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Touch activated switch instead of Push to Make (PTM)

Discussion in 'Electronic Basics' started by Dennis Gaida, Aug 31, 2003.

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  1. Dennis Gaida

    Dennis Gaida Guest

    Hi everyone,

    this has been discussed a lot previously, but I still don't get the
    point and I dont find the circuit for my needs.
    I need a shematic for a simple Push to Make-touch activated switch.
    THat means as long as I press the ONE sensor plate there should be
    current flowing through the switch and power my system.
    I found several promximity sensors or touch activated switches, but
    still... not finding the circuit for my needs and experimenting around
    is just not what I am able to.

    I'm not able to use QTXXX (qprox) chips, since they are not available
    here. So I just wanna use normal parts like this circuit shows:
    .... but it has a 5 second delay and I heard somewhere that it will
    keep the relay triggered and all I want is to trigger it as long as I
    push the button.
    ....doh its 220V and I want it to be 12V max
    ....another one, just too much to choose from.

    I would be very happy if anyone could provide me with a quick
    schematic of such a circuit. To get rid of the 5sec delay in the first
    circuit I would have to get rid of P1, R1 and C1... but I wouldn't
    know how to do that since there are still wires hanging around - So
    could anyone provide me with a complete circuit for a push to make
    touch activated switch?

    Thanks in advance

  2. roma

    roma Guest

  3. If you are going for cool, why not use a Sharp IR (GP2D120 for example)
    sensor and let the wave of your hand activate the light? Its an easy
    circuit, once you've built the frigging power supply that takes 120VAC ->

    Bob Monsen
  4. Dennis Gaida

    Dennis Gaida Guest

    Well that sounds interesting too, actually. Could you provide me with
    a circuit. I admit I didnt look yet for circuits in Google, just for
    the diagram and the chip seems to be intersting. Another way of doing
    what I want to do.
    Hope I get this proximitiy sensor working - what a mess.
  5. Sure. The GP2D120 takes 5VDC and GND, and outputs a voltage which is higher
    when the sensed object is nearer. It senses between 10 and 30cm, I believe.
    You need logic to detect the higher voltage, and to ignore little variations
    when the value is changing.

    The easy way is to connect the output to a PIC port set up as an ADC input.
    I have PIC code to handle the ADC, and to handle the little state machine,
    outputting a logic 1 if the light should be on, and a 0 if the light should
    be off. Its really pretty simple, but I'll send it to you if you want it.
    Email me at rcmonsen at to get it. If there is more interest,
    I'll post it.

    The other way to do it would be to use a comparator with some hysteresis
    (that is, positive feedback with a somewhat high value resistor), and feed
    the output into a binary counter of some kind. A 4017 would work fine. The
    output of the comparator then connects to the clock input, and the Q2 output
    connects to reset. The logic output is Q1.

    Both of these circuits output a logic value. You need to use the logic value
    to turn on and off the light. You can do that by connecting up the gate of a
    triac to the logic output using a 510 ohm resistor. Then, run the power
    through the triac. See the data sheets for triacs for some application
    information. Also, be careful about the triac you choose, making sure that
    it is rated for the current you want to put through it. Also, I think triac
    circuits won't be very useful with fluorescent lights, since they take lots
    of power to start.

    One hard thing about doing this is to get a stable 5VDC supply to run the
    PIC or logic chips off of. See the thread on
    alt.binaries.schematics.electronics for more information on how to do this.

    The cheapest place to get GP2D120s is Arrow electronics, if I recall
    correctly. They are around 8 bucks. They are fun for little robots as well.

    Bob Monsen
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