ECEstream: Switching Circuits

Simple Touch-Sensitive Switch

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Here the touch-sensitive switch is using around NAND gate IC CD4011 and transistor BC547.</div>
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When someone touches plate 1 (which is connected between pin 1 of gate N1 and ground), the RS flip-flop comprising gates N1 and N2 is set. The resulting high output at pin 3 of gate N1 energises the relay via relay driver transistor. The relay, in turn, switches on the load operating on mains.<br />
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<img alt="Simple Touch-Sensitive Switch" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhpzIWs7a9TxW8ytV3zblMf0t5WQ5YNCFrxh5PUOMw8oha6P0TkQJojn9o1Fn3Cp5gjRo3TTSMjLmrFmi_fbCBK0p7tx2e_E8w6ATJQCDMIbKNcoRo7y_EyN2Y_IUTvNbvlQkMVFBfYWokI/s1600/touch-sensistive+switch.jpg" title="Simple Touch-Sensitive Switch" /></div>
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On the other hand, when someone touches plate 2 (which is connected between pin 6 of gate N2 and ground), the RS flip-flop is reset. The resulting low output at pin 3 of gate N1 de-energises the relay via relay driver transistor. The relay, in turn, switches off the load operating on mains. The diode across the relay coil protects the transistor from back e.m.f induced in the relay when it de-energises.</div>
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The circuit works off a 12V power supply. If you want to control heavier loads, the current rating of the relay should be increased accordingly.</div>
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Simple Touch-Sensitive Switch

Here the touch-sensitive switch is using around NAND gate IC CD4011 and transistor BC547. When someone touches plate 1 (which is conne...

Doorbell-Controlled Security Switch

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One way to check whether anybody is at home is to ring the doorbell. Burglars too use this very technique. The circuit described here comes handy in such a situation. It is a simple doorbell-controlled multi-purpose security switch that instantly powers up a connected security device, say, a night-vision door camera, for functioning. The circuit works off 9V DC supply. The input of the circuit is connected in parallel with the 230V AC electric doorbell. An electromagnetic relay is used at the output of the circuit to activate the security device connected to it.</div>
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Fig. 1 shows the circuit of the doorbell-controlled security switch. Working of the circuit is simple: When somebody presses doorbell switch S1, the mains voltage is fed to the doorbell and opto-isolator IC 4N35 (IC1). The doorbell sounds and opto-isolator IC1 provides a high-to-low pulse at its pin 5 as the internal transistor conducts. Pin 5 of IC1 is connected to pin 1 of IC2, so a high-to-low pulse is also produced at pin 1 of gate N1, which triggers IC3 at pin 11. As a result, the Q output (pin 13) of IC3 goes high and the relay energises via relay-driver transistor BC547 (T1). Now the 9V DC supply becomes available at socket J1 to activate the night-vision door camera.</div>
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<img alt="Doorbell-Controlled Security Switch" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhAV4QXWNOPo0BtgiZb5iAqzMUpqnHTuR91VsNzPfrERMFskJ6xW7SSa9lPUgledHKqdQOrSgnscokwgko-IVIM33H3SISaNR6GqbxGrfLaAf6PraBt3TMYXiAIKLlXTEByTUHCtQdAIsoW/s1600/Doorbell-controlled+Security+Switch1.jpg" title="Doorbell-Controlled Security Switch" /></div>
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<i><span style="color: red;">Fig. 1: Circuit for security switch</span></i></div>
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Note that the output socket (J1) works as a security switch and is designed for activating the 9V security device (night-vision door camera). Similarly, you can use this 9V supply to energise another switching relay in an existing home security system. The circuit can be reset by pressing switch S2.</div>
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Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Refer Fig. 2 for pin configurations of opto-isolator 4N35 and transistor BC547 before soldering components on the PCB. Connect the doorbell parallel to the circuit and socket J1 to the night-vision camera.</div>
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<img alt="4N35 and BC547" border="0" height="181" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiTxHZY_DpAf872etWxHWM4m2yERcj6p9pTod_MgmAC96oqrjPXaLMN_1lEpSTQvTUwLOzNzqfHVuXiBrQc2IB5qugRe2a1CTzTAmp4a4UBnKeEPxdxhzDYYR5C2HIcvdtDSQ7QXBpf_Wzo/s200/Doorbell-controlled+Security+Switch2.jpg" title="4N35 and BC547" width="200" /></div>
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<i><span style="color: red;">Fig. 2: Pin configurations of 4N35 and BC547</span></i></div>
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Doorbell-Controlled Security Switch

One way to check whether anybody is at home is to ring the doorbell. Burglars too use this very technique. The circuit described here come...

