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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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    I made some major noob mistakes. but little sleep and well old age and well .... you know.

    Thanks again.


    :D
     
  2. killivolt

    killivolt Well-Known Member

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    Ok, I've had this on the bench now for a week testing this circuit out. I've been allowing differing things static electricity, cell phones, no trouble.

    But the one thing I've found is thermal fluctuations effect the operation. So, room temp will have to maintained for reliable operation.

    How could I apply a differential to the circuit that would allow for Impedance or Capacitance changes due to room temp.

    If it's just impedance maybe I can create a saw tooth pattern to the sensor increasing sensitivity.

    I'm in way over my head.

    Edit: Maybe that will all change when I actually install it to 60ft cable ?
     
  3. Roff

    Roff Well-Known Member

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    I don't know why it would be temperature sensitive. Are you sure you have the PNP pin order correct?
     
  4. dave

    Dave New Member

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  5. killivolt

    killivolt Well-Known Member

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    Yes, although I have an older 2907 in a can?

    This is all they had in stock should I change it.
     
  6. Roff

    Roff Well-Known Member

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    The can should be OK. If the emitter and collector are swapped, you will get low beta, which would cause temperature sensitivity.
     
  7. Space Varmint

    Space Varmint New Member

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    Well you have to do some testing. Is it on the receive side or transmit side? You could try changing one of the amplifiers to self bias which should correct for temperature.
     
  8. killivolt

    killivolt Well-Known Member

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    This leads to an Idea.

    Changing out parts. I might go as far as replacing the 4069's.

    When I installed the bottom 4148 going to drain. I could have affected 2907 too!

    I had a leaky MOSFET, because of it.

    I'm changing out the 2907 first.

    Edit: When I changed the MOSFET I just cut it out of circuit and soldered to the cut offs. Those may have an affect too! I'll have to push it through and solder it down.
     
    Last edited: Feb 11, 2010
  9. Space Varmint

    Space Varmint New Member

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    I think you hit it. Diodes are very temperature sensitive. In fact they make good thermostats.

    edit*

    So what that means is if it causes a transistor to switch with only slight romm temperature variations, you need to re-bias the respective transistor circuit. Apparently it is biased too close to threshold. You must compensate for it.
     
    Last edited: Feb 11, 2010
  10. killivolt

    killivolt Well-Known Member

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    I have a new question.

    20khz should I worry about it? I mean what if someone has a pace maker or some kind of electronic device implanted in them.

    If they touch should I be of concern about it affecting their implants?

    I'm really worried about this, it came to mind when a guy from the shipping dept said jokingly.

    Hey, you don't have the right to pass voltage through my body. What gives you the right?

    Even though he was joking. I thought about it and he's right.
     
  11. Space Varmint

    Space Varmint New Member

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    I hope he don't go to any major airports. They like X-ray people with full body scanners at high intensity. A voltage passing through the body from a touch switch? This clown must not have a computer either. It's the capacitance in your finger that is amplified. An electrostatic field which is partly inductive from the liquid in your body. Nothing passing through. Possibly a mico/millivolt if he comes to work bare footed. From the sound of it, he probably does.
     
  12. Roff

    Roff Well-Known Member

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    Some current has to flow into the body, because the PNP base current has to flow through it. The current is miniscule, and it will probably be distributed throughout the body, making the current density very low.
    I personally won't guarantee that it won't interfere with a pacemaker, but I doubt it.
     
  13. killivolt

    killivolt Well-Known Member

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    I'm still working on this.

    Yesterday, I received some new 4069's it completely squared the wave both top and bottom, it was really bugging me that I didn't have it.

    It increased my Voltage slightly I didn't measure it.

    I have my touch Circuit on the same circuit board, I know I wanted everything to remain on the same board.

    But, when I increase the length to 50'ft I noticed the relay chatters if I shorten it the response is much better. I haven't tried a coax yet, but I have a spool of single conductor about 200ft left on the roll. I'll give that a try before I start messing with the location of the sensor.
     
  14. Space Varmint

    Space Varmint New Member

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    Try putting some capacitance from ground to both ends of the wire. I would say about .01uf but may have to play with it. Too bad your not using shielded cable. But then you would have the low impedance to deal with.
     
  15. Roff

    Roff Well-Known Member

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    The exciter output should not be a square wave, as I think you know.
     
  16. killivolt

    killivolt Well-Known Member

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    Right, no it's not but the bugger has the same freaking voltage. I have replaced everything in the circuit.

    The real reason why is before the 10k resistor I have an excellent square wave. Very good looks just like the the Diagram. Then After I have on the output side of the HEX on the rail is not a Shark tooth but it's a slopping pattern. Also, it expresses in the diagram that it should be a 9v Peak which when I measure it is 4.7v or so?

    If I am to drive 50 touch sensors I will need to get that voltage up or so I think?

    Edit: LIne 4
     
    Last edited: Feb 18, 2010
  17. Roff

    Roff Well-Known Member

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    According to the simulation (I know, it's just a sim), the exciter output should be about 7V p-p.
     
  18. killivolt

    killivolt Well-Known Member

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    Ok, I'm going to get this put on another board separating everything then.

    Maybe I'm getting some sort of weird excitation distortion or something.

    Edit: In my hurry to put it together I think I should have separated it ? I don't know but I know know I should have more voltage.

    I will plan this on a strip board this time and cut out the sections I don't want then run the jumpers after. I hated the way I put this together anyways.
     
    Last edited: Feb 18, 2010
  19. Space Varmint

    Space Varmint New Member

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    No, raising the voltage wouldn't be the solution. Would probaly mess up your threshold voltages.
     
  20. killivolt

    killivolt Well-Known Member

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    Ok, I got it. I didn't need to build another exciter board, I put the relay and exciter on the same board.

    Then built a sensor board with a short wire. My plan is to have 8 total. Chatter is gone.
     

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