm</li>
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<b><span style="color: #3d85c6;">Circuit Diagram:</span></b></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjADzqnokPGt3c8UKIf2JiMup7uUuX65RJnMew7PnGv8eEBe3TyQtXU2gNhUyuXX8VEj9OV-hgGNr-vV5nGaDClblW0nz3wl20VKJLpRsegyVlMa5x28JC30i-6a4I1gkIzXuuu9BCh8K62/s1600/5V_12V_Power_Supply.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img alt="5V & 12V REGULATED POWER SUPPLY" border="0" height="346" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjADzqnokPGt3c8UKIf2JiMup7uUuX65RJnMew7PnGv8eEBe3TyQtXU2gNhUyuXX8VEj9OV-hgGNr-vV5nGaDClblW0nz3wl20VKJLpRsegyVlMa5x28JC30i-6a4I1gkIzXuuu9BCh8K62/s640/5V_12V_Power_Supply.png" title="5V & 12V REGULATED POWER SUPPLY" width="640" /></a></div>
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5V & 12V REGULATED POWER SUPPLY

5V & 12V REGULATED POWER SUPPLY

This regulated power supply can be used to power up TTL and CMOS based projects, it provides 5 VDC & 12 VDC outputs with an onboard ma...

ExpressPCB

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ExpressPCB circuit board layout program is a snap to learn and use. Laying out PCBs is easy, even for the first time user.</div>
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<img border="0" height="483" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEifaX2S1aPosJGD9NjmnfqgIYLJAuhaWJGn12XVjOWAJAwTfEn6Ir8RMMe75tsYgaEihdx4ujcckt6GovhoyYO44oJ3xaDjW3D9EeNrvZoULx8snrbMKb2CLUq3hnyPDbwY3ugZaRrXXiUi/s640/ExpressPCB.gif" width="640" /></div>
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Here are the steps:</div>
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<b><i>Select the Components</i></b></div>
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Begin your layout by adding the components. Select the parts from the Component Manager dialog box. Many components (such as connectors) include Digi-key part numbers to make ordering easy.</div>
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<b><i>Position the Components</i></b></div>
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Drag each component to the desired location on your board. The Snap to grid feature makes it easy to neatly align the parts.</div>
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<b><i>Add the Traces</i></b></div>
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Now add each trace by clicking on the pin of a component and dragging the trace to another pin. If you link your schematic file to the PCB, then the ExpressPCB program highlights the pins that should be wired together in blue.</div>
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<b><i>Edit the Layout</i></b></div>
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Making changes is simple using standard commands such as Copy, Cut and Paste. Rearrange the parts by dragging them with the mouse. Traces always stay connected to their pins, even when you move things around. You can set the properties of items in your layout by double-clicking on them. For example, double-click on a trace to change its layer or width.</div>
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<b><span style="color: #0b5394;">Download</span></b></div>
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<a href="http://www.expresspcb.com/download/expresspcb-3/?wpdmdl=134">Download ExpressPCB for XP, 2000 , NT, Vista, Windows 7 & Windows 8.1</a><br />
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ExpressPCB

ExpressPCB circuit board layout program is a snap to learn and use. Laying out PCBs is easy, even for the first time user. Here ar...

FreePCB

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<b><span style="color: #0b5394;">Introduction:</span></b></div>
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FreePCB is a free, open-source PCB editor for Microsoft Windows, released under the GNU General Public License. It was designed to be easy to learn and easy to use, yet capable of professional-quality work. It does not have a built-in autorouter, but it can use the FreeRoute web-based autorouter at www.freerouting.net. Some of its features are:</div>
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<ul>
<li>1 to 16 copper layers</li>
<li>Board size up to 60 inches by 60 inches</li>
<li>Uses English or metric units (i.e. mils or mm) for most functions.</li>
<li>Footprint libraries courtesy of Ivex Design International, PCB Matrix and the IPC.</li>
<li>Copper fill areas</li>
<li>Footprint Wizard and Footprint Editor for creating or modifying footprints</li>
<li>Imports and exports PADS-PCB netlists</li>
<li>Exports extended Gerber files (RS274X) and Excellon drill files</li>
<li>Design rule checker</li>
<li>Autosave</li>
</ul>
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The layout for a small PCB is shown in the screenshot below;</div>
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<img border="0" height="466" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGToKADAsPQDCQ_nrET3VpQQ88FKzT9ExbUXK4w_mfA0BI3dzqP9MG5BLYpf5865gRCZoN508bh84A9YVdWVPSqav_6FBYJo-x_snqV_GfZvktBbAuRXIS9dLfz3MuSLgo1x9kbjEWfb7B/s640/Freepcb.gif" width="640" /></div>
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<b><span style="color: #0b5394;">Copyright:</span></b></div>
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<span style="font-family: "courier new" , "courier" , monospace;">FreePCB: a PCB layout editor</span></div>
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<span style="font-family: "courier new" , "courier" , monospace;">Copyright (C) 2003-2007 by Allan Wright</span></div>
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<span style="font-family: 'courier new', courier, monospace;">This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.</span></div>
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<span style="font-family: "courier new" , "courier" , monospace;"><br /></span></div>
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<span style="font-family: "courier new" , "courier" , monospace;">This program is distributed in the hope that it will be useful, </span><span style="font-family: "courier new" , "courier" , monospace;">but WITHOUT ANY WARRANTY; without even the implied warranty of </span><span style="font-family: "courier new" , "courier" , monospace;">MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the </span><span style="font-family: "courier new" , "courier" , monospace;">GNU General Public License for more details.</span></div>
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<span style="font-family: "courier new" , "courier" , monospace;">You should have received a copy of the GNU General Public License </span><span style="font-family: "courier new" , "courier" , monospace;">along with this program; if not, write to the Free Software </span><span style="font-family: "courier new" , "courier" , monospace;">Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.</span></div>
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<b><span style="color: #0b5394;">Download:</span></b></div>
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<a href="http://www.freepcb.com/downloads/freepcb_1200_setup.exe">FreePCB_1200_setup.exe</a><br />
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FreePCB

Introduction: FreePCB is a free, open-source PCB editor for Microsoft Windows, released under the GNU General Public License. It was de...

Unijunction Transistor (UJT)

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Unijunction transistor (UJT), also called the double base diode, is a two layer. three terminal solid state (silicon) switching device. The device has unique characteristics that when it is triggered, its emitter current increases regeneratively (due to negative resistance characteristic) until it is restricted by emitter power supply. The low cost per unit, combined with its unique characteristics, have warranted its use in a wide variety of applications. A few include oscillators, pulse generators, sawtooth generators, trigger circuits, phase control. timing circuits, and voltage or current regulated supplies. The device is, in general, a low power absorbing device under normal operating conditions and provides tremendous aid in the continental effort to design relatively efficient systems.</div>
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<b><span style="color: #0b5394;">Construction:</span></b></div>
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The basic structure of a unijunction transistor is shown in Fig. 1. It is essentially consists of a lightly doped N-type silicon bar with piece of heavily doped P-type material alloyed to its one side to produce single P-N junction. The single P-N junction accounts for the terminology unijunction. The silicon bar, at its ends, has two ohmic contacts designated as base-(B1) and base-2(B2), as shown and P type region is termed the emitter (E). The emitter junction is usually located closer to base-1(B2) than base-1(B1) so that the device is not symmetrical, because symmetrical until does not provide optimum electrical characteristics for most of the applications.</div>
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<img alt="UJT Basic Structure" border="0" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi9od7FLq7y6DwBmCEkkLEPum7URCU_ow9r7KDqC7rpjgLXeSeiTvT9N3Ot1u6f_lgpFW8DF3OkKxXsG7ZpF8eo9c6CDnedt227UDygY4h7nAOupSe0Aq9o2CZkHhvZ-ip9TU8yd9GD_Gp2/s400/UJT_strutture.gif" title="UJT Basic Structure" width="321" /></div>
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<b><i>Fig. 1. UJT Basic Structure</i></b></div>
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The symbol for unijunction transistor is shown in Fig. 2. The emitter leg is drawn at an angle to the vertical line representing the N-type  material slab and the arrowhead points in the direction of conventional current when the device is forward biased, active or in the conducting state.</div>
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<img alt="UJT Symbol" border="0" height="160" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEijx6QhfilzyQHpm8bL6XXb4A3fGsYu75_GhQP4lkiYxRVXgDY6oPygLoGBblnFZCpFiXn6s_jBofSaxUz-mWaXt2lExUbX4uMrXppe_V5ntncDvCH7adSY7W-nBvXcAWRdPgAo9kgtHsan/s200/UJT_N_symbol.gif" title="UJT Symbol" width="200" /></div>
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<b><i>Fig. 2. UJT Symbol</i></b></div>
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The basic arrangement for the UJT is shown in Fig. 3.<br />
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<img alt="UJT Basic Arrangement" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhGx2pLTzu4w3rtRYJHCnhTvn9Hra1sJYHppfH6DP6yRHxiR_Dakj5BnXZAeEaxpec3ZLzilnZElyCSvWSSMLzuLRVO5pJWyTBDSSEdiOZjPYrxcTnDCuvwmohU95LVqMB-TCwnFu7rsZFj/s1600/UJT_Basic_Arrangement.png" title="UJT Basic Arrangement" /></div>
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<b><i>Fig. 3. UJT Basic Arrangement</i></b></div>
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A complementary UJT is formed by diffusing an N-type emitter terminal on a P-type base. Except for the polarities of voltage and current, the characteristics of a complementary UJT are exactly the same as those of a conventional UJT.<br />
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The important points about UJT are given below:<br />
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<ol>
<li>The device has only one junction, so it is called the unijunction device.</li>
<li>The device, because of one P-N junction, is quite similar to a diode but it differs from an ordinary diode that it has three terminals.</li>
<li>The structure of a UJT is quite similar to that of an N-channel JFET. The main difference is that P-type (gate) material surrounds the N-type (channel) material in case of JFET and the gate surface of the JFET is much larger than emitter junction of UJT.</li>
<li>In a unijunction transistor the emitter is heavily doped while the N-region is lightly doped, so the resistance between the base terminals is relatively high, typically 4 to 10 K Ohm when the emitter is open.</li>
<li>The N-type silicon bar has a high resistance and the resistance between emitter and base-1 is larger than that between emitter and base-2. It is because emitter is closer to base-2 than base-1.</li>
<li>UJT is operated with emitter junction forward biased while the JFET is normally operated with the gate junction reverse biased.</li>
<li>UJT does not have ability to amplify but it has the ability to control a large ac power with a small signal.</li>
<li>It exhibits a negative resistance characteristic and so it can be employed as an oscillator.</li>
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<b><span style="color: #0b5394;">Device Operation:</span></b></div>
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The device has a unique characteristic that when it is triggered, its emitter current increases regeneratively until it is restricted by emitter power supply. It exhibits a negative resistance characteristic and so it can be employed as an oscillator.</div>
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The UJT is biased with a positive voltage between the two bases. This causes a potential drop along the length of the device. When the emitter voltage is driven approximately one diode voltage above the voltage at the point where the P diffusion (emitter) is, current will begin to flow from the emitter into the base region. Because the base region is very lightly doped, the additional current (actually charges in the base region) causes conductivity modulation which reduces the resistance of the portion of the base between the emitter junction and the B2 terminal. This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected. Overall, the effect is a negative resistance at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuits.</div>
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<img alt="Static Emitter-Characteristic for a UJT" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhbMkoxOS-Pznq8HKgc3osunoZ-r3e8wAK2ojUe_LzsvMEH9B3tdVKwB_zN5_D_XInI67l-Fa78iFq5lCChuoVBWnqlLJ62EW5Ol_b55Ntl9yspg6b5rVj2FIxBS5_5T1SN6R-3toyP0Zkg/s1600/UJT_caratteristica.gif" title="Static Emitter-Characteristic for a UJT" /></div>
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<b><i>Fig. 4. Static Emitter-Characteristic</i></b></div>
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<b><i>for a UJT</i></b></div>
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Unijunction Transistor (UJT)

