Infrared Proximity Detector Alarm

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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

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

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Power Buzzer

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> <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> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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_rms</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> </div>

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...

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General-Purpose Alarm

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> </div> <div style="text-align: justify;"> <div class="separator" style="clear: both; text-align: center;"> <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> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> </div>

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...

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Simple Darkness Activated Alarm

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <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

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

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Model Railway Short-Circuit Beeper

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> </div> <div class="separator" style="clear: both; text-align: center;"> <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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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

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

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Laser Alarm

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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...

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Speed Alarm For Cars

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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

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...

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Night Alert

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Assembly instruction:</b></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Warning:</b></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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> </div>

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...

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Simple Knock Alarm With Piezo Sensor

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> 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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <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> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> 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

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

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Smoke Alarm Battery Life Extender

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> While smoke alarms are quite cheap devices, the cost of 9V batteries quickly exceeds their purchase price. Added to that is the irritation of random beeps from the alarm as the battery reaches the end of its useful life. This circuit allows typical smoke alarms to be powered from the 12V supply in a burglar alarm while still keeping the standard 9V batteries in place. It extends the 9V battery life to that of its "shelf life" as the battery is only required to drive the smoke alarm in the event the 12V supply is removed or shorted out.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Smoke Alarm Battery Life Extender Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj14dIpLWqI8g9shP_kmHwcv-u2Og7tkIfy9H77rE3Is4u7y0I2-eM_ZZWIrByUj8Tsq9yTAE6mZAwBA8cJmlKZo1M6GGsUM8vPDpScrAwMRoKkxiY78QB3YW_eMj1of7YXx8WN_Ekyj7YR/s1600/smoke-alarm-battery-life-extender-circuit-diagram.jpg" title="Smoke Alarm Battery Life Extender Circuit" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> In normal operation, the LM317 supplies 9.7V and this is fed via diode D2, resulting in just over 9V at the smoke alarm supply terminals. Q1 is not biased on, so the 9V battery is disconnected from the circuit. If the 12V supply is removed, the output of the LM317 will be 0V and Q1 will be biased on via the 4.7kO resistor and thus the smoke alarm will continue to be powered. The circuit could be assembled on a piece of Veroboard and fitted inside the smoke alarm. Alternatively, you could house the circuit and 9V battery within a standard electrical flush-mount box which the smoke alarm covers when mounted.</div> </div>

Smoke Alarm Battery Life Extender

While smoke alarms are quite cheap devices, the cost of 9V batteries quickly exceeds their purchase price. Added to that is the irritation...

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Mains Supply Failure Alarm

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> Whenever AC mains supply fails, this circuit alerts you by sounding an alarm. It also provides a backup light to help you find your way to the torch or the generator key in the dark. The circuit is powered directly by a 9V PP3/6F22 compact battery. Pressing of switch S1 provides the 9V power supply to the circuit. A red LED (LED2), in conjunction with zener diode ZD1 (6V), is used to indicate the battery power level.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> Resistor R9 limits the operating current (and hence the brightness) of LED2. When the battery voltage is 9V, LED2 glows with full intensity. As the battery voltage goes below 8V, the intensity of LED2 decreases and it glows very dimly. LED2 goes off when the battery voltage goes below 7.5V. Initially, in standby state, both the LEDs are off and the buzzer does not sound. The 230V AC mains is directly fed to mains-voltage detection optocoupler IC MCT2E (IC1) via resistors R1, R2 and R3, bridge rectifier BR1 and capacitor C1.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> Illumination of the LED inside optocoupler IC1 activates its internal phototransistor and clock input pin 12 of IC2 (connected to 9V via N/C contact of relay RL1) is pulled low. Note that only one monostable of dual-monostable multivibrator IC CD4538 (IC2) is used here. When mains goes off, IC2 is triggered after a short duration determined by components C1, R4 and C3. Output pin 10 of IC2 goes high to forward bias relay driver transistor T1 via resistor R7.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> </div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Mains Supply Failure Alarm" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgtp-4pPcztdU3BnGRZuQiTXYCfw7xnqvVnSg6E7uvs5XLESIH0Be4Ouo0vR3pDCKzGrH9D7vszPg0V0fpesYsYnUwDtw1Xn1Yr5nBBUkoZ8iwuDd1boKZcWXav-MVAFzCCcw5LEeCPeN6E/s1600/mains-supply-failure-alarm-circuit-diagram.gif" title="Mains Supply Failure Alarm" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> Relay RL1 energises to activate the piezo buzzer via its N/O contact for the time-out period of the monostable multivibrator (approximately 17 minutes). At the same time, the N/C contact removes the positive supply to resistor R4. The time-out period of the monostable multivibrator is determined by R5 and C2. Simultaneously, output pin 9 of IC2 goes low and pnp transistor T2 gets forward biased to light up the white LED (LED1).</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> Light provided by this back-up LED is sufficient to search the torch or generator key. During the mono time-out period, the circuit can be switched off by opening switch S1. The ‘on’ period of the monostable multivibrator may be changed by changing the value of resistor R5 or capacitor C2. If mains doesn’t resume when the ‘on’ period of the monostable lapses, the timer is retriggered after a short delay determined by resistor R4 and C3.</div> </div>

