Replacing a pushbutton with circuitry

Discussion in 'The Projects Forum' started by yardleydobon, Nov 10, 2009.

  1. yardleydobon

    Thread Starter Member

    Apr 19, 2009
    I'm hacking an mp3 player that has a pushbutton switch. The input pin of the chip
    that senses button presses is normally pulled up to 3V. When the tactile button
    is pressed the pin gets shorted to ground. I want to replace the mechanical action
    of pressing a button with a signal from another circuit.

    My control line switches from floating to ground when the event of interest
    occurs. How can I make the mp3 player interpret this signal as a button press?

    Is a capacitor between the input pin and the control line good enough?
    Or would it be better to use a pnp in parallel with the pushbutton with control
    line going to the base?
  2. SgtWookie


    Jul 17, 2007
    If the grounds are common, why not just connect your input signal to the input pin of the chip?
  3. yardleydobon

    Thread Starter Member

    Apr 19, 2009
    Well it's obvious when you say it like that!:D

    The grounds are in common.
    I'm sorry I should have been more specific. The pushbutton only momentarily
    brings the input pin to ground, but the control signal _stays_ at ground for a long
    while. So I need to pulse the pin to ground.

    I can think of two ways to do this:
    A capacitor between the input pin and control line would act as a temporary short
    bringing the pin to ground. I think I need a large resistor going directly
    to ground to drain the capacitor too.

    An RC circuit that pulses the base of a transistor driving it to saturation temporarily
    might also work. I think using a FET would be better though.

    It's a pretty simple problem that I'm sure has been solved thousands of times by
    people hacking off the shelf gadgets with pushbuttons, but I couldn't find the answer
    with google or in the forums. I just want somebody to tell me the "right" answer.