Unijunction Transistor (UJT)

Unijunction transistor (UJT), also called the double base diode, is a two layer. three terminal solid state (silicon) switching device. Th...

Fog Lamp Sensor Circuit

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For several years now, a rear fog lamp has been mandatory for trailers and caravans in order to improve visibility under foggy conditions. When this fog lamp is switched on, the fog lamp of the pulling vehicle must be switched of to avoid irritating reflections. For this purpose, a mechanical switch is now built into the 13-way female connector in order to switch of the fog lamp of the pulling vehicle and switch on the fog lamp of the trailer or caravan. For anyone who uses a 7-way connector, this switching can also be implemented electronically with the aid of the circuit illustrated here.</div>
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<b><span style="color: #0b5394;">Circuit Diagram:</span></b></div>
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<img alt="Fog Lamp Sensor Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg6-Zz3BRm4JDlxJDc6g5x1zn8dUSSARhu3X3uauppwt3fPa8Auy_3NrnhYZmi4WB6I_BYzbQ4mWDxqVeZhHCBnWg5dWJ55ZBdM86uJf_GAkiUTRR0UibpxdydnBgfMrT6dLKYlTgeNjehY/s1600/fog-lamp-sensor-circuit-daigram.gif" title="Fog Lamp Sensor Circuit" /></div>
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Here a type P521 optocoupler detects whether the fog lamp of the caravan or trailer is connected. If the fog lamp is switched on in the car, a current flows through the caravan fog lamp via diodes D1 and D2. This causes the LED in the optocoupler to light up, with the result that the photo-transistor conducts and energies the relay via transistor T1. The relay switches of the fog lamp of the car. For anyone who’s not all thumbs, this small circuit can easily be built on a small piece of perforated circuit board and then fitted somewhere close to the rear lamp fitting of the pulling vehicle.</div>
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Fog Lamp Sensor Circuit

Fog Lamp Sensor Circuit

For several years now, a rear fog lamp has been mandatory for trailers and caravans in order to improve visibility under foggy conditions....

Flashing-LED Battery-Status Indicator Circuit

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A Battery-status Indicator circuit can be useful, mainly to monitor portable Test-gear instruments and similar devices. LED D1 flashes to attire the user's attention, signaling that the circuit is running, so it will not be left on by mistake. The circuit generates about two LED flashes per second, but the mean current drawing will be about 200µA. Transistors Q1 and Q2 are wired as an uncommon complementary astable multivibrator: both are off 99% of the time, saturating only when the LED illuminates, thus contributing to keep very low current consumption.</div>
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<b><span style="color: #0b5394;">Circuit operation:</span></b></div>
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The circuit will work with battery supply voltages in the 5 - 12V range and the LED flashing can be stopped at the desired battery voltage (comprised in the 4.8 - 9V value) by adjusting Trimmer R4. This range can be modified by changing R3 and/or R4 value slightly. When the battery voltage approaches the exhausting value, the LED flashing frequency will fall suddenly to alert the user. Obviously, when the battery voltage has fallen below this value, the LED will remain permanently off. To keep stable the exhausting voltage value, diode D1 was added to compensate Q1 Base-Emitter junction changes in temperature. The use of a Schottky-barrier device (e.g. BAT46, 1N5819 and the like) for D1 is mandatory: the circuit will not work if a common silicon diode like the 1N4148 is used in its place.</div>
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<b><span style="color: #0b5394;">Circuit Diagram:</span></b></div>
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<img alt="Flashing-LED Battery-Status Indicator Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhtkAL8n-KsmVfc5q7GZWGo1EwGAI8CNNiY9TqZnyGhrjwwZi_kEev7OkvlxsPAxcMONOUxd1oj5eu-3nzJM_zerlgb3J1cvziw56yruR2jt1_LK7SxfvVXpWPhiJr3w0fb8gH-PtlObBgs/s1600/flashing-led_battery-status_indicator_schematic_circuit_diagram.gif" title="Flashing-LED Battery-Status Indicator Circuit" /></div>
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<b><span style="color: #0b5394;">Parts:</span></b></div>
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R1 = 220R - 1/4W Resistor</div>
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R2 = 120K - 1/4W Resistor</div>
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R3 = 5.6K - 1/4W Resistor</div>
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R4 = 5K - 1/2W Trimmer Cermet</div>
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R5 = 33K - 1/4W Resistor</div>
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R6 = 680K - 1/4W Resistor</div>
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R7 = 220R - 1/4W Resistor</div>
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R8 = 100K - 1/4W Resistor</div>
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R9 = 180R - 1/4W Resistor</div>
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C1 = 4.7uF - 25V Electrolytic Capacitors</div>
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C2 = 4.7uF - 25V Electrolytic Capacitors</div>
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Q1 = BC547 - 45V 100mA NPN Transistor</div>
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Q2 = BC557 - 45V 100mA PNP Transistor</div>
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B1 = 5V to 12V Battery supply</div>
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D1 = BAT46 - 100V 150mA Schottky-barrier Diode</div>
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D2 = LED - Red 5mm.</div>
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<b><span style="color: #0b5394;">Note:</span></b></div>
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Mean current drawing of the circuit can be reduced further on by raising R1, R7 and R9 values.</div>
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Flashing-LED Battery-Status Indicator Circuit

Flashing-LED Battery-Status Indicator Circuit

A Battery-status Indicator circuit can be useful, mainly to monitor portable Test-gear instruments and similar devices. LED D1 flashes to ...

Motor Bike Headlight Controller Circuit

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This circuit automatically turns a motor cycle's headlight on and off, independently of both the light and ignition switches, provided the battery is fully charged. The first stage uses the 22O resistor and ZD1 to hold transistor Q1 off while the motor is not running; it draws about 2mA. Once the battery voltage exceeds 7.0V during charging, Q1 begins to turn on.</div>
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The last stage uses the 22O resistor and ZD2 to turn on transistor Q2, which pulls the base of Q1 down, switching it hard on. In conjunction with the Vbe drop of Q2, ZD2 will turn off Q2 at a battery voltage of about 6.7V. In practice, this means that the headlight will be on most of the time while the motor is running and charging the battery. Heatsinks are required for both transistors. The circuit can be mounted adjacent to the battery with a single lead going to the headlight power feed.</div>
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<b><span style="color: #0b5394;">Circuit Diagram:</span></b></div>
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<img alt="Motor Bike Headlight Controller Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi3x7yPtf-MfPpQmSzvF1IeBBoIjAtkDb-9H18qM9GOaRewHvOdw-fAoa9akEU5EVu97XcF9Xg6RsVzU4rfVakn5AcfjEAVxSLAy7xZ5UNqOJ6TQIIuhhKDwavyf3nBafd9gcOar1RLz1Mw/s1600/motor_bike_headlight_controller_schematic_circuit_diagram.gif" title="Motor Bike Headlight Controller Circuit" /></div>
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<b><span style="color: #0b5394;">Parts:</span></b></div>
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P1 = 5R - 3W</div>
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R1 = 220R -1W</div>
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R2 = 1R - 1W</div>
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R3 = 22R - 5W</div>
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R4 = 22R - 5W</div>
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Q1 = BD139</div>
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Q2 = MJ4502</div>
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D1 = 6.2V - 1WZener</div>
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D2 = 6.2V - 1WZener</div>
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B1 = 6.6V - 7.2V</div>
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F1 = Fuse 15A</div>
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LP1 = DIP Beam 6V-25W Halogen</div>
</div>

Motor Bike Headlight Controller Circuit

Motor Bike Headlight Controller Circuit

This circuit automatically turns a motor cycle's headlight on and off, independently of both the light and ignition switches, provided...