Mains Supply Failure Alarm

Whenever AC mains supply fails, this circuit alerts you by sounding an alarm. It also provides a backup light to help you find your way to...

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Bell Alarm

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> If you use a lamp with a motion sensor for outdoor lighting, the original electrical switch is actually no longer necessary. If you replace the switch with the circuit described here, an acoustical signal will be generated each time the outdoor lamp is switched on. It’s thus somewhere between an alarm and a doorbell. The operating principle is simple. A circuit that causes a voltage drop of only a couple of volts is connected in series with the lamp. As the circuit needs a DC voltage, the current for the lamp is passed through a bridge rectifier.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> The voltage drop across the circuit is determined by R1. The function of C1 is to smooth the raw DC voltage. Note that this is not an example of peak rectification, but instead of averaging. For this reason, the voltage on C1 is lower than you might expect. Ultimately, the DC voltage on C1 reaches the same value as the average voltage across R1. For example, consider what happens with a 100-W lamp. For convenience, we can assume that the lamp has a resistance of 529 R. If we ignore the voltage across the diodes and the voltage across R1, the current is approximately 0.39 A on average (not 0.43 A).</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> This is because the average mains voltage is only 207 V = (230 × v2) ÷ (p/2). This yields a voltage of approximately 8.5 V on C1. As the buzzer and T1 only draw a few milliampères from C1, in practice the voltage will differ from this value by at most a few tenths of a volt. Here you should use a DC buzzer with a large operating voltage range. A good example is the CEP-2260A, which has a voltage range of 3–20 V (available from Digi-Key and other online sources). The charging time of C2 determines how long the buzzer remains energised, and here it will be a few tenths of a second.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Bell Alarm Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjEdvTaMF3-OaZOhyphenhyphenYa-gcVvQoTGBQd_NR3ZdcWBZXLHbdCfyJxcyRb6MjaktdLo_b6WolMFETFKTy22ndh5bBKDBQZ923SbLs9fY3VtVeR7aVQv2xwwwhZfNenNRuEy8H9RF-HWLoM7IVN/s1600/bell-alarm-circuit-diagram.gif" title="Bell Alarm Circuit" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> Depending on how much current the buzzer draws, you can increase the value of R2 in order to extend the time (this is certainly necessary with the above-mentioned buzzer type). Depending on the lamp power, you can consider adjusting the value of R1. This will certainly be necessary if you use a 150-W lamp or larger. In this case, cut the value of R1 in half, primarily because the power dissipation will otherwise be too large. In the example described here, it is around 3 watts. The bridge rectifier also deserves special attention.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> A large current can flow briefly when the lamp is switched on ‘cold’. A 250V, 1.5-A bridge rectifier is adequate for a 100-W lamp, but heavier-duty diodes are necessary with higher lamp power – such as the 1N5408 (1000 V / 3 A). Due to the heat generated by R1, make sure that R1 is located a certain distance away from the other components in the assembled circuit. Also bear in mind that the entire circuit is connected to mains potential. Never make any adjustments while the circuit is connected to the mains! It’s thus a good idea to test the circuit before fitting it into the switch box.</div> </div>

Bell Alarm

If you use a lamp with a motion sensor for outdoor lighting, the original electrical switch is actually no longer necessary. If you replac...

