Touch sensor - without analog reading

Discussion in 'General Electronics Chat' started by Captain E, Jul 11, 2015.

  1. Captain E

    Thread Starter Member

    Jun 16, 2015
    81
    6
    Hello!
    I've got a project in mind, where I'd need many more inputs than the Arduino UNO can provide. I've got a I2C I/O expander, MCP23018, which I'm still learning through my last post, (which I'm not quite done with yet) but I feel very close to what I need from it.

    Back to my project, I would need some kind of switch, and many of them. I can attach, for example, switch buttons to the MCP23018 and read them as digital inputs.

    I'll go with switch buttons if I have to, but what I really want are some capasitive sensors. I've used these before and know how to make, use and understand them.

    But the problem is that I need analog reading for capasity sensors, something the MCP23018 can't provide.

    SO my question is:

    Are there any good way of making capasitive sensors withouts having to read analog inputs?

    Maybe I can change the logic levels of a digital input (from my I2C device) enough so the reading goes from LOW->HIGH with some extra capasitance, from a finger?

    If not, I'll make some cool manual switches, but maybe there is some way :)

    Thanks!
    ~ Cappy E
     
  2. #12

    Expert

    Nov 30, 2010
    16,246
    6,738
    A simple jfet wired as a class A amplifier will turn finger hum into square waves in one stage. My favorite is J210 at 200 ua IDSS. Just put a meg and a nanaofarad in series with the gate and bingo. It's a digital hum sensor.
     
  3. flat5

    Active Member

    Nov 13, 2008
    403
    17
    #12, would you please draw the circuit?
    I don't know if source and/or drain resistors are required and where to tap the output.

    Google does not find a very simple circuit.

    Also did you mean or could we use:
    J201 JFET N-CHANNEL AMPLIFIER TRANSISTOR?
     
    Last edited: Jul 11, 2015
  4. #12

    Expert

    Nov 30, 2010
    16,246
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    Here's one I did in 1975. Follow it with a bipolar transistor like a 2N3904 as a current booster, then a rectifier...or use it as a direct input to a CD series digital chip, maybe even the lower voltage chips by lowering the power supply to 5 volts. Any jfet I ever saw can be used depending on whether you can operate at 200 ua with a J201 or want to be in the 5ma to 15 ma range with a 2N4416. They all have incredibly high gate impedance. You just choose what current range you want to be in by reading the IDSS value on the datasheet. Choose the drain resistor to use up most of the power supply voltage at idle and expect the noise from your finger to turn the jfet off and on.

    Did I just cause more questions than answers?
     
    Last edited: Jul 11, 2015
    flat5 likes this.
  5. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,486
    There are chips out there made by Atmel, among others, that already include everything necessary to implement capacitive switching touch sensors. They are a marvel that compensate for changes in temperature, humidity, power supply fluctuations and electrostatic drift.
     
    #12 likes this.
  6. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Thank you, #12. I have the info I need now.
    I'm interested because I might build a Morse paddle.
    For mechanical simplicity a 2 paddle capacitive switch is what I'm thinking.
    I've tried to do it using Arduino analog inputs but have not gotten it to work reliably without a ground strap on my wrist (actually +5v). Perhaps I need to try other pullup resistors. 100k does not work well. I should try 1M.
     
  7. #12

    Expert

    Nov 30, 2010
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    I'm with Martinez on this one. This jfet circuit was whiz-bang wonderful when Radio Shack was the place to go, but modern chips make it look like monkey business. If this is all you need, fine. If it doesn't work well because you must NOT be grounded, there are a lot of better options.

    Just rember: All jfets are born, "on" and they never match, even in the same bag. You set your drain resistor for the current of the transistor you have in your hand and work from there.
     
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  8. Captain E

    Thread Starter Member

    Jun 16, 2015
    81
    6
    Thanks for the help, guys! :D

    CapE
     
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