Switching Circuits

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<ol>
<li><a href="http://www.ecestream.com/2015/05/doorbell-controlled-security-switch-circuit.html">Doorbell-Controlled Security Switch</a></li>
<li><a href="http://www.ecestream.com/2015/05/simple-touch-sensitive-switch-circuit.html">Simple Touch-Sensitive Switch</a></li>
<li><a href="http://www.ecestream.com/2015/03/on-off-temperature-control-switching-circuit.html">On-Off Temperature Control</a></li>
<li><a href="http://www.ecestream.com/2015/03/automatic-fan-controller-switching-circuit.html">Automatic Fan Controller</a></li>
<li><a href="http://www.ecestream.com/2015/03/clap-switch-circuit.html">Clap Switch Circuit</a></li>
<li><a href="http://www.ecestream.com/2014/11/voltage-comparator-switch.html">Voltage Comparator Switch</a></li>
<li><a href="http://www.ecestream.com/2014/11/remote-controlled-switch-circuit.html">Remote Controlled Switch Circuit</a></li>
<li><a href="http://www.ecestream.com/2014/11/dark-activated-switch.html">Dark Activated Switch</a></li>
<li><a href="http://www.ecestream.com/2014/11/dc-motor-reversing-circuit.html">DC Motor Reversing Circuit</a></li>
<li><a href="http://www.ecestream.com/2014/05/switch-debouncing-circuit.html">Switch Debouncing Circuit</a></li>
<li><a href="http://www.ecestream.com/2014/05/sound-operated-switch.html">Sound Operated Switch</a></li>
<li><a href="http://www.ecestream.com/2014/05/photo-switch-circuit-using-555-timer.html">Photo Switch Circuit with 555 Timer</a></li>
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<img alt="Switch" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjTcljR9UXjZlnrOlJcUEG3sM9NQqGAePt9gwTUzneb2Z85qCkLMpKEoh1v2sQX-gbwkpeC_5TiiziD3-9E05hj5bdEbZUVD4-dhb23baJbMgoLoryhyrZskk95XdDVWLjqdWq3PWgxZBVv/s1600/swtch.jpg" title="Switch" /></div>
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Switching Circuits

Doorbell-Controlled Security Switch Simple Touch-Sensitive Switch On-Off Temperature Control Automatic Fan Controller Clap Switch...

On-Off Temperature Control

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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFV3QCJZZObkO7-wnCMVqVb057RmkUJBpvlaYZxUesc6AWXP3FlFPEHuQZS4hQjyAHCnkpx1zzcuszFYOFYfToeKA6ndXugT1wZa2r_LRSkpxBnIxviMQmP562YDxM516ojC4rZe_9g8IY/s1600/On-Off+Temperature+Control.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img alt="On-Off Temperature Control Circuit" border="0" height="191" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFV3QCJZZObkO7-wnCMVqVb057RmkUJBpvlaYZxUesc6AWXP3FlFPEHuQZS4hQjyAHCnkpx1zzcuszFYOFYfToeKA6ndXugT1wZa2r_LRSkpxBnIxviMQmP562YDxM516ojC4rZe_9g8IY/s1600/On-Off+Temperature+Control.png" title="On-Off Temperature Control" width="400" /></a></div>
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This circuit controls a load (in this case a dc brushless fan) based on a temperature compared with a setpoint. The transduced is a diode in the forward polarization regime. In fact when forward biased, the forward voltage drop across a diode has a temperature dependence, in particular has a negative linear slope. This because of the Boltzmann distribution, causing electrons to pass to the conduction band thermally, lowering the voltage drop across the diode.<br />
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Anyway this circuit compares a precise voltage reference (zener) with the forward voltage drop of the diode forward biased with 11mA of current.<br />
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The comparator is simply a LM158/258/358 working in open-loop mode, the inverting input is connected to the diode sensor, and the non-inverting to the reference voltage. Se when the temperature rises above the setpoint, the forward voltage drops under the voltage reference and the comparator output is Vcc turning on the transistor and so the fan.<br />
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Higher power transistor can be substituted for bigger fans, or you can substitute a relay, IGBT, mosfet, etc to control higher loads (and higher voltages).<br />
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The setpoint is adjusted with the potentiometer, and you can use a LM3914 led driver to make a temperature setpoint indicator (needs careful calibrations and the use of excel to calculate slope and intercept).<br />
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Many modifications can be done, but the circuit works very well in its basic form.<br />
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<strong><span style="color: #3d85c6;">Important Points</span></strong><br />
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The comparator can distinguish 10uV differences so approx. 0.01°C differences (carefully adjusting the potentiometer can allow to feel body heat from 1/2 cm from the sensor, or feel ambient heat, making to turn the fan on and off continuosly).<br />
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You can control temperatures up to 140°C (150 max diode temperature), but linearity is not ensured.<br />
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Possible uses? Heatsink cooling, computer emergency cooling (but a linear device would be better than a on-off), metal cooling when drilling etc.</div>
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On-Off Temperature Control

This circuit controls a load (in this case a dc brushless fan) based on a temperature compared with a setpoint. The transduced is a dio...