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Water Alarm Circuit Diagram

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> Have you ever seen the stairs to one of the upper stories in your house turn into a waterfall? Or maybe you’ve come home to find your aquarium fish trying to swim across the carpet? For your sake, we hope not, because the consequences are usually fairly dramatic. With a handful of electronic components, you can at least ensure that you will be warned before you have to put on your waders. It’s better to prevent water problems than to have to correct them. But no how many precautions you take, an occasional leak can still happen.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> A burst water supply hose for the washing machine, a bath tap that someone forgot to turn off, a broken aquarium wall, or a leaking boiler or central heating tank – anything is possible. In such cases, it’s nice to be warned as quickly as possible, for example by an acoustic water alarm. Then you can at least limit the damage. If you’re handy with a soldering iron and know the difference between an IC and a PC, you’ll no doubt enjoy building the electronic water alarm described here.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> The circuit takes advantage of the fact that ‘normal’ water is always slightly contaminated, even if only slightly, and thus conducts electricity to a certain extent. It is built around an popular IC from the somewhat antiquated 4000-series logic family: the 4093. This IC contains four inverted-output AND gates (NAND gates) with Schmitt-trigger inputs. If water is detected between the probes, it emits an intermittent and rather irritating beeping tone.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Water Alarm Circuit Diagram" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgLy5RhT4EebzZVVQH3gezcPd0hqcgJcg54SSeQoh5KbV7w1a537sELkiPwbW2Zra7wvpPR0AirN_CoTMhvaMIXdLi9JKewAQuFmnBbuql1nslTDnwLNZW70BJxkAr7s7CAge0sZnuUtML-/s1600/water-alarm-circuit-diagram.png" title="Water Alarm Circuit Diagram" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> The conductivity of the water is used to active the circuit built around IC1a. The two electrodes (probes) are fitted at the lowest point where water will come to stand. They can be two tinned copper wires, but you can also use two pieces of circuit board with the copper surface coated with solder. The combination of IC1a, resistor R2 and capacitor C2 forms a simple oscillator that produces the intermittent (on/off) effect of the alarm. If no water is present between the probes, the input of IC1a is held low by R1 and the output of IC1b is also low.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> The oscillator is not active in this state. If moisture is sensed, the supply voltage pulls input 1 of gate IC1a high via the conductive water, causing the gate to start oscillating. Whenever the output of IC1b is high, the tone generator built around IC1c is enabled, and in turn it energizes buzzer BZ1. The net result is a periodic, intermittent beeping tone. You can adjust the intermittent effect of the sound produced by the water alarm to suit your taste by simply adjusting the value of R2 or C2. You can also set the pitch of the sound with P1.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> The closer the pitch is to the resonant frequency of buzzer BZ1, the louder the tone will be. You should set the sound to the most irritating level possible. Gate IC1d is used to boost the amount of power than can be pumped into the buzzer. It inverts the output signal from IC1c to double the voltage applied to the buzzer. Naturally, the circuit of the alarm must be fitted somewhere that will remain high and dry. Use a pair of thin twisted wires to connect the electrodes (probes) to the board. Naturally, you should use insulated, flexible wire for this purpose.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> Twisting the wires together makes the relatively long connection between the probes and the circuit less sensitive to false alarms due to external electromagnetic interference. The current consumption is very low (less than 0.1µA) when everything is dry. When the buzzer is energized, the current consumption can rise to around 2mA. We measured 3mA with the frequency set to the maximum value. The battery will thus last for several years as long as no water is detected. Of course, you should bear in mind that the battery might start leaking after a while…</div> </div>

Water Alarm Circuit Diagram

Have you ever seen the stairs to one of the upper stories in your house turn into a waterfall? Or maybe you’ve come home to find your aqua...

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Midnight Security Light Circuit Schematic