Car-Bulb Flasher

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This astonishingly simple circuit allows one or two powerful 12V 21W car bulbs to be driven in flashing mode by means of a power MosFet. Devices of this kind are particularly suited for road, traffic and yard alerts and in all cases where mains supply are not available but a powerful flashing light are yet necessary.</div>
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<b><span style="color: #0b5394;">Schematic:</span></b></div>
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<img alt="Car-Bulb Flasher Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh2LCsvuFIMnrX5cHgvBHaCA59vcsTb1lqdpcKjS30XPJBl5HHsUwV4xoOeRX70xYm9vBMN31_G2-nZigyMl2U63J3DixL5tTqFcUd-EzrlbcydnL2lj-Bxg-IJo29usiXnW89bZ1bFMH-n/s1600/carbulbflasher.gif" title="Car-Bulb Flasher Circuit" /></div>
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<b><span style="color: #0b5394;">Parts:</span></b></div>
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R1 = 6.8K</div>
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R2 = 220K</div>
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R3 = 22K</div>
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C1 = 100uF-25V</div>
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C2 = 10u-25V</div>
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D1 = 1N4002</div>
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Q1 = BC557</div>
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Q2 = IRF530</div>
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LP1 = 12V-21W Car Filament Bulb (See Notes)</div>
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SW1 = SPST Switch (3 Amp minimum)</div>
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<b><span style="color: #0b5394;">Notes:</span></b></div>
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Flashing frequency can be varied within a limited range by changing C1 value.</div>
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As high dc currents are involved, please use suitably sized cables for battery and bulb(s) connections.</div>
</div>

Car-Bulb Flasher

Car-Bulb Flasher

This astonishingly simple circuit allows one or two powerful 12V 21W car bulbs to be driven in flashing mode by means of a power MosFet. D...

Fog Lamp Switch Circuit

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In most countries it is now mandatory or at least recommended to have a rear fog light on a trailer with the additional requirement that, when the trailer is coupled to the car, the rear fog light of the towing car has to be off. The circuit shown here is eminently suitable for this application. The circuit is placed near the rear fog light of the car. The 12-V connection to the lamp has to be interrupted and is instead connected to relay contacts 30 and 87A (K1, K3). When the rear fog light is turned on it will continue to operate normally.</div>
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<b><span style="color: #0b5394;">Circuit Diagram:</span></b></div>
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<img alt="Fog Lamp Switch Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEihpwAb0oI3SjELEjyvNFjxkwEKsAOSR8SUp-qqnn1bzwbRILdO_dFtDzNNoYm7_JDO5vCy3P7MeiaH7gsUqmlAn-6l6ZU9_OFn7-nJwdr4c_x6zE8Svds5JfJZ5IsX93Wc_6SaMkrPgsuG/s1600/fog-lamp-switch-circuit-diagram.gif" title="Fog Lamp Switch Circuit" /></div>
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If a trailer with fog light is now connected to the trailer connector (7- or 13-way, K2), a current will flow through L1. L1 is a coil with about 8 turns, wound around reed contact S1. S1 will close because of the current through L1, which in turn energizes relay RE1 and the rear fog light of the car is switched off. The fog light of the trailer is on, obviously. The size of L1 depends on reed contact S1. The fog lamp is 21 W, so at 12 V there is a current of 1.75 A. L1 is sized for a current between 1.0 and 1.5 A, so that it is certain that the contact closes. The wire size has to be about 0.8 mm. The relay RE1 is an automotive relay that is capable of switching the lamp current. The voltage drop across L1 is negligible.</div>
</div>

Fog Lamp Switch Circuit

Fog Lamp Switch Circuit

In most countries it is now mandatory or at least recommended to have a rear fog light on a trailer with the additional requirement that, ...

Automatic Windshield Washer Control

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Most, if not all, recent cars have an impressive amount of electronics, whether it be ABS brake systems, engine control with injection calculators, airbag activation, or other various functions, called comfort functions. Among them is one which we tend to forget because it has become so common today. It turns on the windshield wipers automatically for a few seconds after the windshield cleaner. This practice is almost indispensable because it avoids any dripping of excess rinse product right in the middle of a just-cleaned windshield.</div>
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Unfortunately, many ‘low end’ cars or some of the older cars are not equipped with this automatic function which is a very nice convenience to have. So, since all that is required is a handful of components that any electronics hobbyist worthy of the name already has in his/her drawer, we will discuss the circuit proposed here. This project is super simple and simply keeps the windshield wiper activated for a few seconds after the windshield washer control contact has been released.</div>
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While the windshield washer pump is operating, the 12 volts delivered by the battery are present at the terminals and are therefore charging capacitor C1. Once the windshield washer has stopped, this capacitor can only discharge through R2, P1, R3, and the T1 emitter-base junction, due to the presence of diode D1. It thus keeps T1 in the conductive state during a certain time, the exact period of which depends on the setting of P1. T1 in turn saturates T2, which then does the same for T3.</div>
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<b><span style="color: #0b5394;">Circuit Diagram:</span></b></div>
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<br /></div>
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<img alt="Automatic Windshield Washer Control Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEimd3QAuI0wwi-8dspzEGxCA8yy2RP2Z7tcItqRvJNzllzXH0WMAFN0NZwP_P2SvHictP_8K0T09F1dr6AfA0AIf2TCtWyWqxDWmwJyFsdmYXZYge43pYANGj5gQb145VsWtmGvUT1Z6mJF/s1600/automatic-windshield-washer-control-circuit-diagram.gif" title="Automatic Windshield Washer Control Circuit" /></div>
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The RE1 relay is therefore connected which maintains the windshield wiper in operation because its work contact is wired in parallel to the control switch. Once C1 is sufficiently discharged, T1 is blocked, which then blocks T2 and T3 and deactivates relay RE1. The type of components is not really critical, even if we indicate specific reference numbers for T3, any low-power npn transistor with a gain over 25 will work. However, considering the amount of power consumed by the windshield wiper motor, relay RE1 will imperatively be an ‘automobile’ relay.</div>
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You can find very low-priced ones at many car accessory shops (and even at some component retailers). These relays maintain contact under 12 volts and often do not have more than one work contact but they are, in general, capable of cutting off about 20 amps. Finally, the only delicate point of this project is to properly identify the control wire for the windshield pump on one hand, and the windshield wiper motor on the other. Observing what is happening at the various connections with a simple voltmeter, should get it right without too much difficulty.</div>
</div>

Automatic Windshield Washer Control

Automatic Windshield Washer Control

Most, if not all, recent cars have an impressive amount of electronics, whether it be ABS brake systems, engine control with injection cal...

Simple Headlight Reminders

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These two headlight reminder circuits are easy to install and operate on the KISS (Keep It Simple Stupid) principle. The simple circuit involves adding just a 12V piezo buzzer between the lights circuit and a door switch. The buzzer sounds if the lights are left on and you open a door. The disadvantage of this simple circuit is that it's annoying to have the buzzer sound continuously if you want to leave the door open while the lights are on. The improved circuit overcomes that problem by adding a 1000µF capacitor and a parallel 100 K Ohm resistor in series with the buzzer. Now, when a door is opened, the buzzer gives a brief burst of sound only, while the 1000µF capacitor charges. The 100 K Ohm resistor discharges the capacitor when the lights are switched off.</div>
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<b><span style="color: #0b5394;">Circuit Diagram:</span></b></div>
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<br /></div>
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<img alt="Simple Headlight Reminders Circuit Diagram" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjiThwQbx44dWgvilX9zE8wxAyV86pIwO3v5UKfww-Q4bJrv58nNyliT88MW7PkOny3NsSX0fKaD-QiQ_TIfNX4g45SAX2lmyWZb7KUm0t4n_kUnhdhRo5KKcHe1fqJgeP1f_vKGo-qBjix/s1600/simple-headlight-reminders-circuit-diagram-2.gif" title="Simple Headlight Reminders Circuit" /></div>
</div>

Simple Headlight Reminders

Simple Headlight Reminders

These two headlight reminder circuits are easy to install and operate on the KISS (Keep It Simple Stupid) principle. The simple circuit in...

LED Bike Light Circuit

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On my mountain bike I always used to have one of those well-known flashing LED lights from the high street shop. These often gave me trouble with flat batteries and lights that fell off. As an electronics student I thought: “this can be done better”. First I bought another front wheel, one which has a dynamo already built in the hub. This supplied a nice sine wave of 30 Vpp (at no load). With this knowledge I designed a simple power supply. The transistors that are used are type BD911.</div>
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These are a bit of an over-kill, but there were plenty of these at my school, so that is why I used them. Something a little smaller will also work. The power supply is connected to an astable multi-vibrator. This alternately drives the front light and the rear light. The frequency is determined by the RC time-constant of R3 and C3, and R2 and C4. This time can be calculated with the formula: t = R3×C3 = 20×103×10×10-6 = 0.2 s You can use a 22k (common value) for R2 and R3, that doesn’t make much difference. On a small piece of prototyping board are six LEDs with a voltage dropping resistor in series with each pair of LEDs.</div>
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<b><span style="color: #0b5394;">Circuit Diagram:</span></b></div>
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<br /></div>
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<img alt="LED Bike Light Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEibBUf04PstL6Hr19-_63Gcv0rkTouYxAxnArZ3A85nOzBEEMVxl1ZN1Ur9vKnn83crGSP8x4c15LlSnHmUl5bAKZq8kEsp2Q8v9gpAFx1y4_aUsiJVR-E474yuBLnUFDR-3AtCIGHBOyRw/s1600/simple-led-bike-light-circuit-diagram-2.gif" title="LED Bike Light Circuit" /></div>
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Such a PCB is used for both the front and the rear of the bike. Of course, you use white LEDs for the front and red ones for the rear. The PCB with the main circuit is mounted under the seat, where it is safe and has been working for more than a year now. There are a few things I would change for the next revision. An on/off switch would be nice. And if the whole circuit was built with SMD parts it could be mounted near the front light. This would also be more convenient when routing the wiring. Now the cable from the dynamo goes all the way to the seat and from there to the front and rear lights.</div>
</div>

LED Bike Light Circuit

LED Bike Light Circuit

On my mountain bike I always used to have one of those well-known flashing LED lights from the high street shop. These often gave me troub...

