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Circuit Description
The touch alarm described here is a wonderful electronic device. When you just touch a copper plate peculiar sounds are produced. This can be used at many place for theft protection.
The circuit comprise two stages. The first stage having T1 and T2 transistors forms a trigger circuit. When the copper plate is touched the transistor T1 conducts and produces feeble pulses which are amplified by T2.The second circuit is and audio oscillator made by T3 and T4.The signal form the trigger part becomes an audio signal which is converted into sound by the loudspeaker.
Circuit Diagram
Components Required
1. Transistors-
T1- BC 149
T2- BC 149
T3- BC 147
T4- AC 128
2. Resistances- T2- BC 149
T3- BC 147
T4- AC 128
R1- 5MΩ
R2- 100kΩ
R3- 220kΩ
R4- 10kΩ
R5- 1.2kΩ
3. Capacitor- R2- 100kΩ
R3- 220kΩ
R4- 10kΩ
R5- 1.2kΩ
C1-1µF 6v
C2- 0.01µF (disc ceramic)
4. Loudspeaker 80Ω 500 mWC2- 0.01µF (disc ceramic)
5. 9V DC Battery
Other Related Circuit:
The purpose of this project is to trigger/switch an LED with a simple touch. The skins resistance is about 600K-1M. So if you try building a circuit with just an LED, power supply and touch conductors, the LED doesn't glow. This is because, high skin's resistance prevents enough current to flow through the circuit. So the only option we have is to amplify the tiny current flowing through the skin to a magnitude enough for an LED to glow. For that purpose, we have to use transistors. The circuit will also work with just one transistor, but the LED doesn't glow bright enough. So we have used two. Tiny electric current flows through the skin when the end touch conductors are touched. This tiny current is amplified in magnitude by the first and second transistors and it finally passes through the LED. Refer to the circuit diagram for better understanding.Circuit Diagram
Components Required
1. Transistors BC 547
2. Resistors: 270 Ohm, 100 KOhm
3. LED
4. 6V Power Supply
5. Touch Plate
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