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> Most thefts happen after midnight hours when people enter the second phase of sleep called ‘paradoxical’ sleep. Here is an energy-saving circuit that causes the thieves to abort the theft attempt by lighting up the possible sites of intrusion (such as kitchen or backyard of your house) at around 1:00 am. It automatically resets in the morning. The circuit is fully automatic and uses a CMOS IC CD 4060 to get the desired time delay. Light-dependent resistor LDR1 controls reset pin 12 of IC1 for its automatic action. During day time, the low resistance of LDR1 makes pin 12 of IC1 ‘high,’ so it doesn’t oscillate.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> After sunset, the high resistance of LDR1 makes pin 12 of IC1 ‘low’ and it starts oscillating, which is indicated by the fashing of LED2 connected to pin 7 of IC1. The values of oscillator components (resistors R1 and R2 and capacitor C4) are chosen such that output pin 3 of IC1 goes ‘high’ after seven hours, i.e., around 1 am. This high output drives triac 1 (BT136) through D5 and R3. Bulb L1 connected between the phase line and M2 terminal of triac 1 turns on when the gate of triac 1 gets the trigger voltage from pin 3 of IC1. It remains ‘on’ until pin 12 of IC1 becomes high again in the morning. Capacitors C1 and C3 act as power reserves, so IC1 keeps oscillating even if there is power interruption for a few seconds. Capacitor C2 keeps trigger pin 12 of IC1 high during day time, so slight changes in light intensity don’t affect the circuit.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> Using preset P1 you can adjust the sensitivity of LDR1. Power supply to the circuit is derived from a step-down transformer T1 (230V AC primary to 0-9V, 300mA secondary), rectifed by a full-wave rectifer comprising diodes D1 through D4 and fltered by capacitor C1. Assemble the circuit on a general-purpose PCB with adequate spacing between the components. Sleeve the exposed leads of the components. Using switch S1 you can turn on the lamp manually. Enclose the unit in a plastic case and mount at a location that allows adequate daylight.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Midnight Security Light Circuit Schematic" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj_KLIgCVm70k4SJ2Sgy3MWBQ0XvVQ5AsZWIwTcjs9L5zu7WnD_ylK7izh-98NGgpKTzAXQ6C7RzTd93Ti-ZRpwU-Gms5zAJq3Q79wTftYC5rELI-Rviuk6pdq8GWjAXKzoweNSya5ynJDH/s1600/midnight_security_light_schematic_circuit_diagram.gif" title="Midnight Security Light Circuit Schematic" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Parts:</b></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> P1 = 100K</div> <div style="text-align: justify;"> R1 = 120K</div> <div style="text-align: justify;"> R2 = 1M</div> <div style="text-align: justify;"> R3 = 100R</div> <div style="text-align: justify;"> R4 = 100R</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> C1 = 1000uF-25V</div> <div style="text-align: justify;"> C2 = 100uF-25v</div> <div style="text-align: justify;"> C3 = 100nF-63V</div> <div style="text-align: justify;"> C4 = 1uF-25V</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> D1 = 1N4001</div> <div style="text-align: justify;"> D2 = 1N4001</div> <div style="text-align: justify;"> D3 = 1N4001</div> <div style="text-align: justify;"> D4 = 1N4001</div> <div style="text-align: justify;"> D5 = Red LED</div> <div style="text-align: justify;"> D6 = Green LED</div> <div style="text-align: justify;"> IC = CD4060</div> <div style="text-align: justify;"> TR = BT136</div> <div style="text-align: justify;"> T1 = 9v 300mA Transformer</div> <div style="text-align: justify;"> L1 = 230V-60W Bulb</div> <div style="text-align: justify;"> SW = On/Off Switch</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <div> <b>Caution:</b></div> <div> 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. We will not be responsible for any kind of resulting loss or damage.</div> </div> </div>

Midnight Security Light Circuit Schematic

Most thefts happen after midnight hours when people enter the second phase of sleep called ‘paradoxical’ sleep. Here is an energy-saving c...

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Low Cost Burglar Alarm For Boats

