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Touch activated Switches

Discussion in 'General Electronics Chat' started by killivolt, Jan 23, 2010.

  1. killivolt

    killivolt Well-Known Member

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    So, I need to have some switches at the students desks I thought this might work.

    It claims wire length should work the room is 30ft by 20ft But actual wire length is only about 10ft per row.

    If this looks like a good circuit. what are the drawbacks 8 students per row 32 total.

    Thanks in advance.

    kv
     

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    Last edited: Jan 23, 2010
  2. Space Varmint

    Space Varmint New Member

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    What's the oscillator for? The touch switch looks pretty good. They are using a JFET which is high Z and that should send out a pulse when you touch it. I have found that in the gate if you attach it to a double sided copper clad board that it will act like a huge capacitor and makes a nice pad for the touch switch.
     
  3. killivolt

    killivolt Well-Known Member

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    I'm not an engineer.

    I'm hoping I can build it without to much difficulty.

    That's the very question or questions needed to pick out the inconsistency's of the circuit.
     
  4. dave

    Dave New Member

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  5. Roff

    Roff Well-Known Member

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    The oscillator is required because the touch switches need AC to operate. With DC, they would just give a short pulse when touched.
    The receiver uses a MOSFET, not a JFET (as stated by SV).
    It appears to me that the EXCITER signal needs to be distributed on one wire to all remote touch sensor circuits, and all the SWITCH OUTs need to be bussed onto another wire and routed to the receiver input.
     
    Last edited: Jan 23, 2010
  6. Space Varmint

    Space Varmint New Member

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    Yeah I noticed it was a MOSFET after I typed it. Maybe you need to describe what this thing is supposed to do. I gather you want several students to be able to activate a device by them each having their own touch switch? I would use a JFET instead of a MOSFET. I thought the clock might be some sort of sequencing circuit because as you said Roff, the MOSFET circuit would deliver a pulse which is all that is needed. That pulse could be used to toggle say a J-K flip flop off and on.
     
  7. killivolt

    killivolt Well-Known Member

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    If the MOSFET will deliver a long enough pulse to trigger a relay momentary contact. It is what I'm looking for.

    So, each Student touch will deliver a single pulse the exciter will momentary contact a relay it wil activate my secondary existing circuit for ON when they are done touch again and will turn OFF.

    So, I guess would the pulse be long enough to drive a momentary contact relay closed ?

    kv

    Edit: Or do I still need the JFET
     
    Last edited: Jan 23, 2010
  8. Roff

    Roff Well-Known Member

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    I'm not sure the touch switch would be reliable without the pulsed drive. A toggle FF would be sensitive to bounce, and would also change state every time someone touched a switch. How would you process that?
    A JFET would require a negative gate voltage to turn it off. Why do you think a JFET would be better than a MOSFET?

    Dave Johnson (the designer) is a very bright guy. I have looked at several of his designs, and they all seemed to have been very well thought out. That doesn't mean this circuit is appropriate for killivolt's purpose.

    @killivolt: What are the switches to be used for?
     
  9. killivolt

    killivolt Well-Known Member

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    To drive a momentary contact relay. I need closure of the contacts with each pulse.

    Edit: I might just use the MOSFET to pull a single 5v line to ground now that I think about it.
     
    Last edited: Jan 23, 2010
  10. Roff

    Roff Well-Known Member

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    So you just need to activate the relay when any student pushes his button?
     
  11. Space Varmint

    Space Varmint New Member

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    If you need a longer pulse just use a one shot multi-vibrator like a 74hc123.
     
  12. killivolt

    killivolt Well-Known Member

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    Yes.

    Edit: I also wouldn't want any bounce in it I need complete closure and release.


    If this is what I'm looking for I'll give a try. If it doesn't seem to work properly I'll look into some other Idea's.
     
    Last edited: Jan 23, 2010
  13. Space Varmint

    Space Varmint New Member

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    Sounds good. Still I would use a JFET. They have very high impedance and will be more sensitive to touch. A one shot will give you any variation of pulse width because of the RC time constant but if you just want it to toggle off and on I would use a J-K flip flop. Tie J and K high and use the clock input.

    edit*

    Be sure to tie preset and clear high also
     
    Last edited: Jan 23, 2010
  14. killivolt

    killivolt Well-Known Member

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    So, is this going to deliver a series of pulses until touched again or just a single pulse ?

    If so, then I would need to implement the one shot into the circuit.

    Or do I have it all wrong ?
     
  15. Space Varmint

    Space Varmint New Member

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    It will be a single pulse though you will get a damped wave effect (a few smaller pulses). So yes, if you need a long pulse then use a one shot. I believe you could get pulse widths up tp a couple of minutes. If you use a one shot, than disregard the after pulses. One shots are often used to de-bounce a switch.
     
  16. Roff

    Roff Well-Known Member

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    A MOSFET's gate impedance is at least as high as a JFET's in this application. Keep in mind that the FET is not being used to sense touch. That's the job of the PN2907.
     
  17. killivolt

    killivolt Well-Known Member

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    So, with that I will build the circuit bread board it and check it with the Scope then add the receiver.

    Can I just tie the 5v line to D junction. All I need to do is pull it down which is what the momentary would do anyway.
     
  18. Space Varmint

    Space Varmint New Member

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    You might need one transistor amplifier. Check it and see. I doubt it but may be hit and miss.
     
  19. killivolt

    killivolt Well-Known Member

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    I'm not sure if I'll be able to draw up a diagram Monday or not but as soon as I can I'll do it.

    Meanwhile, I'll see if I can get the approval for a 4069 from the Boss and get everything I need.
     
    Last edited: Jan 23, 2010
  20. killivolt

    killivolt Well-Known Member

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    Where will it be added if needed. In the one shot or on the D junction of the MOSFET?

    Edit: I think I understand. I need to Amp the 5v line on the Drain.
     
    Last edited: Jan 23, 2010
  21. Roff

    Roff Well-Known Member

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    Don't tie the drain (D) of the MOSFET to +5V! You should be able to drive the relay off the drain, with the other end of the coil connected to +V. Be sure you add a diode (1N4148) across the coil, anode to the drain of the MOSFET, to protect the MOSFET from the flyback voltage which is generated when the MOSFET turns off.
     

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