Automatic Fan Controller

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Th1, the 50K thermistor, is a standard type. Mine was a bar or rectangular looking thingy. Available from Tandy/Radio-Shack. Almost any type will do. I experimented with different models from 22K to 100K and all worked fine after replacing the trimmer pot. The one used in the above circuit diagram was a 50K model. This 50K was measured at exactly 25 °C and with 10% tolerance. The resistance increases as the surrounding temperature decreases. Tolerance for my application (cooling a large power supply cool rib) is 10%. Another name for this thing is 'NTC'. NTC stands for "Negative Temperature Coefficient" which means when the surrounding temperature decreases the resistance of this thermistor will increase. I replaced my thermistor for a 60K hermetically sealed glass type since the environment for my application may contain corrosive particles which may affect performance on a future date. P1 is a regular Bourns trimmer and adjusts a wide range of temperatures for this circuit. <br />
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<img alt="Automatic-Fan-Controller-Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiO1IN8oOY3mpT_50PXsuM-8XuUXTfCb28ML6bQC8EiBpYJGEVqyrPHXQTvp6nPaCe17ti4lLFYYdfkXUVE50Zxj_dNXyXrWSOBH7pk6O2EWbhwvDbQaXc1UtyUvdRhb2maka1gKEdgYk4K/s1600/AUTOMATIC+FAN+CONTROLLER.jpg" title="Automatic Fan Controller" /></div>
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I used the 10-turn type for a bit finer adjustment but the regular type will work for your application. R1 is a 'security' resistor just in case the trimmer pot P1 is adjusted all the way to '0' ohms. At which time the thermistor would get the full 12 volt and it will get so hot that it puts blisters on your fingers. R3 feeds a bit of hysteresis back into the op-amp to eliminate relay 'chatter' when the temperature of the thermistor reaches its threshold point. Depending on your application and the type you use for Q1 and Re1, start with 330K or so and adjust its value downwards until your satisfied. The value of 150K shown in the diagram worked for me. Decreasing the value of R2 means more hysteresis, just don't use more then necessary. Or temporarily use a trimmer pot and read off the value. 120K worked for me.<br />
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Transistor Q1 can be a 2N2222(A), 2N3904, NTE123A, ECG123A, etc. Not critical at all. It acts only as a switch for the relay so almost any type will work, as long as it can provide the current needed to activate the relay's coil. D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering. For my application the 1N4148 was good enough since the tiny relay I used was only 1 Amp. <br />
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However, you can use a large variety of diodes here, my next choice would be a regular purpose 1N4001 or something and should be used if your relay type can handle more then 1 amp. If you like to make your own pcb, try the one above. The pcb is fitted with holes for the relay but may not fit your particular relay. It was designed for a Aromat HB1-DC12V type. <br />
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The variety and model of relays is just to great. How to mount it then? Well, I left ample space on the pcb to mount your relay. You can even mount it up-side-down and connect the wires individually. Use Silicon glue, cyanoacrylate ester (crazy glue), or double-sided tape to hold the relay in place. Works well. Note that the pcb and layout is not according to the circuit diagram in regards to the hookup of the fans. The PCB measures approximately 1.5 x 3 inches (4.8 x 7.6mm) If you print the pcb to an inkjet printer it is probably not to scale. Try to fit a 8-pin ic socket on the printed copy to make sure it fits before making the pcb.</div>
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Automatic Fan Controller

Th1, the 50K thermistor, is a standard type. Mine was a bar or rectangular looking thingy. Available from Tandy/Radio-Shack. Almost any ty...

Clap Switch Circuit

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In order to make a clap switch, we have to combine the  touch sensor (<a href="http://www.ecestream.com/2014/12/the-555-as-monostable-multivibrator.html" target="_blank">monostable mode</a> of <a href="http://www.ecestream.com/2014/11/the-555-timer.html" target="_blank">555</a>) and the transistorized dark sensor circuits. Whenever there is loud sound produced near the electret condenser mic, pin 2 of 555 gets triggered and it switches on the <a href="http://www.ecestream.com/2014/11/light-emitting-diodes-leds.html" target="_blank">LED</a> D1. The LED remains on for a definite time determined by R1 and C1. We can adjust the sensitivity of the circuit using a variable <a href="http://www.ecestream.com/2014/11/resistor.html" target="_blank">resistor</a> in place of fixed resistor R5. You can check the circuit with 1K or 10K.</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh4-zTqyts_g6vSkdR0j7gtPK8oZ_Ou4gNwiDkvntrPRuKLaF_SgLFcnut0FM3c3k5C7XsIgaJhnOapnZixMCoHhSPT6EuRDTlcM1akAU-akFshQRV-Y3lOcT4MWVM4jROOkRyY1TwqpyO-/s1600/clap-switch.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img alt="Clap-Switch-Circuit-ecestream" border="0" height="284" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh4-zTqyts_g6vSkdR0j7gtPK8oZ_Ou4gNwiDkvntrPRuKLaF_SgLFcnut0FM3c3k5C7XsIgaJhnOapnZixMCoHhSPT6EuRDTlcM1akAU-akFshQRV-Y3lOcT4MWVM4jROOkRyY1TwqpyO-/s1600/clap-switch.jpg" title="Clap Switch Circuit" width="640" /></a></div>
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<b><i>Clap Switch Circuit</i></b></div>
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Clap Switch Circuit