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> This low-cost burglar alarm employs a 12V strobe light and a truck reversing horn as the visible and audible alarm outputs while the alarm itself is a 12V horn relay and some pressure mat switches. This simple approach has the benefit that the alarm continues to operate even if the entry point is immediately closed and it draws no current while in the armed condition. To make it independent of the boat supply it runs from a single 12V or two 6V alkaline lantern batteries connected in series. These should last well in excess of two years. An advantage of a lantern battery is that it will last less than an hour while powering the alarm in its active role.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> This means the alarm system will not seriously contravene noise pollution laws. If there are very strict noise regulations in your area, you can fit an alarm timer, available from some electronic shops, in the circuit between the battery positive and the key switch. The key switch can be installed in the cockpit bulkhead and connects to two normally open (NO) switches wired in parallel. One switch is a pressure mat placed on the cockpit floor near the entry, underneath a suitable piece of carpet or pliable cover. If a pressure mat is unsuitable, the main entry can be protected by a mechanical switch such as an automobile boot lid or door courtesy light switch.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Schematic</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Low Cost Burglar Alarm For Boats" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGnLd1eDGhBee80a0GAeaLBc85bNkCbmKmXuE-m9MUnkshWmm6-0bLsX85bZWHTRpFZO3IozvvUF9d2t9UA_wMAQkOFYnhjFEd0Q8baclGBz3QhqTOjN1Qh4T9bvKeZYKVxWJ5yVeJlPFq/s1600/low-cost-burgar-alarm-for-boats.jpg" title="Low Cost Burglar Alarm Circuit For Boats" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> The second switch can be used to protect against entry through a forward hatch or second wheelhouse door. Any number of normally open (NO) switches can be installed in this system, all wired in parallel. If the alarm is tripped, the relay closes and latches on due to the wiring of its contacts and the horn and strobe light are powered. The suggested truck reversing horn is the Hella 6023 intermittent reversing buzzer which has an 85dB sound level and a current drain of 0.9A at 12V. The suggested strobe is a Hella 1657 which has an amber lens in a weatherproof housing. This strobe flashes about once per second and the current drain at 12V is 0.25A. The strobe can be installed outside on deck or in the main cabin where its flash will be seen through most ports and windows. Wiring to both the warning horn and strobe light should be concealed.</div> </div>

Low Cost Burglar Alarm For Boats

This low-cost burglar alarm employs a 12V strobe light and a truck reversing horn as the visible and audible alarm outputs while the alarm...

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Water Level Alert

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> This circuit will emit an intermittent beep (or will flash a LED) when the water contained into a recipient has reached the desired level. It should be mounted on top of the recipient (e.g. a plastic tank) by means of two crocodile clips, acting also as probes. If a deeper sensing level is needed, the clips can be extended by means of two pieces of stiff wire (see pictures).</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Operation:</b></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> IC1, a 555 CMos timer chip, is wired as an astable multivibrator whose operating frequency is set by C1, R1 and R2, plus the resistance presented by water across the probes. If the resistance across the probes is zero (i.e. probes shorted), the output frequency will be about 3Hz and the sounder will beep (or the LED will flash) about three times per second. As water usually presents a certain amount of resistance, the actual oscillation frequency will be lower: less than one beep/flash per second. As probes will be increasingly immersed in water, the resistance across them will decrease and the oscillation frequency of IC1 will increase.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> This means that a rough aural or visual indication of the level reached by water will be available. If a LED is chosen as the alert, C2, D1 and D2 must be added to the circuit in order to double the output voltage, thus allowing proper LED operation (see the rightmost part of the schematics). Interesting features of this circuit are 1.5V supply and ultra-low current consumption: 40µA in stand-by and 0.5mA in operation. This allows a single AAA alkaline cell to last several years and the saving of the power on/off switch.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Schematic:</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Water Level Alert" border="0" height="205" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg9_Ko1cuQRaxMmvIWr44YiPHWF0EvZ9wLuHrncgS2yoJj_69UZQMA74M7RqTPOfZNlBgynPMhqhbPOgvfjHk935g4JYrHXgtoHFvtiIB4oW3WZLfLkaMf7OJVNkAO2N78ClCf4z8PLo0on/s400/water_level_alert_schematic_circuit_diagram.gif" title="Water Level Alert Circuit" width="400" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Parts List:</b></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> R1 = 1K - 1/4W Resistor</div> <div style="text-align: justify;"> R2 = 100K - 1/4W Resistor (See Notes)</div> <div style="text-align: justify;"> C1 = 2.2uF-50V Electrolytic Capacitor</div> <div style="text-align: justify;"> C2 = 220µF - 25V Electrolytic Capacitor (See Notes)</div> <div style="text-align: justify;"> D1 = 5 or 10mm. Ultra-bright red LED (See Notes)</div> <div style="text-align: justify;"> D2 = 1N5819 - 40V 1A Schottky-barrier Diode (See Notes)</div> <div style="text-align: justify;"> IC = 7555 or TS555CN CMos Timer IC</div> <div style="text-align: justify;"> BZ = Piezo sounder (incorporating 3KHz oscillator)</div> <div style="text-align: justify;"> B1 = 1.5V Battery (AAA or AA cell etc.)</div> <div style="text-align: justify;"> Two small crocodile clips</div> <div style="text-align: justify;"> Two pieces of stiff wire of suitable length</div> <div style="text-align: justify;"> Battery socket, etc.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Notes:</b></div> <div style="text-align: justify;"> </div> <ol> <li>If a LED alert is needed instead of the beeper, R2 value must be changed to 10K, the Piezo sounder can be omitted and D1, D2 and C2 must be added, as shown in the rightmost part of the schematics.</li> <li>A common red LED can be used for D1, but ultra-bright types are preferred.</li> <li>Any Schottky-barrier type diode can be used in place of the 1N5819, e.g. the BAT46, rated @ 100V 150mA.</li> <li>Wipe the probes regularly to avoid excessive resistance variations due to partial oxidization.</li> </ol> </div>

