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touch alarm circuit schematic

Simple Touch Alarm Circuit

Touch the sensor of the alarm with your finger and it starts beeping, goes on for some time and then stops. Touching it again, and it goes again! This little and flexible circuit consists of a touch sensor and a directly coupled transistor amplifier with a small loudspeaker as the output load. Two sensor strips of metal are mounted side by side on a 1×1 cm size micasheet and connected to the input of the circuit.

How the touch alarm works

Under normal conditions no current flows through switching transistors T1 and T2 and the potential across capacitor C1 is almost 0V. If the two touch plates are touched together by a finger, the alarm circuit is enabled by T1 and T2. Transistors T3and T4 form a complimentary pair amplifier, with positive feedback (regeneration) provided to the base of first transistor T3 via R3 and C2.

The sound generator oscillates at a frequency determined by C2 and R3 and the bias voltage of T3. The bias voltage of T3 can be changed by changing the values of R2 (and R1). The alarm will provide quite loud audio output into an 8 Ohm speaker with a 9V battery at a current drain of less than 100mA. Needless to say, this is an ultra simple hobby circuit. You can experiment with different component values (R1,R2,C1,C2,R3 and R4, etc) to make your own special alarm!

Touch Alarm Circuit Schematic

touch alarm circuit schematic

77 Comments

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  • Nikil

    Hy I hv to make it for my assignment plz help..i don’t know how to make it..

  • Sunil

    Sir,seems to be good circuit. Can I use 12V/6V relay in place of speaker? Can I use 12 V power supply?And what modifications are needed? Thank you.

  • kalai

    Please send ur circuit diagram to my mail id.please………..tnx

  • Precious

    Pls sir, i need the full details of touch alarm circuit using b556 (single touch plate)
    tnx

  • KB khan

    Pl.send me this project in detail with diagram

  • chetan

    sir what is d costing of touch sensitive alarm means costing???

  • shwetha

    How can I fix touch plates on breadboard?? Or can we use just a small single stand wire for this circuit in place of touchplates??please reply me soon

  • shwetha

    How can I fix touch plates on breadboard?? Or can we use just a small single stand wire for this circuit??please reply me soon

  • bharagava

    Sir I am in chitoor district of andhrapradesh please tell me where the touch plates are available I dont now pls

  • Tyler

    Hey guys my name is Tyler and im a 16 year old high school student. I used the schematics of this circuit for a physics assignment, however had trouble with the exact schematics and details listed above, so i got creative. As it says you can play with and change the values of the resistors but you can go further. This circuit is far more complex than it needs to be in terms of components, there are much more basic and common components that give the same effect.

    I’ll be more than happy to give more detail as to what i have done in terms of changing this circuit for it to work, and mind you it does work very well.

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