In order to make a clap switch, we have to combine the touch sensor ( monostable mode of 555 ) and the transistorized dark sensor circui...

Voltage Comparator Switch

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This circuit will provide an indication whenever the input voltage differs from two defined limits, V1 and V2. The limits are adjustable and the circuit made to trigger from the adjustable "window". The supply voltage, Vcc must be higher than the highest input voltage by at least 2 volts. One application here is to monitor a 12V car battery. V1 could be set to 14V and V2 to 11V thus giving an indication of over charge or a weak battery.</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiyHYLxIS-fx2lR5-n_FiSXofDzAw6nvbKepqhOFA2IxBFJKroVtZf-byIElIiLyuh04q-aQTk5mnYwUR_YKpfNEanbFHXBiMBPm64Q6yxPRQJpIvlkwW5vM_ZCefhdS6xZzIW1NShBY-J-/s1600/VOLTAGE+COMPARATOR+SWITCH.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiyHYLxIS-fx2lR5-n_FiSXofDzAw6nvbKepqhOFA2IxBFJKroVtZf-byIElIiLyuh04q-aQTk5mnYwUR_YKpfNEanbFHXBiMBPm64Q6yxPRQJpIvlkwW5vM_ZCefhdS6xZzIW1NShBY-J-/s1600/VOLTAGE+COMPARATOR+SWITCH.jpg" style="font-family: inherit;" /></a></div>
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<b style="font-family: inherit;">Important Points</b></div>
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<span style="font-family: inherit;">Only pins 7 and 4 (power supply), pin 6 (output) and pins 2 and 3 (inverting and non-inverting inputs) are used. All other pins are not connected. The pinout for the CA3140 is shown below.</span><br />
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<span style="font-family: inherit;">The <b>op-amps</b> used here are <b>MOSFET CA3140</b>. They are used to advantage as they have very little output offset voltage and can switch down to near 0 volts. If any other op-amp is used such as <b>LF351 </b>or <b>CA741</b> then it will be necessary to have an offset null control. This is just a 10k preset contacted between pins 1 and 5, the wiper connected to the negative supply rail or op-amp pin 4. With this circuit either op-amp will light the LED if the input voltage goes out of limits, the two 1N4148 diodes forming an "AND"-gate at the output. The input voltage to be monitored is fed via a series 10k resistor to inputs of both op-amps. If the input voltage is greater than the limit set by V1 then the CA3140 will swing its output to almost the full supply voltage and light the LED. Similarly, if the input voltage is less than the limit defined by V2 then this op-amp will swing towards Vcc and light the LED.</span></div>
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Voltage Comparator Switch

This circuit will provide an indication whenever the input voltage differs from two defined limits, V1 and V2. The limits are adjustable...

Remote Controlled Switch Circuit

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This is a versatile<i> remote controlled switch</i> that can ON or OFF any appliance connected to it using a TV remote. <i><b>IR remote sensor IC TSOP 1738</b></i> is used for receiving the signal. Normally when no signal is falling on IC3 the output of it will be high. This makes Q1 OFF. When a signal of 38 KHz from the TV remote falls on the IC3 its output goes low. This makes Q1 conduct and a negative pulse is obtained at pin 2 of IC 1 <b>NE 555</b>. Due to this IC1 wired as a monostable multivibrator produces a 4 Sec long high signal at its output. This high output is the clock for IC 2 which is wired as a flipflop and of , its two outputs pin 3 goes low and pin 2 goes high. The high output at pin 2 is amplified to drive the relay. For the next signal the outputs of IC2 toggles state.  Result, we get a relay toggling on each press on the remote. Any appliance connected to this circuit can be switched ON or OFF.</div>
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<b>Remote Controlled Switch Circuit Diagram with Parts List</b></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjoCbGPywbwsveu_-CkvmkvvB632LBiSMoygn5CH_jOC_g-Asj_mIjD_wXMFT5HSwPcFxdr7YdF6lYUXfd41ZpWPmD2tcjS3-L9akdS73FIddwrW7mdpx1TLI6YQwxG7jmduPLRbFlQRuf5/s1600/Remote+Controlled+circuit.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="355" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjoCbGPywbwsveu_-CkvmkvvB632LBiSMoygn5CH_jOC_g-Asj_mIjD_wXMFT5HSwPcFxdr7YdF6lYUXfd41ZpWPmD2tcjS3-L9akdS73FIddwrW7mdpx1TLI6YQwxG7jmduPLRbFlQRuf5/s1600/Remote+Controlled+circuit.png" width="640" /></a></div>
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<i>IC1, IC2 = NE 555 / Timer</i></div>
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<i>IC3 = TSOP 1738 / IR remote sensor</i></div>
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<i>Q1 = BC 158 / Transistor</i></div>
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<i>Q2 = SL 100 / Transistor</i></div>
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<b>Important Points</b></div>
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<li>Before wiring the circuit make sure that the carrier frequency of the TV remote you have is 38 kHz. For that wire the sensor part only, point your remote to the TSOP1738 and press any switch. If output of TSOP1738 goes low then OK, your remote is of 38Khz type. Nothing to worry almost all TV remote are of this type.</li>
<li>You can use any switch of the remote because for any switch the code only changes, the carrier frequency remains same. We need this carrier frequency only.</li>
<li>Assemble the circuit on a good quality PCB or common board.</li>
<li>The appliance can be connected through NO or NC and C contacts of the relay.</li>
<li>Use a regulated 6V power supply for the circuit.</li>
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Remote Controlled Switch Circuit