Water Level Alert

This circuit will emit an intermittent beep (or will flash a LED) when the water contained into a recipient has reached the desired level....

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Two-Tone Siren Using One IC

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> This circuit is intended for children fun, and can be installed on bicycles, battery powered cars and motorcycles, but also on models and various games and toys. With SW1 positioned as shown in the circuit diagram, the typical dual-tone sound of Police or Fire-brigade cars is generated, by the oscillation of IC1A and IC1B gates. With SW1 set to the other position, the old siren sound increasing in frequency and then slowly decreasing is reproduced, by pushing on P1 that starts oscillation in IC1C and IC1D.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> The loudspeaker, driven by Q1, should be of reasonable dimensions and well encased, in order to obtain a more realistic and louder output. Tone and period of the sound oscillations can be varied by changing the values of C1, C2, C5, C6 and/or associated resistors. No power switch is required: leave SW1 in the low position (old-type siren) and the circuit consumption will be negligible.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Schematic:</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Two-Tone Siren Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhnZvyPlcmALi8fPp7k6GQwGXWTI0EvQJg8idlI2EavPU2vGIgMB5K3YhHOf4HvdXIQiWsUHB9ljHiuA_g1Mqh-bXzGKiPbHmdRxl-p4EG2spSgz1mQ-bXGs4z9-WvzJyoaJbjP6GS6WVTY/s1600/two_sirens_circuit_diagram.gif" title="Two-Tone Siren Circuit" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Parts:</b></div> <div style="text-align: justify;"> R1 = 470K - 1/4W Resistors</div> <div style="text-align: justify;"> R2 = 680K - 1/4W Resistor</div> <div style="text-align: justify;"> R3 = 470K - 1/4W Resistors</div> <div style="text-align: justify;"> R4 = 82K - 1/4W Resistor</div> <div style="text-align: justify;"> R5 = 330K - 1/4W Resistor</div> <div style="text-align: justify;"> R6 = 10K - 1/4W Resistor</div> <div style="text-align: justify;"> R7 = 33K - 1/4W Resistor</div> <div style="text-align: justify;"> R8 = 3.3M - 1/4W Resistor</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> C1 = 10µF - 25V Electrolytic Capacitors</div> <div style="text-align: justify;"> C2 = 10nF - 63V Polyester Capacitors</div> <div style="text-align: justify;"> C3 = 100nF - 63V Polyester Capacitor</div> <div style="text-align: justify;"> C4 = 100µF - 25V Electrolytic Capacitor</div> <div style="text-align: justify;"> C5 = 10µF - 25V Electrolytic Capacitors</div> <div style="text-align: justify;"> C6 = 10nF - 63V Polyester Capacitors</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> D1 = 1N4148 - 75V 150mA Diodes</div> <div style="text-align: justify;"> D2 = 1N4148 - 75V 150mA Diodes</div> <div style="text-align: justify;"> D3 = 1N4148 - 75V 150mA Diodes</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> Q1 = BC337 - 45V 800mA NPN Transistor</div> <div style="text-align: justify;"> P1 = SPST Pushbutton</div> <div style="text-align: justify;"> B1 = 6V Battery (4 AA 1.5V Cells in series)</div> <div style="text-align: justify;"> IC1 = 4093 - Quad 2 input Schmitt NAND Gate IC</div> <div style="text-align: justify;"> SW1 = DPDT Switch</div> <div style="text-align: justify;"> SPKR= 8 Ohm Loudspeaker</div> </div>

Two-Tone Siren Using One IC

This circuit is intended for children fun, and can be installed on bicycles, battery powered cars and motorcycles, but also on models and ...