Bicycle Speedometer With Hub Dynamo

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<b><i>Bicycle Speedometer With Hub Dynamo</i></b> | Basic bicycle speedometer circuit consists of two parts: the cycle computer itself and a transmitter that is mounted on the front fork. A small magnet is attached to the spokes so that the transmitter sends out a pulse for every revolution of the wheel (as long as everything has been fitted properly). Since the range of the transmitter is limited (about 75 cm), you’ll be lucky if it works straight away. And when the voltage of the battery starts to drop you can forget it. The following circuit gets round these problems.</div>
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<img alt="Bicycle Speedometer With Hub Dynamo" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhR9tdVaMw8sA5vmzM5RhNHR3fTzWAAcbaYnPUli7VziKwm5ZoHeGLskTvEGve3d5wAiT36v6FddVsqC_EEDi8OvbZenWRM8LLtNlNGSP_aFttQLRCV5hyphenhyphensDY85NJ-LR-VlfsLKAuQKp6QV/s1600/bicycle-speedometer-with-hub-dynamo.gif" title="Bicycle Speedometer With Hub Dynamo" /></div>
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A Shimano NX-30 hub dynamo has 28 poles. This results in 14 complete periods of a 6 V alternating voltage per revolution (when loaded by a lamp; under no load the voltage is much higher). C1, C2, D1 and D2 double the voltage of the AC output. Regulator IC2 keeps the voltage to the transmitter and the divider IC at a safe level (12 V, the same as the original battery). The divider chip (IC1) divides the frequency of the signal from the dynamo by 14, so that a single pulse goes to the transmitter for every revolution of the wheel.</div>
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<b><span style="color: #0b5394;">Circuit Diagram:</span></b></div>
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<img alt="Bicycle Speedometer With Hub Dynamo Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh8AW8wu7qdGW7YhltZXNClEISo88a4c3Y4_CxnHn4WOJysNmamgSsM7ydaOf3arduC2Bt66vJups5Liqhhyphenhyphen3lCrgCHZSl3kNRz0AH11ce3s1pJrvrbfUlxTJ4HLHRb-JLJU3ueDLbjwyaE/s1600/bicycle-speedometer-with-hub-dynamo-circuit-diagram.gif" title="Bicycle Speedometer With Hub Dynamo Circuit" /></div>
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This pulse enters the circuit at the point where the reed contact was originally. The circuit is built inside the front light, since it has enough room and a cable from the dynamo is already present. The distance to the cycle computer is smaller as well in that case. The following tip can be used if you want to save yourself a few components. In the author’s prototype the counter divided by 16 and the setting for the size of the wheel was adjusted to 16/14th of the real size in the setup of the cycle computer. In that case you can leave out D4, D5 and D8.</div>
</div>

Bicycle Speedometer With Hub Dynamo

Bicycle Speedometer With Hub Dynamo

Bicycle Speedometer With Hub Dynamo | Basic bicycle speedometer circuit consists of two parts: the cycle computer itself and a transmitte...

Complementary Symmetry Push-Pull Amplifiers

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The use of transformers, at input as well as at output ends, in the push-pull amplifier makes it bulky and expensive especially in this age of the integrated circuits. Another drawback of the circuit is is that it needs two out-of-phase signals which necessitates an input tapped transformer or phase inverter, and thus makes the driver circuitry quite complicated.</div>
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The above two drawbacks of an ordinary push-pull amplifier have been overcome in the complementary symmetry push-pull amplifier.</div>
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Thus arrangement uses two transistors having complementary symmetry (one NPN and another PNP). The term complementary arises from the fact that one transistor is the NPN type and the other is PNP type. They have symmetry as they are made with the same material and technology and are of same maximum rating.</div>
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</div>
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<img alt="Complementary Symmetry Push-Pull Amplifier" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjEvYwbU5HEOaOthfeTjsozU3LGvvz4Obfpfm0qji1hkWqvGilZg3MfgmwKEnDsCa7Ndkn7ZihFasouY8y0cyYWV2oH462DHpytlTSHhalH5KYS98WzuxrNkol_ApfrngqE0t-HpKlpat0H/s1600/Complementary+symmetry+push+pull+amplifier.gif" title="Complementary Symmetry Push-Pull Amplifier" /></div>
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<b><i>Fig. 1</i></b></div>
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The circuit of a complementary push-pull class AB power amplifier is shown in Fig. 1. The resistors R1 and R2 provide the voltage divider bias to forward bias the emitter-base junction of transistors Q1 and similarly resistors R3 and R4 provide the voltage divider bias for emitter junction of transistor Q2. The resistors are so selected that under zero signal condition, the operating point is at cutoff and so no collector current flows.</div>
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The signal applied at the input goes to the base of both transistors. The two transistors conduct in opposite half cycles of the input. For example during the positive half cycle of the input signal, the NPN transistor Q1 is forward biased and conducts while the PNP transistor Q2 is reverse biased and so does not conduct. This results in a half cycle of output voltage across the load, resistor <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">R<sub>L</sub></span>. Similarly during the negative half cycle only the PNP transistor Q2 is forward biased and conducts and develops second half cycle of the voltage voltage across the load resistor <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">R<sub>L</sub></span>. Transistor Q1 being reverse biased does not conduct during the negative half cycle of the input cycle of the input signal. Thus, during a complete cycle of input, a complete cycle of the output signal is developed.</div>
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Since collector current from each transistor flows through the load during alternate half cycles of the input signal, no centre tapped output transformer is required. However, for impedance matching an output transformer (without centre tapping) can be used.</div>
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<div style="text-align: justify;">
One obvious drawback of this circuit arrangement is the requirement for two supply voltages. Another drawback of this circuit is difficulty of obtaining matched complementary transistors. If there is an unbalance in the characteristics of the two transistors, even harmonics will no longer be cancelled and considerable distortion will be introduced. Very often negative feedback is used in power amplified to reduce nonlinear distortion.</div>
</div>

Complementary Symmetry Push-Pull Amplifiers

Complementary Symmetry Push-Pull Amplifiers

The use of transformers, at input as well as at output ends, in the push-pull amplifier makes it bulky and expensive especially in this ag...

Phase Inverter / Phase-Splitter Circuit for Push-Pull Amplifier

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In a push-pull amplifier circuit an input transformer is used to provide the polarity inversion between the two input signals to the push-pull circuit, but the transformer is quite expensive and bulky. The same result may be achieved by using a circuit called the <i>phase-splitter</i> or <i>phase inverter</i>. Such a circuit is shown in Fig. 1.</div>
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<img alt="Phase Inverter / Phase-Splitter Circuit for Push-Pull Amplifier" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi9m44wuRruLEE8YP7Mzpeevz0j_BxjSrzoIAePtf-lx-zamw4DjlXcsz2mq8gMqWFqRwgkZUHoaI2mSkdwlgxT1BP1_QH2e1q5QqqViam_gCoI1jFewxguI-CGPt-WWXyn2AwifhjH1FZs/s1600/phase-splitter.gif" title="Phase Inverter / Phase-Splitter Circuit" /></div>
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<b><i>Fig. 1. Phase Inverter Circuit</i></b></div>
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<div style="text-align: justify;">
The signal appearing at the collector is of polarity opposite to that of the input signal. During positive half cycle of input voltage, base current, and therefore, collector current increases causing increase in <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">I<sub>C</sub>R<sub>C</sub></span> drop and consequently decrease in collector-emitter voltage <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">V<sub>CE</sub></span>. Thus we get negative half cycle of <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">V<sub>CE</sub></span> i.e., phase of signal at collector of a CE transistor is reversed. The output from the emitter is of same polarity as the input. Thus the two output signals obtained from this circuit are of opposite polarity. Values of resistors <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">R<sub>C</sub></span>, <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">R<sub>E</sub></span> and <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">β</span> are chosen so as to make the voltage gain for the output signal at collector to be unity. The gain for signal at emitter is 1. Thus the circuit gives two signals of equal magnitude but of opposite polarities. The use of input transformer can thus be eliminated.</div>
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However, the circuit explained above has three shortcomings as follows:</div>
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</div>
<ul>
<li>Since this circuit is of the emitter-follower type, there is no voltage gain.</li>
<li>Since the emitter and collector currents are not equal, the output voltages will have some imbalance.</li>
<li>This phase inverter could not be used to drive a class B push-pull amplifier because output voltages are only equal if driving a resistance load over the entire cycle.</li>
</ul>
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The circuit can be used on a class A push-pull circuit, where the load is constant on each output.</div>
</div>

Phase Inverter / Phase-Splitter Circuit for Push-Pull Amplifier

Phase Inverter / Phase-Splitter Circuit for Push-Pull Amplifier

In a push-pull amplifier circuit an input transformer is used to provide the polarity inversion between the two input signals to the push-...