This is a versatile remote controlled switch that can ON or OFF any appliance connected to it using a TV remote. IR remote sensor IC TSO...

Dark Activated Switch

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This circuit will activate a relay when light falls to a preset level. Light level can be adjusted with VR1 and the relay contacts may be used to operate an external light or buzzer. In bright light the resistance of the photocell is very low and resistance of photocell increases with increase in darkness. Tht photocell used here is<b> ORP12</b>.</div>
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<br /></div>
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<b>Circuit Diagram</b><br />
<b><br /></b></div>
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<img border="0" height="421" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgAhNUd1i76wPs6NLR7AtSkmz0V55-kAvb1cA7RkbTQV8H1IVXNpKek6LGJywiYlZ0QzedYZPkrQYo__jIClNzpL-92gGuCrAS-UCyCcZLWKAGdgvLp2O3geweyFKuDMUUs5WwHDTheshbb/s1600/Dark+Activated+Switch.jpg" width="640" /></div>
<b><br /></b>
<b>Note</b></div>
<div style="text-align: justify;">
The light sensor used is the ORP12 photocell. In bright light the resistance of the ORP12 can be as low as 80 ohm and at 50lux (darkness) the resistance increases to over 1 Mohm. The 1M control should provide a wide range for light intensities, if not its value may be increased. The op-amp senses the voltage difference between pins 2 and 3. The control VR1 is adjusted so that the relay is off, the output of the op-amp will be around 2 Volts. When light falls, the resistance of the photocell increases and the difference in input voltage is amplified by the op-amp, the output will swing towards full supply and drive the transistor and relay. The 270k resistor provides a small amount of hysteresis, so that the circuit switches on and off with slightly different light levels. This eliminates relay chatter. Take great care if you decide to wire the relay to activate a mains lamp. Make sure the relay contacts provide adequate isolation and have ample rating for the load.</div>
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<b>Parts List</b></div>
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<b><br /></b></div>
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<table align="left" border="1" cellpadding="0" cellspacing="0" class="MsoNormalTable" style="border-collapse: collapse; border: none; margin-left: 6.75pt; margin-right: 6.75pt; mso-border-alt: solid windowtext .5pt; mso-border-insideh: .5pt solid windowtext; mso-border-insidev: .5pt solid windowtext; mso-padding-alt: 0in 5.4pt 0in 5.4pt; mso-table-anchor-horizontal: margin; mso-table-anchor-vertical: margin; mso-table-left: left; mso-table-lspace: 9.0pt; mso-table-rspace: 9.0pt; mso-table-top: 16.5pt;">
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<o:p> ORP12</o:p></div>
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<o:p> Photocell (1)</o:p></div>
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<o:p> RLY1</o:p></div>
</td>
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<o:p> 12V SPDT</o:p></div>
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<o:p> U1</o:p></div>
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<o:p> UA741 (1)</o:p></div>
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<o:p> Q1</o:p></div>
</td>
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<o:p> BC109 NPN (1)</o:p></div>
</td>
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<tr style="height: 19.75pt; mso-yfti-irow: 4;">
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<o:p> D1</o:p></div>
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<o:p> IN002 DIODE (1)</o:p></div>
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<o:p> F1</o:p></div>
</td>
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<o:p> 1A (1)</o:p></div>
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<o:p> VR1</o:p></div>
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<o:p> 1M RESISTOR (1)</o:p></div>
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<o:p> ORP12</o:p></div>
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<o:p> 500K RESISTOR (1)</o:p></div>
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<o:p> R1</o:p></div>
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<o:p> 10K RESISTOR (1)</o:p></div>
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<o:p> R2</o:p></div>
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<o:p> 10K RESISTOR (1)</o:p></div>
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  <td style="border-top: none; border: solid windowtext 1.0pt; height: 19.75pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; width: 45.9pt;" valign="top" width="61"><div class="MsoNormal">
<o:p> R3</o:p></div>
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<o:p> 10K RESISTOR (1)</o:p></div>
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<o:p> R4</o:p></div>
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<o:p> 270K RESISTOR (1)</o:p></div>
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<o:p> R5</o:p></div>
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<o:p> 4.7K RESISTOR (1)</o:p></div>
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<o:p> R6</o:p></div>
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<o:p> 1K RESISTOR (1)</o:p></div>
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Dark Activated Switch