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Fridge Door Alarm Circuit

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> This circuit, enclosed into a small box, is placed in the fridge near the lamp (if any) or the opening. With the door closed the interior of the fridge is in the dark, the photo resistor R2 presents a high resistance (up to 200K) thus clamping IC1 by holding pin 12 high. When a beam of light enters from the opening, or the fridge lamp illuminates, the photo resistor lowers its resistance (less 2K), pin 12 goes low, IC1 starts counting and, after a preset delay (20 seconds in this case) the piezo sounder beeps for 20 sec. then stops for the same lapse of time and the cycle repeats until the fridge door closes. D2 connected to pin 6 of IC1 allows the piezo sounder beeping 3 times per second.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Schematic:</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Fridge Door Alarm Circuit" border="0" height="257" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhTL9bAJ52FjQfZ75LSA4K66kGyHKTOved8aJJ3bvKJpxfu4d22YrkqYS3sqlcLRVaWpjOx07ypXYy2oLby76jHsmMmf0UdkryS8U8zegsmftjG6nIH5Zhafb1QWpmGDt-b7vvm1xCtQL25/s640/fridge_door_alarm_circuit_diagram.gif" title="Fridge Door Alarm Circuit" width="640" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Parts List:</b></div> <div style="text-align: justify;"> R1 = 10K</div> <div style="text-align: justify;"> R2 = LDR</div> <div style="text-align: justify;"> R3 = 100K</div> <div style="text-align: justify;"> R4 = 100K</div> <div style="text-align: justify;"> D1 = 1N4148</div> <div style="text-align: justify;"> D2 = 1N4148</div> <div style="text-align: justify;"> Q1 = BC337</div> <div style="text-align: justify;"> C1 = 10nF-63V</div> <div style="text-align: justify;"> C2 = 100uF-25V</div> <div style="text-align: justify;"> B1 = 3V Battery</div> <div style="text-align: justify;"> IC1 = 4060 Ripple Counter & Oscillator IC</div> <div style="text-align: justify;"> BZ1 = Piezo Buzzer Incorporating 3KHZ Oscillator</div> <div style="text-align: justify;"> SW1 = Miniature SPST Slider Switch</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Notes:</b></div> <div style="text-align: justify;"> </div> <ol> <li>Connecting D1 to pin 2 of IC1 will halve the delay time.</li> <li>Delay time can be varied changing C1 and/or R3 values.</li> <li>Any photo resistor type should work.</li> <li>Quiescent current drawing is negligible, so SW1 can be omitted.</li> <li>Place the circuit near the lamp and take it away when defrosting, to avoid circuit damage due to excessive moisture.</li> <li>Do not put this device in the freezer.</li> </ol> </div>

Fridge Door Alarm Circuit

This circuit, enclosed into a small box, is placed in the fridge near the lamp (if any) or the opening. With the door closed the interior ...

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Broken Charger Connection Alarm Circuit

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> The above circuit can be useful to detect if the load of any battery charger or plug-in adapter supply is not properly connected. The load can be a set of batteries to be charged or any other type of battery or low dc voltage operated device. The circuit can safely operate over a 3 to 15V range and 1A max. Current, provided the supply voltage is about one volt higher than the voltage required by the load. The circuit is inserted between the supply and the load; therefore, until a trickle-charging current of at least 100µA is flowing towards the load, D1 and D2 will conduct. The forward voltage drop (about 1V) available across the Diodes drives Q2 into conduction and, consequently, Q1 will be cut-off. If no appreciable load is connected across the circuit's output, Q2 will become cut-off, Q1 will conduct and the Piezo-sounder will beep.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Schematic:</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Broken Charger Connection Alarm Circuit" border="0" height="227" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjSvQ1l6HM-d79xnYsqbmowRSXHyO2gL5fwFTWD2Ede1xQ4Vvhiw931NeRhA2OK-3CZkexPyW8p3Q2V7sd_2cu7I1dNHhII3mJPzMyRO9DFrVaCLD2Lbwi89qHZUXe9KjMW3qgB9RswrZNr/s640/broken_charger_connection_alarm.gif" title="Broken Charger Connection Alarm Circuit" width="640" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Parts List:</b></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> R1 = 10K</div> <div style="text-align: justify;"> R2 = 1K</div> <div style="text-align: justify;"> R3 = 1K</div> <div style="text-align: justify;"> Q1 = BC557</div> <div style="text-align: justify;"> Q2 = BC557</div> <div style="text-align: justify;"> D1 = 1N4007</div> <div style="text-align: justify;"> D2 = 1N4007</div> <div style="text-align: justify;"> D3 = Red LED</div> <div style="text-align: justify;"> BZ1 = Piezo Sounder</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Notes:</b></div> <div style="text-align: justify;"> </div> <ol> <li>An optional LED and its series limiting resistor can be wired in parallel to BZ1, as shown in dotted lines in the circuit diagram.</li> <li>In this case you may omit the Piezo-sounder in order to obtain a visual alert only.</li> </ol> </div>