Class B Push-Pull Amplifier

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The circuitry for the class B push-pull operation is the same as that for the class A operation except that the devices are biased at cutoff. The transistor in class B amplifier circuit is essentially at cutoff if the base is shorted to the emitter. The advantages of class B operation over class A operation are given below:</div>
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<li>It is possible to obtain greater power output - class B push-pull amplifier provides practically 4-times the power supplied by a single ended amplifier provided load resistance remains the same.</li>
<li>It gives higher operating efficiency (theoretical 78.5%). It is primarily due to the fact that no power is drawn by the circuit under zero signal condition.</li>
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Because of above advantages, a push-pull class B transistor circuit is preferred in systems where the power supply is limited, such as those operating from solar cells or a battery. The automatic cancellation of all even-order harmonics from the output makes class B push-pull amplifiers highly desirable for communication sound equipment.</div>
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The drawbacks of class B push-pull amplifiers are that the harmonic distortion is higher, self-bias cannot be used, and the supply voltages must have good regulation.</div>
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Class B push-pull amplifiers are widely employed for audio work in portable record-players, as stereo amplifiers and in high-fidelity radio receivers.</div>
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<b><span style="color: #0b5394;">Circuit Arrangement</span></b></div>
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Circuitry for a class B push-pull is shown in Fig. 1. As illustrated in the figure, the bases of the two CE-connected identical transistors Q1 and Q2 have been connected to the opposite ends of the secondary of the input transformer T1. Load resistance <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">R<sub>L</sub></span> is coupled to the collector terminals of the two transistors through output transformer T2. The emitter terminals of the transistors are connected to the centre tap on the primary of output transformer. This is done in order to have balanced circuits. Since zero bias is required for cutoff, the two buses of the two transistors have been grounded.<br />
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<img alt="Class B Push-Pull Amplifier" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi18oBIaS-ANgdOxxcNfVlA21zu-RtHWyW-vlUx1qAeJR9Fc63xIx0OizIQ1PGYm89-P0DPGS_xG1kBHaxlVt8_4c4V4R3p1jWQZqQxSXgs3ekcD-QuGRY29WWt97hcVJcgkSqqY2WvjLDW/s1600/class-b-push-pull-amplifier.gif" title="Class B Push-Pull Amplifier" /></div>
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<b><i>Fig. 1. Class B Push-Pull Amplifier</i></b></div>
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<b><span style="color: #0b5394;">Circuit Operation:</span></b></div>
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When the input signal is applied, the centre tapped secondary of the input transformer develops two signals which are identical but in phase opposition. The transistors Q1 and Q2 are driven by these two signals. Thus when V1 is going positive, V2 is going negative, so that transistor Q2 is being biased further off when transistor Q1 is being biased on. As the collector current in Q1 increases from zero, it produces a half sine wave of voltage across the upper half of the primary of the output transformer, as shown. When the positive half cycle of the input signal to Q1 begins to go negative, the signal at Q2 base is commencing to go positive. Thus, as Q1 becomes biased off again Q2 is biased on and a half cycle of voltage waveform is generated across the lower primary winding of output transformer.</div>
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The effect of the half cycles in separate halves of the primary of output transformer is to produce a magnetic flux in the transformer core, which flows first in one direction and then in the opposite direction. This flux links with the secondary of the output transformer and generates a complete sine wave output, which is passed on to the load. In the class B circuit, the two output transistors are said to be operating in push-pull.</div>
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As explained above, a full sine wave is developed at the output of the transformer. When no signal is present, both transistors remain off, and thus no current is drawn from dc supply source <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">V</span><sub style="font-family: Calibri, sans-serif; line-height: 115%;">CC</sub>. Hence there is no dissipation of power with zero signal. Some power is dissipated in each transistor only while the transistor is conducting.</div>
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The load resistance (usually a loudspeaker) is connected across the secondary of the output transformer. The turn-ratio 2N1:N2 of the transformer is chosen so that load resistance <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">R<sub>L</sub></span> is matched with the output impedance of the transistor. Undermatched conditions, maximum power is delivered to the load by the amplifier.</div>
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Class B Push-Pull Amplifier

Class B Push-Pull Amplifier

The circuitry for the class B push-pull operation is the same as that for the class A operation except that the devices are biased at cuto...

Infrared Proximity Detector Alarm

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This circuit can be built from readily available low-cost components. The indicated value of 22 Ω for resistor R1 causes an average current of about 65 mA through infrared emitter D1. Because the IRED is pulsed at a duty factor of about 50% through the action of T1 and IC1, a peak current of 128 mA flows during every half cycle. This may seem a lot but in fact is well within the safe speciﬁcation of the LD274. The LM567 PLL IC is configured to supply a switching frequency of about 20 kHz. When the infrared beam emitted by D1 is reflected by a nearby object, IC1, through receiver diode D2 and transistor T2, receives the recovered 20 kHz signal at its input, pin 3.</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg2Y-daoaTLjcVdRcaf2Yog2skUJGZ0bDI7kmNMBT9VeYZW1uVR1-Scttl5IbPhT-wu2qtpYSZaa9MYdMUbE_QJWTKS_MKFdaRq8hVKBz-7oz9SQy9BYYmHWXfG4ZW3LqvpDZOK2RYC1JP0/s1600/infrared_proximity_detector_alarm_circuit_diagram.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img alt="Infrared Proximity Detector Alarm" border="0" height="551" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg2Y-daoaTLjcVdRcaf2Yog2skUJGZ0bDI7kmNMBT9VeYZW1uVR1-Scttl5IbPhT-wu2qtpYSZaa9MYdMUbE_QJWTKS_MKFdaRq8hVKBz-7oz9SQy9BYYmHWXfG4ZW3LqvpDZOK2RYC1JP0/s640/infrared_proximity_detector_alarm_circuit_diagram.gif" title="Infrared Proximity Detector Alarm" width="640" /></a></div>
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Because the ‘567 PLL is then locked, the IC output (pin 8), drops low, triggering the 555 chip in monostable mode (IC2) and so causing acoustic actuator Bz1 to sound. The monostable remains on as long as the reﬂected signal is being received. Because of the presence of T3, capacitor C5 is allowed to charge only when no signal is being received. In that condition, the 555 is turned off automatically after a time determined by R9-C5. Using the component values shown, this will be about 5 seconds. Obviously D1 and D2 should be mounted such that the latter can only pick up reﬂected infrared light.</div>
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The choice of the two infrared components used in this circuit will be uncritical but they must be ‘band’ compatible, i.e., generate (D1) and respond to (D2) the same wavelength. The operating point of the receiver input circuit is rather dependent on ambient day-light levels and the value of R4 may need to be adjusted a little to ensure a voltage of between 1.5 V and 4 V on the collector of T1 when no signal is being received. Some dc buzzers cause a lot of back-emf so it may be necessary to insert a diode in series with the output of IC1. If necessary, this diode should preferably be a Schottky type because of the inherent low voltage drop of about 0.4 V as opposed to 0.65 V for a typical small-signal silicon diode.</div>
</div>

Infrared Proximity Detector Alarm

Infrared Proximity Detector Alarm

This circuit can be built from readily available low-cost components. The indicated value of 22 Ω for resistor R1 causes an average curren...

Power Buzzer

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<span style="background-color: white; font-family: Verdana, Arial, Tahoma, sans-serif; font-size: 13px; line-height: 18.2000007629395px; text-align: left;">How often on average do you have to call members of your family each day to tell them that dinner is ready, it’s time to leave, and the like? The person you want is usually in a different room, such as the hobby room or bedroom. A powerful buzzer in the room, combined with a push-button at the bottom of the stairs or in the kitchen, could be very handy in such situations. </span>The heart of this circuit is formed by IC1, a TDA2030. This IC has built-in thermal protection, so it’s not likely to quickly give up the ghost. R1 and R2 apply a voltage equal to half the supply voltage to the plus input of the op-amp. R3 provides positive feedback. Finally, the combination of C2, R4 and trimmer P12 determines the oscillation frequency of the circuit.</div>
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<img alt="Power Buzzer Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgEUt5zxBZMl-huxGMDck_r5PUvP99ghrp3rkF5t5yvQkL61pvV54RCeYrM0REc8yzYTrQhFWEpST6IJSWRh-EHgz8yAdSBmJe5am6iPR57xr9FkIdbwujs4llFRlO7X8k3veITNbEje0OM/s1600/power_buzzer_circuit_diagram.gif" title="Power Buzzer Circuit" /></div>
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The frequency of the tone can also be adjusted using P1. There is no volume control, since you always want to get attention when you press push-button S1. Fit the entire circuit where you want to have the push-button. The loudspeaker can then be placed in a strategic location, such as in the bedroom or wherever is appropriate. Use speaker cable to connect the loudspeaker. Normal bell wire can cause a signiﬁcant power loss if the loudspeaker is relatively far away. The loudspeaker must be able to handle a continuous power of at least 6 W (with a 20-V supply voltage).</div>
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The power quickly drops as the supply voltage decreases (P = <span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">V<sub>rms</sub></span> 2 / RL). The power supply for this circuit is not particularly critical. However, it must be able to provide sufficient current. A good nominal value is around 400 mA at 20 V. At 4 V, it will be approximately 25 mA. Most likely, you can find a suitable power supply somewhere in your hobby room. Otherwise, you can certainly ﬁnd a low-cost power supply design in our circuits archive that will ﬁll the bill!</div>
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Power Buzzer

How often on average do you have to call members of your family each day to tell them that dinner is ready, it’s time to leave, and the li...

General-Purpose Alarm

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The alarm may be used for a variety of applications, such as frost monitor, room temperature monitor, and so on. In the quiescent state, the circuit draws a current of only a few microamperes, so that, in theory at least, a 9 V dry battery (PP3, 6AM6, MN1604, 6LR61) should last for up to ten years. Such a tiny current is not possible when ICs are used, and the circuit is therefore a discrete design. Every four seconds a measuring bridge, which actuates a Schmitt trigger, is switched on for 150 ms by a clock generator. In that period of 150 ms, the resistance of an NTC thermistor, R11, is compared with that of a fixed resistor. If the former is less than the latter, the alarm is set off.</div>
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When the circuit is switched on, capacitor C1 is not charged and transistors T1–T3 are off. After switch-on, C1 is charged gradually via R1, R7, and R8, until the base voltage of T1 exceeds the threshold bias. Transistor T1 then comes on and causes T2 and T3 to conduct also. Thereupon, C1 is charged via current source T1-T2-D1, until the current from the source becomes smaller than that flowing through R3 and T3 (about 3 µA). This results in T1 switching off, so that, owing to the coupling with C1, the entire circuit is disabled. Capacitor C1 is (almost) fully charged, so that the anode potential of D1 drops well below 0 V. Only when C1 is charged again can a new cycle begin.</div>
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It is obvious that the larger part of the current is used for charging C1. Gate IC1a functions as impedance inverter and feedback stage, and regularly switches on measurement bridge R9–R12-C2-P1 briefly. The bridge is terminated in a differential amplifier, which, in spite of the tiny current (and the consequent small transconductance of the transistors) provides a large amplification and, therefore, a high sensitivity. Resistors R13 and R15 provide through a kind of hysteresis a Schmitt trigger input for the differential amplifier, which results in unambiguous and fast measurement results. Capacitor C2 compensates for the capacitive effect of long cables between sensor and circuit and so prevents false alarms.</div>
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<b>Circuit Diagram</b></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEipE269fCi8Rv1LElT9dB4Q4DDtPPLEvdvJkxCgO_ISJRp7YfF50n7VIVJ6rdDgTH7F6J5JdtsCiEGmH2xot21phfk2hxG-2nOKY_bytIiCrh7el_JCbzJh-WD-7HRkm_V36fBwDxW-JX1k/s1600/general_purpose_alarm_circuit_diagram.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img alt="General Purpose Alarm Circuit" border="0" height="376" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEipE269fCi8Rv1LElT9dB4Q4DDtPPLEvdvJkxCgO_ISJRp7YfF50n7VIVJ6rdDgTH7F6J5JdtsCiEGmH2xot21phfk2hxG-2nOKY_bytIiCrh7el_JCbzJh-WD-7HRkm_V36fBwDxW-JX1k/s640/general_purpose_alarm_circuit_diagram.gif" title="General Purpose Alarm" width="640" /></a></div>
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If the sensor (R11) is built in the same enclosure as the remainder of the circuit (as, for instance, in a room temperature monitor), C2 and R13 may be omitted. In that case, C3 will absorb any interference signals and so prevent false alarms. To prevent any residual charge in C3 causing a false alarm when the bridge is in equilibrium, the capacitor is discharged rapidly via D2 when this happens. Gates IC1c and IC1d form an oscillator to drive the buzzer (an a.c. type). Owing to the very high impedance of the clock, an epoxy resin (not pertinax) board must be used for building the alarm. For the same reason, C1 should be a type with very low leakage current. If operation of the alarm is required when the resistance of R11 is higher than that of the fixed resistor, reverse the connections of the elements of the bridge and thus effectively the inverting and non-inverting inputs of the differential amplifier.</div>
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An NTC thermistor such as R11 has a resistance at –18 °C that is about ten times as high as that at room temperature. It is, therefore, advisable, if not a must, when precise operation is required, to consult the data sheet of the device or take a number of test readings. For the present circuit, the resistance at –18 °C must be 300–400 kΩ. The value of R12 should be the same. Preset P1 provides fine adjustment of the response threshold. Note that although the prototype uses an NTC thermistor, a different kind of sensor may also be used, provided its electrical specification is known and suits the present circuit.</div>
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General-Purpose Alarm