This circuit will activate a relay when light falls to a preset level. Light level can be adjusted with VR1 and the relay contacts may ...

DC Motor Reversing Circuit

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<b style="font-family: inherit;">Description</b></div>
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<span style="font-family: inherit;">When the forward button is pressed and released the motor will run continuously in one direction. The reverse button will cause the motor to run continuously in the opposite direction. The motor cannot be switched from forward to reverse unless the stop switch is pressed first and vice </span>verse<span style="font-family: inherit;">. Putting a motor straight into reverse would be quite dangerous, because when running a motor develops a back emf voltage which would add to current flow in the opposite direction and probably cause arcing of the relay contacts.</span></div>
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<br /></div>
<div style="text-align: justify;">
<span style="font-family: inherit;"><b>Circuit Diagram</b></span><br />
<span style="font-family: inherit;"><b><br /></b></span></div>
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<img border="0" height="402" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjE_HUmOkHZQbI06-wIOtGhFWeAZkuE2wgibmfVPX1OHs_hYRvvPQojbjRxVy_5Ox0QOu2iPeog9bwLoJD616c1C8zn9zW5wGWuuVFPzqI33oKaLPmsz4xpzxTHGqZDPoAwrczkv2N9Y-pR/s1600/DC+Motor+Reversing+Circuit.jpg" width="640" /></div>
<span style="font-family: inherit;"><b><br /></b></span></div>
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<span style="font-family: inherit;"><b>Note</b></span></div>
<div style="text-align: justify;">
<span style="font-family: inherit;">At first glance this may look over-complicated, but this is simply because three non-latching push button switches are used. When the forward button is pressed and released the motor will run continuously in one direction. The Stop button must be used before pressing the reverse button. The reverse button will cause the motor to run continuously in the opposite direction, or until the stop button is used. Putting a motor straight into reverse would be quite dangerous, because when running a motor develops a back emf voltage which would add to current flow in the opposite direction and probably cause arcing of the relay contacts. This circuit has a built-in safeguard against that condition. </span></div>
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<br /></div>
<div style="text-align: justify;">
<span style="font-family: inherit;"><b>Circuit Operation</b> </span></div>
<div style="text-align: justify;">
<span style="font-family: inherit;">Assume that the motor is not running and that all relays are unenergized. When the forward button is pressed, a positive battery is applied via the NC contacts of B1 to the coil of relay RA/2. This will operate as the return path is via the NC contacts of D1. Relay RA/2 will operate. Contacts A1 maintain power to the relay even though the forward button is released.</span></div>
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<br /></div>
<div style="text-align: justify;">
<span style="font-family: inherit;">Contacts A2 apply power to the motor which will now run continuously in one direction. If now the reverse button is pressed, nothing happens because the positive supply for the switch is fed via the NC contact A1, which is now open because Relay RA/2 is energized. To Stop the motor the Stop switch is pressed, Relay D operates and its contact D1 breaks the power to relays A and B, (only Relay A is operated at the moment). If the reverse switch is now pressed and released. Relay B operates via NC contact A1 and NC contact D1. Contact B1 closes and maintains power so that the relay is now latched, even when the reverse switch is opened. Relay RC/2 will also be energized and latched. Contact B2 applies power to the motor but as contacts C1 and C2 have changed position, the motor will now run continuously in the opposite direction. Pressing the forward button has no effect as power to this switch is broken via the now open NC contact B1. If the stop button is now pressed. Relay D energizes, its contact D1 breaks power to relay B, which in turn breaks power to relay C via the NO contact of B1 and of course the motor will stop. All very easy. The capacitor across relay D is there to make sure that relay D will operate at least longer than the time relays A,B and C take to release. </span></div>
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<br /></div>
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<span style="font-family: inherit;"><b>Correction to Diagram</b></span></div>
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<span style="font-family: inherit;">In the original circuit when the Stop switch was pressed, relay RA remains energized, its holding current path is through relay coils RB and RC. To fix this relay D has two contacts, D2 now breaks power to the relay coil.</span></div>
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DC Motor Reversing Circuit

Description When the forward button is pressed and released the motor will run continuously in one direction. The reverse button will...

