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    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

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    63 Responses to "Simple Touch Alarm Circuit"

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    1. I tried that circuit.. But i couldn’t got exact output. Without touching those 2 plates i heared the buzzer sound…!

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      • Hi pradeep. Im Shivu. IM also getting the same problem that is when I switch on the circuit im getting the buzzer sound directly without touching the Touch plates How did u solve that problem. Can u plz reply

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    2. T.K.Hareendran T.K.Hareendran says: on February 6, 2011 at 3:44 am

      The sounder is not a buzzer.It is an ordinary Loudspeaker!
      Recheck your circuit for any wrong wiring at the front end ie T1,T2 etc.Finally try to increase the value of R1.This circuit is highly sensitive& even a small bridge between the touch points ,formed by solder flux in PCB will trigger it.

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    3. diploma in electronics and communication (2nd year)

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    4. satwinder hundal says: on April 24, 2011 at 1:35 pm

      i’ve made it with a single touch plate

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    5. sarathy says: on May 12, 2011 at 11:28 am

      sir,
      can produce me the full detailed explanation of its working .

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    6. twinkle thakur says: on July 5, 2011 at 12:41 pm

      plz send me principle and full details

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    7. shreyas b says: on September 7, 2011 at 2:03 pm

      thnx a lot sir!! It really worked!! I wanted to ask if we want another type of sound from speaker,what changes should be made??

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    8. can this work with a single plate…
      plz post the ckt detail.can it be used for lie detector,door knob alarm,vechicle alarms also..
      plz do reply…

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    9. pls give more information on how the project works….plsss…

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    10. USMAN ASLAM FROM TUF ENGINEER says: on January 24, 2012 at 6:39 am

      i hv made this project . its very simple n gud ij real life…. implement in home

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    11. plzzzz send touch alarm circuit

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    12. sir ,
      please send me a pdf and ppt oftouch alarm circuit.

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    13. Naeemuddin says: on May 29, 2012 at 8:05 am

      this is very good project and have made it ……very very simple and interesting project………….

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    14. rockin yar. i did it for the exhibition and won 1st place

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    15. it really worked.thanks for posting this circuit diagram.it will help many students

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    16. Student says: on July 30, 2012 at 2:17 pm

      Where will we get the touch plates. And can u please send me the full details of this. I have an exhibition.

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    17. Student says: on July 30, 2012 at 2:18 pm

      Can u do this on a bread board or pcb

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      • Jim Keith Jim Keith says: on July 30, 2012 at 2:52 pm
          member

        The touch plates are simply two small strips of foil or metal that your finger can bridge–two bare wires should function if your skin is moist.

        This can be made on a breadboard or PCB. On a PCB, the touch plates could be etched onto the surface.

        Appears to be a fun project.

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      • can you give to me a fool documentation or details how to make that circuit…

        t.y.

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      • hm how can i attached the two plates on pcb or in breadboard?? and what kind of toochplates should i buy to do this kind of project?? thanks for answering sir.

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      • Jim Keith Jim Keith says: on October 13, 2012 at 2:43 pm
          member

        Actually any conductive material may be used for touch plates. How about two machine screws? Experiment and use imagination.

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    18. i tried it but not working,,, may be the diagram i too complicated ,,,, please please send me its simple diagram please please

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    19. the circuit is really nice. i want to make it. can u please send me a simpler circuit diagram? also give me information how can it be connected on a bread board? i mean the touch plates and speaker? please provide me information?

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    20. burntoffering says: on September 20, 2012 at 4:23 pm

      can you send me full details of how the touch alarm works please

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    21. Sir what is the alternative for BC547 B?

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    22. Sir,
      I wanted to know if I could use a LED in place of the speaker n what changes will be required for this to work??

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    23. Dear T. K.Hareendrn I’m a technology Student EC PLZ Sent for me the interesting circuit technology

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    24. weeee! i made it! hoping for a good grade tomorrow! my classmates also liked it and they always touch the sensor. haha!

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    25. sir, when i simulate the circuit, i have getting the output in buzzer as 0V. when see graph the trigger became DC (direct current). actually there must be AC (alternatif current). so how to set the trigger to became AC from DC. plz help me sir.

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    26. sori i wrongly send to this section

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    27. hey can you suggest me another capacitor instead of c3 10nf as it is not available

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    28. i want application related simple touch alram using

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    29. 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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    30. Vamshi Krishna says: on October 12, 2013 at 7:12 am

      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

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    31. 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

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

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

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    33. 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

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      • hmmm, can u explain to me how each of the components works? and its function?

        ty in advance

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    34. 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 .

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      • 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.

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    35. dody cherian says: on March 29, 2014 at 9:24 am

      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

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    36. dody cherian says: on March 29, 2014 at 9:25 am

      help me
      urgent

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    37. dody cherian says: on March 29, 2014 at 9:26 am

      is it only work if i touch with fingers

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    38. simple touch alarm says: on April 20, 2014 at 10:36 am

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

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