Touch on/off switch circuit

Discussion in 'General Electronics Chat' started by hazim, Apr 21, 2010.

  1. hazim

    hazim Thread Starter Active Member

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    Hello,

    I'm trying to make a touch on/off switch circuit with one plate; i.e. when I touch the plate it switches on, when I touch it again then it switches off. I built the attached circuit for a friend from several months, it worked fine with 12V power source, but with 9V or less it didn't. I want to use the circuit with 5V source, so I tried to increase the 10M resistor but it didn't worked. I think with 12V the current through the 10M resistor and the human body to the earth decreases the voltage on pin 2 and triggers the IC.. with 5V the 555 triggers only if I touched the ground (0V) with my second hand at the same time I touch the plate.
    Anyway, my aim is to turn a circuit on and off by touching a plate as I described. You my suggest another way, other than the attached circuit if it consents my purpose. Any ideas are appreciated.

    Thanks,
    Hazim

    [​IMG]

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  2. Bosparra

    Bosparra Member

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    Have you tried lowering the value of the 10M? With the decreased voltage comes decreased current for the same resistor value.
  3. rjenkins

    rjenkins AAC Fanatic!

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    That circuit requires contact with the sensor to pull the trigger input below it's operating threshold.

    It won't be reliable with a single contact, it requires definite earth (0V) contact (albeit via a high resistance) to operate.

    You would be better off with a capacitive style switch that does not rely on a dc circuit.

    Example of a 5V one here:
    http://www.discovercircuits.com/DJ-Circuits/5vmom1.htm
  4. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    How about this circuit?

    [​IMG]
  5. hazim

    hazim Thread Starter Active Member

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    Yes I tried lowering the value of 10M, as I expected, it went from bad to worse. A capacitive style switch should work fine, but the circuit will be bigger and more complex, I think I can find a simpler way using a timer or maybe an op-amp/comparator.
    The circuit Bill provided seems to be ok, but it uses a momentary push button switch instead of a touch contact :confused:
  6. kingdano

    kingdano Member

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    perhaps you could try his circuit using your touch-contact switch - and see if it works?
  7. hazim

    hazim Thread Starter Active Member

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    kingdano, the touch-contact switch isn't working properly.
  8. John P

    John P Senior Member

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    I think the right way to do this is to use an oscillator where the frequency depends on the value of a small capacitor connected to ground. Touch the wire connected to the capacitor, and you'll add your own capacitance in parallel with the existing cap, and it'll change the oscillator frequency, which can be detected and used to switch the circuit. But I can't deny that it's a step up in complication.

    Wikipedia says:
    "The Human Body Model for capacitance, as defined by the Electrostatic Discharge Association (ESDA) is a 100pF capacitor in series with a 1.5kΏ resistor."
    hazim likes this.
  9. rjenkins

    rjenkins AAC Fanatic!

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    The simple capacitive touch switches use an oscillator fed via a high resistance to the touch plate.

    The touch plate also connects to a simple rectifier / comparator.

    When the plate is touched, the level of the AC signal is reduced due to body capacitance.
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  10. John P

    John P Senior Member

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    OK, there you go. Use an oscillator. I don't think you need a rectifier exactly, more like a pulse and filter arrangement, where each cycle puts a bit of charge on a capacitor, and it drains off through a resistor. The faster the pulses come in, the higher the voltage will go, and then you use a comparator to test it.
  11. hazim

    hazim Thread Starter Active Member

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    Ok. Thanks for you all. It's clear that capacitive based touch switch is better.
    I've got this circuit. It's simple and easy, but I have a problem with building it..
    [​IMG]

    I can't find the mosfet transistor used or even any small one (like TO-92) here in Lebanon. I have only TO-220 power mosfet transistors and I don't want to use them. I have two small 5V relays I may use instead, but I'd like to ask about other possible solutions.
    Also, is it ok to use 74ls74 with 5V supply instead of 4013?

    Regards,
    Hazim

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  12. retched

    retched AAC Fanatic!

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    Do you have an old TV or radio or cell phone or inverter or etc.. That you can scavenge for parts? You may get lucky there.

    As for the 4013 and 7474, they are both d-type flip-flops, so either should do. Watch for spec differences, if any.
    hazim likes this.
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