Switch Debouncing Circuit

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<span style="font-family: inherit;"><b><i>Switch debouncing</i></b> is one of those things you generally have to live with when playing with switches and digital circuits. If you want to input a manual switch signal into a digital circuit you'll need to debounce the signal so a single press doesn't appear like multiple presses. This circuit will remove the transient spikes and contact bounces from a non-latching push button switch.</span><br />
<span style="font-family: inherit;"><br /></span></div>
<div style="text-align: justify;">
</div>
<div style="text-align: left;">
<span style="font-family: inherit;"><b>Circuit Diagram</b></span></div>
<span style="font-family: inherit;"><br /></span>
<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj2lcz551Ie75Hmp7_n_Tv7tkCo3Glq0fWNvLuDV0alN3GTKGflDJroaOG9DoobstVb-kV46Hg3-8TGes-gpCjG3iAatIafR-hPzGX8fCiDhJhR2QtBUlWIB6_XPjynTfVaWb-20xB4nF9S/s1600/SWITCH+DEBOUNCING+CIRCUIT.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: inherit;"><img border="0" height="626" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj2lcz551Ie75Hmp7_n_Tv7tkCo3Glq0fWNvLuDV0alN3GTKGflDJroaOG9DoobstVb-kV46Hg3-8TGes-gpCjG3iAatIafR-hPzGX8fCiDhJhR2QtBUlWIB6_XPjynTfVaWb-20xB4nF9S/s1600/SWITCH+DEBOUNCING+CIRCUIT.png" width="640" /></span></a></div>
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<span style="font-family: inherit;"><br /></span></div>
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<span style="font-family: inherit;"><b>Notes</b></span></div>
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<span style="font-family: inherit;">Using a 555 timer as a monostable circuit, it is easy to build a good switch debouncer circuit. There are many circuits for SPDT debouncing, but not many for a normally open, push-to-make press button switch (PBS). The 555 monostable gives an output pulse of around 20 ms with component values shown. The formula for determining the output pulse is:</span></div>
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<span style="font-family: inherit;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: inherit;">Tout = 1.1 R1 C1</span></div>
<div style="text-align: justify;">
<span style="font-family: inherit;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: inherit;">With the values in my circuit this equates to :</span></div>
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<span style="font-family: inherit;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: inherit;">Tout = 1.1*1.8*</span>10<sup><span style="font-family: inherit; font-size: 11pt; line-height: 16.8666667938232px;">6</span></sup><span style="font-family: inherit;">*10*10</span><sup><span style="font-size: 11pt; line-height: 115%;"><span style="font-family: inherit;">-9</span></span></sup><span style="font-family: inherit;"> = 0.0198 sec or 19.8 ms</span></div>
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<span style="font-family: inherit;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: inherit;">The 555 circuit can be re-triggered if the input is held low longer than the output pulse. To prevent this happening, I have included a further timing circuit comprised of the 1Meg resistor and 47n capacitor. Normally, the 47n capacitor is discharged via the 1 Meg resistor. When the switch is pressed the capacitor quickly charges and provides a brief negative pulse to the 555 input. When the capacitor is fully charged, the potential across the voltage divider formed by the 10k and 1Meg resistors is insufficient to retrigger the monostable. Releasing the switch quickly discharges the capacitor. The output of a 555 monostable is suitable for connecting to TTL and CMOS logic circuits.</span></div>
</div>

Switch Debouncing Circuit

Switch debouncing is one of those things you generally have to live with when playing with switches and digital circuits. If you want to ...