Broken Charger Connection Alarm Circuit

The above circuit can be useful to detect if the load of any battery charger or plug-in adapter supply is not properly connected. The load...

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Fire Alarm Using Thermistor

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> In this fire alarm circuit, a Thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the Thermistor (TH1) is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature increases beyond 100 C. The circuit uses readily available components and can be easily constructed on any general-purpose PCB.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Circuit Diagram:</b></div> <div style="text-align: justify;"> <br /></div> <div class="separator" style="clear: both; text-align: center;"> <img alt="Fire Alarm Using Thermistor" border="0" height="241" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh3Lut4NKHp0pZ-5HJSDg2WENnkH025-b1bzjKEg7rDcwxe040WaeB9mu_U7ez6ST1F1XvjoWPK6bqieZvfACLxUbMazDH-a12U8syJJLvcBpu_WKNV8_ROsvRAI4Rx3lSAjKFfAHRivMGa/s640/fire_alarm_using_thermistor.gif" title="Fire Alarm Using Thermistor Circuit" width="640" /></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <b>Parts:</b></div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> R1 = 470R</div> <div style="text-align: justify;"> R2 = 470R</div> <div style="text-align: justify;"> R3 = 33K</div> <div style="text-align: justify;"> R5 = 560R</div> <div style="text-align: justify;"> R4 = 470R</div> <div style="text-align: justify;"> R6 = 47K</div> <div style="text-align: justify;"> R7 = 2.2K</div> <div style="text-align: justify;"> R8 = 470R</div> <div style="text-align: justify;"> C1 = 10uF-16V</div> <div style="text-align: justify;"> C2 = 0.04uF-63V</div> <div style="text-align: justify;"> C3 = 0.01uF-63V</div> <div style="text-align: justify;"> Q1 = BC548</div> <div style="text-align: justify;"> Q2 = BC558</div> <div style="text-align: justify;"> Q3 = SL100B</div> <div style="text-align: justify;"> D1 = Red Led</div> <div style="text-align: justify;"> D2 = 1N4001</div> <div style="text-align: justify;"> IC1 = NE555 </div> <div style="text-align: justify;"> SPKR = 1W-8R</div> <div style="text-align: justify;"> TH1 = Thermistor-10K</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <div> <b>Circuit Operation:</b></div> <div> <br /></div> <div> Timer IC NE555 (IC1) is wired as an astable multivibrator oscillating in audio frequency band. Switching transistors Q1 and Q2 drive multivibrator IC1. The output of IC1 is connected to NPN transistor Q3, which drives the loudspeaker (SPKR) to generate sound. The frequency of IC1 depends on the values of resistors R6, R7 and capacitor C2. When Thermistor TH1 becomes hot, it provides a low-resistance path to extend positive voltage to the base of transistor Q1 via diode D2 and resistor R3. Capacitor C1 charges up to the positive voltage and increases the ‘on’ time of alarm. The higher the value of capacitor C1, the higher the forward voltage applied to the base of transistor Q1. Since the collector of transistor Q1 is connected to the base of transistor Q2, transistor Q2 provides positive voltage to reset pin 4 of IC1. R5 is used such that IC1 remains inactive in the absence of positive voltage. D2 stops discharging of capacitor C1 when the Thermistor connected to the positive supply cools down and provides a high-resistance (10k) path. It also stops the conduction of Q1. To prevent the Thermistor from melting, wrap it up in mica tape. The circuit works off a 6V-12V regulated power supply. D1 is used to indicate that power to the circuit is switched on.</div> </div> </div>

Fire Alarm Using Thermistor

In this fire alarm circuit, a Thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. A...

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