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Capacitor for momentary switch?

Discussion in 'Electronic Design' started by MrQuan, Feb 21, 2007.

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  1. MrQuan

    MrQuan Guest

    Dear all,

    I'm a complete novice at electronics and need help selecting a
    capacitor (I guess this is what I need).

    I have a momentary switch that is wired to a controller board. The
    controller has a resolution of one second, in that it scans it's
    inputs once a second only. Sometimes when I push the button too fast,
    the controller misses it and doesn't register the input.

    I'm hoping there is a way to connect a capacitor into the switch
    circuit so that it's held closed for longer than a second no matter
    how fast I push the button, and therefore registering the input on the
    controller without fail. I've measure 5VDC across the switching
    connection, incase that information helps.

    Can anyone share a solution for this problem I'm having? ...and
    perhaps even suggest the right capacitor to use, if in fact a
    capacitor will solve this?

    Many thanks,
  2. John Barrett

    John Barrett Guest

    Capacitor wont do it -- you need a pulse stretcher, which may be more
    trouble than its worth.. you can make one with a 555 timer easy enough..
    look for "one-shot" circuits where, when triggered, the circuit turns on for
    a specific period of time, then resets and waits for the next trigger.
  3. EdV

    EdV Guest

    Using a toggle switch might work. Of course this may make operation a
    little goofy but it is better than it not working at all.
  4. John Fields

    John Fields Guest

  5. MrQuan

    MrQuan Guest

    Well, that's fine for me in the workshop, but this solution was really
    aimed more at the people whos offices and buildings these are
    installed in. The controller is used for HVAC and access control. I
    have seen some of my colleagues label push buttons with "push and hold
    for one second", I was just hoping for a more elegant approach!
  6. MrQuan

    MrQuan Guest

    I was actually looking at different switch alternatives, but I think
    for aesthetic reasons a push button style is my only option as it will
    be mounted to a small panel on a wall.
  7. MrQuan

    MrQuan Guest

    I see how this solution would work, however as you say it looks like
    more trouble than it's worth... I'll have to look at alternative
    solutions I guess. I have been mounting neon indicators next to the
    push buttons, so the user gets feed back whether they have actually
    switched anything - and this seems to be working. I was just hoping,
    if it were easy enough, to make it totally robust. Thanks
  8. John Barrett

    John Barrett Guest

    OK -- solution number 2 -- get a dual flip-flop chip (74LS74)... button
    hooks up to the Set line, which will "remember" the button press until the
    processor reads the data and deliberatly resets the flip-flop by pulsing the
    Reset line.. each chip will handle 2 buttons and no messy R/C timing needed.
  9. Rich Grise

    Rich Grise Guest

    Then that is definitely the way you want to leave it - it protects the
    system from being turned on/off accidentally when somebody just bumps
    the button.

  10. Have the pushbutton set a flip flop and the sampler reset it.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  11. amdx

    amdx Guest

    There are ways to use an SCR to stretch your pulse.
    Might take 1-cap 2-resistors and an SCR. Off the top of my head.
  12. amdx

    amdx Guest

    I posted a schematic on alt.binaries.schematics.electronic
    of a circuit I used to debounce a switch, but I think you can
    modify it to stretch your pulse.
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