Sound Operated Switch

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<span style="font-family: inherit;">A sound operated switch with a relay driver. We have described a sound operated switch circuit which will switch ON the light by just listening sound. Another advantage of this circuit is that you cannot get any electrical shock as we did not have to use any mechanical switch.This sensitive sound operated switch can be used with a dynamic microphone insert as above, or be used with an electret (ECM) microphone. If an ECM is used then R1 (shown dotted) will need to be included. A suitable value would be between 2.2k and 10kohms.</span><br />
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<span style="font-family: inherit;"><b>Circuit Diagram</b></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjdCsMMUJDXy5vvtf8BlJIjvdrfVz_ZaxKlKucqhugP7mE0cLcqu9Txmy5X4lMSqSjwBfJXMeeKtlGldJS4rSkJEB2I5SbL2ZIYAXUkJ3JkTpfbBflvnHLSE-6R6A-36AD9-_58gIhCtYyL/s1600/SOUND+OPERATED+SWITCH.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: inherit;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjdCsMMUJDXy5vvtf8BlJIjvdrfVz_ZaxKlKucqhugP7mE0cLcqu9Txmy5X4lMSqSjwBfJXMeeKtlGldJS4rSkJEB2I5SbL2ZIYAXUkJ3JkTpfbBflvnHLSE-6R6A-36AD9-_58gIhCtYyL/s1600/SOUND+OPERATED+SWITCH.png" /></span></a></div>
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<span style="font-family: inherit;"><b>Notes</b></span></div>
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<span style="font-family: inherit;">This sensitive sound operated switch can be used with a dynamic microphone insert as above, or be used with an electret (ECM) microphone. If an ECM is used then R1 (shown dotted) will need to be included. A suitable value would be between 2.2k and 10kohms.</span></div>
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<span style="font-family: inherit;">The two BC109C transistors form an audio preamp, the gain of which is controlled by the 10k preset. The output is further amplified by a BC182B transistor. To prevent instability the preamp is decoupled with a 100u capacitor and 1k resistor. The audio voltage at the collector of the BC182B is rectified by the two 1N4148 diodes and 4.7u capacitor. This dc voltage will directly drive the BC212B transistor and operate the relay and LED. It should be noted that this circuit does not "latch". The relay and LED operate momentarily in response to audio peaks.</span></div>
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<span style="font-family: inherit;">The gain of the circuit and sensitivity is controlled by the 10k variable resistor on the emitter of the first (left hand side) transistor. A preset may be used if gain is fixed, a potentiometer should be used to trigger at different sound levels.</span></div>
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<span style="font-family: inherit;">The relay contacts close and then open (momentary action) in response to audio peaks, these can be used to switch other circuit. The diode across the relay is the usual back emf diode and a 1N4003 or 1N4004 will work well here, preventing damage to the transistor.</span></div>
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Sound Operated Switch

A sound operated switch with a relay driver. We have described a sound operated switch circuit which will switch ON the light by just list...

Photo Switch Circuit using 555 Timer

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<span style="font-family: inherit;">This is a <b><i>simple photo switch</i></b> suitable for home and industrial purpose. The circuit switches on a relay when the light intensity falling on the sensor crosses the set limit.The circuit is based on a NE 555 and two transistors for driving the relay. The resistor R1 is adjusted such a way that under normal condition the voltage across the <b>LDR</b> is less than 1/3 the supply voltage. In this condition output of IC is high .This makes transistor Q1 on. Transistor Q2′s base is connected to collector of Q1. So Q2 will be off keeping the relay de energized. When light falls the voltage across LDR rises above 2/3 Vcc. This makes the output of IC go high making the relay to energized. The result we get a relay that operates according to the intensity of light falling on the LDR.</span><br />
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<span style="font-family: inherit;"><b>Circuit Diagram and Parts List</b></span></div>
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<span style="font-family: inherit; margin-left: 1em; margin-right: 1em;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhK5_JBrEHCMcEoYDsbNWq0jTY0W6zu7kyu8WzeaCbUVdUUZ_ry99b_QeOvQFjOXaoUjcMjM4AXm6r-a4C3mzp-vDQeeW4pv5_tngv_DTCiMuN8_ghT8whiX8m86uNh-vm8OZTKeq9dCZ7A/s1600/PHOTO+SWITCH+CIRCUIT+USING+NE+555.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="446" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhK5_JBrEHCMcEoYDsbNWq0jTY0W6zu7kyu8WzeaCbUVdUUZ_ry99b_QeOvQFjOXaoUjcMjM4AXm6r-a4C3mzp-vDQeeW4pv5_tngv_DTCiMuN8_ghT8whiX8m86uNh-vm8OZTKeq9dCZ7A/s1600/PHOTO+SWITCH+CIRCUIT+USING+NE+555.jpg" width="640" /></a></span></div>
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<b style="font-family: inherit;">Important Points</b></div>
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<li><span style="font-family: inherit;">Assemble the circuit on a good quality PCB or common board.</span></li>
<li><span style="font-family: inherit;">To setup the circuit switch on power supply, connect a multimeter across LDR and adjust R1 so that voltage across LDR is just below 1/3 supply voltage. Ensure that relay is off in this condition. If not reduce voltage across LDR further more by adjusting R1 to make relay off.</span></li>
<li><span style="font-family: inherit;">Now cover the top of LDR using a black paper and see the relay gets activated. If not, make further adjustments with R1. Nothing to care after some trial and error you will get the correct set piont of R1 where the circuit works. This circuit  is nothing great, but some simple basics. I have tested this and got good results.</span></li>
<li><span style="font-family: inherit;">If there is some chattering in the relay, that may be because off the difference between the turning on and turning off voltages. This can be avoided by connecting a resistor whose value is equal to 1/2 the resistance of LDR at illuminated condition.</span></li>
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Photo Switch Circuit using 555 Timer

This is a simple photo switch suitable for home and industrial purpose. The circuit switches on a relay when the light intensity falling ...

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