General-Purpose Alarm

The alarm may be used for a variety of applications, such as frost monitor, room temperature monitor, and so on. In the quiescent state, t...

Electronics Projects/Circuits

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<ol style="text-align: left;">
<li><a href="http://www.ecestream.com/2014/05/seven-segment-intruder-alarm-circuit.html">Seven Segment Intruder Alarm Circuit</a></li>
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<li><a href="http://www.ecestream.com/2014/05/electronic-fuse-circuit.html">Electronic Fuse Circuit</a></li>
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<li><a href="http://www.ecestream.com/2014/11/designing-rf-probe.html">Designing An RF Probe</a></li>
<li><a href="http://www.ecestream.com/2014/11/protection-for-your-electrical.html">Protection For Your Electrical Appliances</a></li>
<li><a href="http://www.ecestream.com/2014/11/multipurpose-circuit-for-telephones.html">Multipurpose Circuit for Telephones</a></li>
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<li><a href="http://www.ecestream.com/2014/11/rain-alarm-circuit.html">Rain Alarm Circuit</a></li>
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<li><a href="http://www.ecestream.com/2014/11/automatic-water-tap-faucetvalve.html">Automatic Water Tap (Faucet/Valve) Controller</a></li>
<li><a href="http://www.ecestream.com/2015/03/led-chaser-circuit-using-cd4017-and.html">LED Chaser Circuit using CD4017 and NE555 Timer</a></li>
<li><a href="http://www.ecestream.com/2015/03/led-vu-meter-with-lm3916.html">LED VU Meter with LM3916</a></li>
<li><a href="http://www.ecestream.com/2015/03/triac-based-lamp-dimmer-circuit.html">Triac based lamp dimmer</a></li>
<li><a href="http://www.ecestream.com/2015/03/led-chaser-circuit.html">LED Chaser Circuit</a></li>
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Electronics Projects/Circuits

Electronics Projects/Circuits

Seven Segment Intruder Alarm Circuit Speed Checker for Highways Sensitive Intruder Alarm Circuit Electronic Fuse Circuit Simple Code...

Simple Darkness Activated Alarm

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Most darkness activated alarms employ op-amps and some logic ICs. Here, a less expensive approach is shown based on the eternal 555, this time in monostable multivibrator mode. Components R2 and C1 represent a one-second network. When the LDR (light dependent resistor) is in the dark, its resistance is high, pulling pin 2 of the 555 to ground. This triggers the monostable and the (active!) 6-volt piezo buzzer will sound. Preset P1 is adjusted depending on ambient light levels. The circuit may be fitted on a wall in your home. Assuming P1 has been set for the existing ambient light level, the shadow cast by anybody entering the room or hallway will trigger the alarm.</div>
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<b>Circuit Diagram</b></div>
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<img alt="Simple Darkness Activated Alarm Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi_whB1_Eerk5xdQsllFFNCuc_SBBiZUGXEY7JPOMiKKhKr4cBIH45mmXDZkv2CPaC8WhVba3jUhAb6r8zRxbV_AlLT109L8N2I0z7wC26uesIs4kcT3ATESSFAzvOnkpZvAB5DERxX-VpP/s1600/simple-darkness-activated-alarm-circuit-diagram.gif" title="Simple Darkness Activated Alarm" /></div>
</div>

Simple Darkness Activated Alarm

Simple Darkness Activated Alarm

Most darkness activated alarms employ op-amps and some logic ICs. Here, a less expensive approach is shown based on the eternal 555, this ...

Model Railway Short-Circuit Beeper

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Short circuits in the tracks, points or wiring are almost inevitable when building or operating a model railway. Although transformers for model systems must be protected against short circuits by built-in bimetallic switches, the response time of such switches is so long that is not possible to immediately localise a short that occurs while the trains are running, for example. Furthermore, bimetallic protection switches do not always work properly when the voltage applied to the track circuit is relatively low. The rapid-acting acoustic short-circuit detector described here eliminates these problems. However, it requires its own power source, which is implemented here in the form of a GoldCap storage capacitor with a capacity of 0.1 to 1 F. A commonly available reed switch (filled with an inert gas) is used for the current sensor, but in this case it is actuated by a solenoid instead of a permanent magnet.</div>
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An adequate coil is provided by several turns of 0.8–1 mm enamelled copper wire wound around a drill bit or yarn spool and then slipped over the glass tube of the reed switch. This technique generates only a negligible voltage drop. The actuation sensitivity of the switch (expressed in ampère-turns or A-t)) determines the number of turns required for the coil. For instance, if you select a type rated at 20–40 A-t and assume a maximum allowable operating current of 6 A, seven turns (40 ÷ 6 = 6.67) will be sufficient. As a rule, the optimum number of windings must be determined empirically, due to a lack of specification data. As you can see from the circuit diagram, the short-circuit detector is equally suitable for AC and DC railways. With Märklin transformers (HO and I), the track and lighting circuits can be sensed together, since both circuits are powered from a single secondary winding.</div>
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<b>Circuit Diagram</b></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiEahYT0GQbjmt5fAPeg0LvHYH1d4iOkzH4HplppYU2MpjA55Kub2tK4oHNbEgeLoHiOUVoSePUmvNpI3B2kVyFhE9UTB0XHJhyKjeCXKC7mYF4Rj-mAp6GszAjZlhA6wCMIMgLBYxPqtE1/s1600/model-railway-short-circuit-beeper-circuit-diagram.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img alt="Model Railway Short-Circuit Beeper Circuit" border="0" height="422" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiEahYT0GQbjmt5fAPeg0LvHYH1d4iOkzH4HplppYU2MpjA55Kub2tK4oHNbEgeLoHiOUVoSePUmvNpI3B2kVyFhE9UTB0XHJhyKjeCXKC7mYF4Rj-mAp6GszAjZlhA6wCMIMgLBYxPqtE1/s640/model-railway-short-circuit-beeper-circuit-diagram.gif" title="Model Railway Short Circuit Beeper Circuit" width="640" /></a></div>
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Coil L1 is located in the common ground lead (‘O’ terminal), so the piezoelectric buzzer will sound if a short circuit is present in either of the two circuits. The (positive) trigger voltage is taken from the lighting circuit (L) via D1 and series resistor R1. Even though the current flowing through winding L1 is an AC or pulsating DC current, which causes the contact reeds to vibrate in synchronization with the mains frequency, the buzzer will be activated because a brief positive pulse is all that is required to trigger thyristor Th1. The thyristor takes its anode voltage from the GoldCap storage capacitor (C2), which is charged via C2 and R2.</div>
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The alarm can be manually switched off using switch S1, since although the thyristor will return to the blocking state after C2 has been discharged if a short circuit is present the lighting circuit, this will not happen if there is a short circuit in the track circuit. C1 eliminates any noise pulses that may be generated. As a continuous tone does not attract as much attention as an intermittent beep, an intermittent piezoelectric generator is preferable. As almost no current flows during the intervals between beeps and the hold current through the thyristor must be kept above 3 mA, a resistor with a value of 1.5–1.8 kΩ is connected in parallel with the buzzer. This may also be necessary with certain types of continuous-tone buzzers if the operating current is less than 3 mA. The Zener diode must limit the operating voltage to 5.1 V, since the rated voltage of the GoldCap capacitor is 5.5 V.</div>
</div>

Model Railway Short-Circuit Beeper

Model Railway Short-Circuit Beeper

Short circuits in the tracks, points or wiring are almost inevitable when building or operating a model railway. Although transformers for...

