Calculator Buttons..?

Discussion in 'General Electronics Chat' started by Ian Gilchrist, Sep 11, 2016.

  1. Ian Gilchrist

    Thread Starter New Member

    Sep 11, 2016
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    0
    Hi Gang,

    I have a auto cat feeder.. it has buttons on it's little circuit board - kind like those in a calculator (see image below). Those buttons have a kind of contact pad which pushes down on to them etc - how does that work..?

    i.e. what are they sensing between the PCB contact patches - capacitance, resistance or..? I think if I just short circuit these they don't work (believe I have tried this in my long past)

    Ultimately - I want to use a raspberry Pi to control the feeding timing (as the onboard controller sucks) - but I don't want to remove the controller.. just be able to use the rPi to effectively 'press' the manual feed button. I was thinking, I could just use a relay and have the rPi close that but I think the control circuit for these needs to see something else - as opposed to short circuit.

    Please reply on topic - not looking for alternative ideas to the problem - just how these buttons work, hoping someone knows :)

    thanks,

    Ian


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  2. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,144
    203
    they are effectively a conductive elastomer keyboard. http://www.eecoswitch.com/wp-content/uploads/2011/08/ElastomerKeypads.pdf

    Many times the contacts are gold plated for long life. The "rubber" that makes contact is conductive.

    Usually the conductive rubber wears off or they get dirty. There is a repair kit.

    The keypad is typical of a MATRIX design. With x and y. If the designer is smart, they will make certain keys not able to be easily hit.
    two rows and one column is an illegal key.

    You can measure the resistance of the elastomer. They should activate with a short and sometimes with a resistor. try 200 ohms to 1K ohms.

    You can use an OPTO FET instead of a relay to activate the keys. Likely only a matrix would be required, not the entire set. These are high impedance inputs usually. You holding a wire may get different results than a relay or opto-FET.
     
  3. Ian Gilchrist

    Thread Starter New Member

    Sep 11, 2016
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  4. Ian Gilchrist

    Thread Starter New Member

    Sep 11, 2016
    3
    0
    Great stuff - I like it.. I've never use an OPTO FET.. in fact not much of a FET guy(for no good reason) - if I have a high output from the rPi (TTL if I recall) how would I wire that with the FET etc - thanks!
     
  5. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    The repair kit includes some carbon paint to make a resistive layer when it dries.
     
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