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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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  • simple touch alarm

    sir, I want to the circuit diagram of simple touch alarm and list of materials required.

  • dody cherian

    is it only work if i touch with fingers

  • dody cherian

    help me
    urgent

  • dody cherian

    it is not working
    where to put the earth wires
    is it important to connect it to the plate to make it work
    i connected the two earth wires and two wire to the plates together to know is it working i dont hear any sound
    my circuit is correct

  • FJEG

    How can i make it using only one touch plate?
    and how can i increase the volume of the buzzer, what components will i change?
    please answer.
    thank you .

    • Kaz

      try looking in others,…there are lots of circuits of this,.. And if u want to increase the volume of the buzzer, u nid to change the load watts into higher value,…or try to changes the value of resistor connected to it.

  • VIVEK

    i have tried and i had successful in this expirement and i hope that you will also happy

    so i request bring more expirements and i will try my level my best okkkk

    thank you

  • lucy

    sir, how long does the beep sound remain? ,please tell me immediately sir….

    • Kaz

      it depend on how much voltage was stored in your capacitor in which the speaker is connected,….

  • Kaz

    Pls help me on this, how it works? and to change it with 1 touch plate only…nid it for my project, nid it this week

  • Vamshi Krishna

    I am very much impressed with this post.
    It helped me a lot.I took it as my mini project.It works really.I cant believe it.I thank the person who had posted this.But I have a suggestion.You can make this post more useful if you provide the information and usage of every capacitor and resistor.You did not mentioned that the given pair of amplifier is Darlington pair.
    Any ways thank you for your valuable information

  • akhila

    im trying to make this for my mini project this year and can any 1 help in making this circuit on a breadboard i would like some help it and it would help alot.

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