Laser Alarm

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This circuit is a laser alarm system like the one we see in various movies. It uses a laser pointer beam to secure your valuables and property. Essentially, when the beam gets interrupted by a person, animal or object, the resistance of a photodiode will increase and an alarm will be activated. The laser and the receiver can be fitted in same box, sharing a common power supply. As the receiver draws less than 10 mA on average, you’ll soon find that the laser is the most current hungry device! Mirrors are used to direct the beam in whatever setup you require. Examples of a passage and an area protected by the alarm are shown in the diagram.</div>
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In the circuit diagram we find a TL072 op-amp (IC1.A) configured as voltage comparator between the voltage reference provided by the adjustable voltage divider P1/R4 and the light-dependent voltage provided by the voltage divider consisting of photodiode D1 and fixed resistor R3. When the laser beam is interrupted, the voltage on comparator pin 2 drops below that at pin 3, causing the output to swing to (almost) the positive supply voltage and indicating an alarm condition. This signal can drive a siren, a computer or a light that hopefully will deter the intruder.</div>
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<b>Circuit Diagram</b></div>
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<img alt="Laser Alarm Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhWQsPv2quxFsI06g_mcEb61vZRWjfmt9GoBsANmM-ozCUbYOXwDFq1e8kaDP9Skvs9GMei5Y9WPQpSnuOCr4KDGBq1Bl4fPj6NgRO5csZ7eV4dz31ItzhsjHLtnuhSq5CnqSpfgy9bsZ8t/s1600/laser-alarm-killer-circuit-diagram.gif" title="Laser Alarm Circuit" /></div>
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Alternatively it can be used to ‘silently’ trigger a more sophisticated alarm. Resistor R2 provides some hysteresis to prevent oscillation when the two comparator input voltages are almost equal. Capacitor C1 makes the circuit immune to short, accidental interruptions of the beam, e.g., by flying insects. If you want your circuit to have faster responses you can reduce its value to 1 µF. The operation of the circuit is illustrated by the waveform diagram, which also proves the hysteresis action that sets an upper and a lower threshold on the input voltage. You can also see the delay introduced by capacitor C1.</div>
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The circuit is simple and could be assembled on a piece of breadboard. After assembling the circuit and testing it, you should mount it in a black box that has just a small hole. You may decide to put the laser in the same box but only if you are sure there is no way the photodiode can ‘see’ the laser beam directly. The small hole should be filled with a black drinking straw so that only light from the direction of the laser beam can enter. With the appropriate setup of the box and the mirrors, the laser beam is so intense that even direct sunlight cannot affect the operation of the photodiode.</div>
</div>

Laser Alarm

This circuit is a laser alarm system like the one we see in various movies. It uses a laser pointer beam to secure your valuables and prop...

8051 Circuits

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<b>8051 Projects/Circuits:</b></div>
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</div>
<ol>
<li><a href="http://www.ecestream.com/2014/11/line-follower-robot-using-8051.html">Line Follower Robot using 8051</a></li>
<li><a href="http://www.ecestream.com/2014/11/interfacing-hex-keypad-to-8051.html">Interfacing Hex Keypad to 8051</a></li>
<li><a href="http://www.ecestream.com/2014/11/interfacing-dot-matrix-led-display-to.html">Interfacing Dot Matrix LED Display to 8051</a></li>
<li><a href="http://www.ecestream.com/2014/11/delay-using-8051-timer.html">Delay Using 8051 Timer</a></li>
<li><a href="http://www.ecestream.com/2014/05/water-level-controller-using-8051.html">Water Level Controller using 8051</a></li>
<li><a href="http://www.ecestream.com/2014/05/voltmeter-using-8051-microcontroller.html">Voltmeter using 8051 Microcontroller</a></li>
<li><a href="http://www.ecestream.com/2014/05/random-number-generator-using-8051.html">Random Number Generator Using 8051</a></li>
<li><a href="http://www.ecestream.com/2014/05/object-counter-using-8051.html">Object Counter using 8051</a></li>
</ol>
</div>
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<ul>
<li><a href="http://www.ecestream.com/search/label/Intel%208051%20Microcontroller">8051 Tutorials</a></li>
</ul>
</div>
<hr style="text-align: justify;" />
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<img alt="8051 Projects" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjdSmSRF42ulG6geSgd4UtHGrFpUGJ3tGT7EauuObZsMbHPnX2yl2snOPAYn6_Ad8yIkvxCGJ9ZcOrNc7qeSaPj_qjmpsrcoUPTyfFkcQzd7UR3fwCLSVN54OmK5RgFMZxR7Vc6w-_GoKAd/s1600/8051-Microcontroller-Projects.gif" title="8051 Projects" /></div>
</div>

8051 Circuits

8051 Projects/Circuits: Line Follower Robot using 8051 Interfacing Hex Keypad to 8051 Interfacing Dot Matrix LED Display to 8051 D...

Speed Alarm For Cars

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In normal suburban driving you pass through so many different speed zones that it can be a nuisance having to switch speed settings. The speed display can also be a distraction. This circuit eliminates the display and the need for speed selection. Each time you exceed a particular speed setting (eg, 40km/h, 50km/h, etc), a piezo buzzer will beep. Speed pulses are fed to the base of Q1 and the resulting waveform at its collector is fed via an RC network to the input of an LM2917 frequency-to-voltage converter, IC1. The resulting voltage is fed to three comparators (IC2d-IC2b) which have the reference voltages at their inverting inputs set by 10-turn trimpots VR1, VR2 & VR3. The output of each comparator is applied via another RC network to the gate of an SCR. The anodes of the three SCRs are commonly connected to the inverting input of the remaining comparator, IC2a.</div>
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<b>Circuit Diagram</b></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhMY9wSfSsm-7Pg0ZrhQHoyQfTVAClV-6i-V2eU2O-3Sw9bHc_0t-gDqhhxWaRpRlnS1NbCewAFdajCIdqJBH4lZSEo6fgI6L5kZU4qdj7b3oABDhRZ3ge0H-ODTnHMJqdLL03ZapCwcphh/s1600/speed-alarm-for-cars.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img alt="Speed Alarm For Car Circuit" border="0" height="302" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhMY9wSfSsm-7Pg0ZrhQHoyQfTVAClV-6i-V2eU2O-3Sw9bHc_0t-gDqhhxWaRpRlnS1NbCewAFdajCIdqJBH4lZSEo6fgI6L5kZU4qdj7b3oABDhRZ3ge0H-ODTnHMJqdLL03ZapCwcphh/s640/speed-alarm-for-cars.jpg" title="Speed Alarm For Car Circuit" width="640" /></a></div>
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Its non-inverting input is set to +2.3V by trimpot VR4. In use, once you exceed the speed setting for a particular comparator, its associated SCR briefly conducts to pull pin 2 of IC2a low and a short beep is emitted by the piezo buzzer. Then, as you exceed the next speed setting, another beep will be heard. The idea is make each speed setting a few km/h higher than actual so that if you are driving at the correct speed in a given zone, the buzzer will not sound. But as you increase speed, the buzzer will beep once as you exceed the speed setting for each zone. In this way, there is no need to continually switch speed settings as you drive through different zones and you can choose to ignore beeps that are not "illegal".</div>
</div>

Speed Alarm For Cars

Speed Alarm For Cars

In normal suburban driving you pass through so many different speed zones that it can be a nuisance having to switch speed settings. The s...

Night Alert

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Idea of this circuit came to me at midnight when my pet dog started barking continuously on sensing a moving shadow, perhaps that of an intruder. Dogs have a night adaptation capability to maximise the sensitivity of vision in low light. They are well adapted to see moving objects rather than stationary ones in darkness. This circuit turns a lamp ‘on’ for a short duration when the dog barks, giving an impression that the occupants have been alerted.</div>
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The condenser microphone fitted in the dog’s cage senses barking sound and generates AC signals, which pass through DC blocking capacitor C1 to the base of transistor BC549 (T1). Transistor T1 along with transistor T2 amplifies the sound signals and provides current pulses from the collector of T2. The input trigger pulse is applied to the collector of transistor T3 and coupled by capacitor C3 to the base of transistor T4 causing T4 to cut off. The collector voltage of transistor T4 forward biases transistor T3 via resistor R8.</div>
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<b>Circuit Diagram</b></div>
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<img alt="Night Alert Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjdJm5eYqdRWXBS1xB_lHfn6WPc9jwox6d-RK4D_OUMRm8jlaU3T1oUW1NkUhJU-Edjp0SP0XmYfKhoeh1rDVqwOnti-8nHZ_Wqvp7OfCHk8Mdy1nNiYj0Zn6jbvqh7Zc5lskrhOH6X__uE/s1600/night-alert-circuit-diagram.gif" title="Night Alert Circuit" /></div>
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Transistor T1 conducts and capacitor C3 discharges to keep transistor T4 cut-off. Transistor T4 remains cut-off until capacitor C3 charges enough to enable it to conduct. When transistor T4 conducts, its collector voltage goes low to drive transistor T3 into cut-off state. Resistor R9 and capacitor C3 are timing components. When fully charged, capacitor C3 takes about two minutes to discharge. So when sound is produced in front of the condenser mic, TRiAC1 (BT136) fires and the bulb (B1) glows for about two minutes.</div>
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<b>Assembly instruction:</b></div>
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Assemble the circuit on a general-purpose PCB and enclose in a plastic cabinet. Power to the circuit can be derived from a 12V, 500mA step-down transformer with rectifier and smoothing capacitor. Solder the triac ensuring sufficient spacing between the pins to avoid short circuit. Fix the unit in the dog’s cage, with the lamp inside or outside as desired. Connect the microphone to the circuit using a short length of shielded wire. Enclose the microphone in a tube to increase its sensitivity.</div>
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<b>Warning:</b></div>
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Since the circuit uses 230V AC, many of its points are at AC mains voltage. it could give you lethal shock if you are not careful. So if you don’t know much about working with line voltages, do not attempt to construct this circuit. EFY will not be responsible for any kind of resulting loss or damage.</div>
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Night Alert

Idea of this circuit came to me at midnight when my pet dog started barking continuously on sensing a moving shadow, perhaps that of an in...

Simple Knock Alarm With Piezo Sensor

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This circuit uses a thin piezoelectric sensor to sense the vibrations generated by knocking on a surface; e.g, a door or table. Basically, it amplifies and processes the signal from the sensor and sounds an alarm for a preset period. In operation, the piezoelectric sensor converts mechanical vibration into an electrical signal. This sensor can be attached to a door, a cash box, cupboard, etc using adhesive. A 1-1.5m long shielded cable can then be connected between the sensor plate and the input of the circuit. The signal generated by the sensor is amplified by transistors Q1-Q3 which are wired as common-emitter amplifiers.</div>
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<b>Circuit Diagram</b></div>
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<img alt="Simple Knock Alarm With Piezo Sensor Circuit" border="0" height="268" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiZbsq9CW-t00l_Jrl3ssBKPxZjh-gkuCVkWT6OOfiD8pAevgI829Cdt5H6Jy9tc-DuT5qy8pWtywHKJV2psi4zXrxR3IOjXUS8z61za2bSX4SaRLQnUXwVzI-tQ0RjRBtHhnDuXEnMF87B/s640/simple-knock-alarm-with-piezo-sensor.jpg" title="Simple Knock Alarm With Piezo Sensor" width="640" /></div>
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The signal is then rectified by diode D1 and amplified by transistors Q4-Q6. As shown, the output from Q6's collector is fed to pin 4 (reset) of 555 timer IC1. This is wired as an astable multivibrator. Each time Q6 turns on, its collector goes high and IC1 activates and produces an alarm tone in the speaker. The alarm automatically turns off 10s after knocking ceases - ie, the time taken for the 22µF capacitor on Q4's emitter to discharge. Finally, note that it may be necessary to adjust the 470O resistor in Q6's collector circuit to ensure that IC1 remains off in the absence of any perceptible knock. A value somewhere between 220O and 680O should be suitable.</div>
</div>

Simple Knock Alarm With Piezo Sensor

Simple Knock Alarm With Piezo Sensor

This circuit uses a thin piezoelectric sensor to sense the vibrations generated by knocking on a surface; e.g, a door or table. Basically,...

Class B Power Amplifiers

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In class B operation the transistor is so biased that zero signal collector current is zero. Hence class B operation does not need any biasing system. The operating point is set at cutoff, as illustrated in Fig. 1 & 2. It remains forward biased for only half cycle of the input signal i.e., its conduction angle is 180<span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">°</span>.</div>
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<img alt="Class B Operation" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjsYzyuASpoZQPRRpIi9trAYFPuiohN5TYaaepklFsU6fuoPPcDaJoROTvB0iAFg1Xx1IKjJVvcFPfIGafze8A3L_ZhaPYaceAMf63aFab6Ok87UkPxQocriZEH3SFGAlp9ptt9Swr5mtje/s1600/Class+B+Operation.png" title="Class B Operation" /></div>
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<b><i>Fig. 1</i></b></div>
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<img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjqcore7wD62yo9WdlaNnV7qqWLLKQQgU6NSWpp2kyNzOHRA1rBQbz6c1iodqPyKtCX3ovIujuk-H78Bf72iq5hq-XyRBOMwd7V9eWXd6E1yeBCYfW-ybYyUcoQ3I1wNLrjl7EqBIBGV1aT/s1600/Class+B+Operation1.png" /></div>
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<b><i>Fig. 2</i></b></div>
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As illustrated in Fig. 1 & 2, during the positive half cycle of the input ac signal, the circuit is forward biased and, therefore, collector current flows. On the other hand, during negative cycle of the input ac signal, the circuit is reverse biased and no collector current flows.<br />
<br />
<b>Advantages:</b><br />
(<i>i</i>) Very low standing bias current. Negligible power consumption without signal.<br />
(<i>ii</i>) Can be used for much more powerful outputs than class A<br />
(<i>iii</i>) More efficient than Class A.<br />
<br />
<b>Disadvantages:</b><br />
(<i>i</i>) Creates Crossover distortion.<br />
(<i>ii</i>) Supply current changes with signal, stabilized supply may be needed.<br />
(<i>iii</i>) More distortion than Class A.</div>
</div>

Class B Power Amplifiers

Class B Power Amplifiers

In class B operation the transistor is so biased that zero signal collector current is zero. Hence class B operation does not need any bia...

Class A Push-Pull Amplifier

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A class A push-pull amplifier circuit is shown in Fig.1. A push-pull amplifier means that current flows in the output of the active device (each transistor) for the entire of the input cycle.</div>
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<b><span style="color: #0b5394;">Circuit Arrangement:</span></b></div>
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The circuitry of a typical push-pull amplifier uses two identical transistors, say Q1 and Q2 is shown in Fig. 1. The emitter terminals of two transistors Q1 and Q2 connected together. The input signal is applied to the inputs of two transistors through centre tapped step up transformer T1 which provides opposite polarity signals to the two transistor inputs. The collector terminal of both the transistors are connected to end terminals of centre tapped primary of output transformer T2. The power supply Vcc is connected between the emitter terminals and the centre tap of primary of primary of output transformer. Resistors R1 and R2 are used to provide the proper bias for the circuit. The load Rl is connected across the secondary of the output transformer T2. The turn ratio (2N1:N2) of the output transformer is chosen so as to match the load with the output impedance of the amplifier and, therefore, transfer maximum power. The quiescent currents of the two transistors, which are equal in magnitude, flow in opposite directions through each half of primary of the output transformer T2, so no saturation of the magnetic core occurs.<br />
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<img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhS7p6Cu8638x6hMBeW7k6B_ntpbcYo-pHjKWCqfbd2I5RDYxjDDTqlFc7XB04-lWM4GAZw1DYvXvSu8_vp8ll9oLMpiudGk70RFBFoXfKNNYG5Y3HOYkQ8ZYgZjXdCOsTm3-GHiqquM4DO/s1600/class-A-pull-pull-amplifier.gif" /></div>
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<b><i>Fig. 1. Class A Push-Pull Amplifier</i></b></div>
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<b><span style="color: #0b5394;">Circuit Operation:</span></b></div>
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When the base current of one transistor is being driven positive with respect to the quiescent point Q, the collector current increases, thus causing a decrease in collector potential relative to ground. At the same time, however, a reverse action takes place in the base circuit of the second transistor, i.e., base current decreases causing a drop in the collector current with a consequent rise in collector potential w.r.t. ground. This means that the ac current flowing through the transformer primary winding is in the same direction. As I1 increases (i.e. pulls), the current I2 decreases (i.e. pushes). Hence the name <i>push-pull amplifier</i>. The overall operation results in ac voltage induced in the secondary of the output transformer and thus ac power is delivered to the load. The difference of two collector currents is illustrated in Fig. 2.<br />
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<img alt="Difference of two Collector Currents" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZO3fSpTc9PLu2nOU9d-DJRaIAgYTcfk95SPzXyCsKf5DVgwl5R7i3YyPIROwySuCiBYbZYDrbYPaOjze_01RWCNk22VXnxRE3AQxno_IoYd31G7AKLA4JdOhqj7cfz3wPO-H44gDMO9c5/s1600/class-A-pull-pull-amplifier-difference-of-collector-currents.gif" title="Difference of two Collector Currents" /></div>
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<b><i>Fig. 2</i></b></div>
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<b><span style="color: #0b5394;">Advantages:</span></b></div>
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<ul>
<li>Because of absence of even harmonics in the output of the push-pull amplifier, such a circuit gives more output per active device for a given amount of distortion or less distortion for a given power output power transistor.</li>
<li>As already mentioned earlier, the dc components of the collector current oppose each other magnetically in the transformer core. This eliminates any tendency toward core saturation and consequent non-linear distortion that may arise from the curvature of the transformer magnetization curve.</li>
<li>Another advantage of this system is that the effects of ripple voltage that may be continued in the power supply because of inadequate filtering will be balanced out. This cancellation results because the currents produced by the ripple voltage are in opposite directions in the transformer winding, and so will not appear in the load. Of course, the power supply hum will also act on the voltage amplifier stages, and so will be part of the input to power stage. This hum will not be eliminated by the push-pull circuit.</li>
</ul>
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<b><span style="color: #0b5394;">Disadvantages:</span></b></div>
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The drawbacks of push-pull amplifiers are:</div>
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(i) requirement of two identical or matched pair transistors</div>
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(ii) need of use of driver stage to furnish two equal and opposite voltages at the input and</div>
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(iii) need of bulky and expensive transformers.</div>
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Class A Push-Pull Amplifier

Class A Push-Pull Amplifier

A class A push-pull amplifier circuit is shown in Fig.1. A push-pull amplifier means that current flows in the output of the active device...

Push Pull Amplifiers

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<span style="color: red;"><i>Double-ended or push-pull amplifier makes use of two identical transistors in a single stage. It consists of two loops in which the transistor collector currents flow in opposite directions but add in the load.</i></span></div>
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Push-pull amplifiers eliminate all of three drawbacks. These drawbacks are; (i) The total harmonic distortion is unacceptability high for most audio applications except in cheaper amplifiers. The second harmonic distortion is usually the greatest offender, (ii) Maximum output power is delivered at a load resistance that is different from the value for minimum harmonic distortion. Consequently, the amplifier is designed to operate into a load that provides slightly less than maximum output power, but a significantly lower total distortion, (iii) The output transformer is subject to saturation problems due to flow of direct current in transformer primary.</div>
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The push-pull is a power amplifier and is frequency used in the output stage of electronic circuits. It is employed whenever high output at high efficiency is required. The audio power amplifiers used in transistor receivers, tape recorders, record-players, public address systems etc. make use of such circuits. These systems are usually operated by batteries or cells and, therefore, amplifier efficiency is of prime importance.</div>
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<b>Basic Principle of Working of Push-Pull Amplifier</b></div>
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The basic principle on which a push-pull amplifier operates is that the input signal is converted before amplification, into two separate signals, which are identical except for a 180<span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">°</span> phase difference. Each of these signals is applied as the input to one of the transistors of the push-pull amplifier. Since the inputs to the two transistors are 180<span style="font-family: Calibri, sans-serif; font-size: 13.5pt; line-height: 115%;">°</span> out of phase, so the output of the transistors. The output transformer is connected in the collector circuits of the transistors in such a way that the collector currents of the two transistors combine to provide an overall signal having the same waveform as the original input signal. The magnitude of this combined output signal is larger than the input signal. Push-pull amplifiers can be operated in class A, class AB or class B mode.<br />
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<img border="0" height="269" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEinARhAuqUoHSCx1DRC6kkgDkOhH7d22Oa5yeFV7xH8lrg-Or9_zEcvC70he_w0JU0lLcklEcsjADHC3kxaDRfuSzW1z7ByCM8kDlVj4b4buQVtC1t155SRCpoQcV_gvNDcKJAKL39inpFy/s320/complementary-symmetry-push-pull-amplifier.gif" width="320" /></div>
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<b><i>Fig. Complementary Symmetry Push-Pull Amplifier</i></b></div>
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Push Pull Amplifiers

Push Pull Amplifiers

Double-ended or push-pull amplifier makes use of two identical transistors in a single stage. It consists of two loops in which the transi...

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