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Switch de-bounce circuit questions.

Discussion in 'Electronic Basics' started by CFoley1064, Nov 6, 2004.

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  1. CFoley1064

    CFoley1064 Guest

    Subject: Switch de-bounce circuit questions.
    Hi, Andrew. The 555 trigger input is level-triggered rather than
    edge-triggered. That gives you some options, depending on how your circuit is

    Here's the worst choice (view in fixed font or M$ Notepad):

    T .---------.
    --- | |
    ..--o o-----o-----|2 |
    | | |
    | | |
    '----------o--. | 555 |
    | | |
    | | |
    === | |
    GND | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    A standard 555 has an input that may permit this, but it will false trigger all
    over the place.

    This setup is somewhat better:

    | |
    | |
    T '-' .---------.
    --- || | | |
    ..--o o---o--||-o--o2 |
    | || | |
    | .01uF | |
    '--------o--. | 555 |
    | | |
    | | |
    === | |
    GND | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    The lower value you can make the pullup resistor, the better. The cap keeps
    the input signal from exceeding the length of the one shot.

    A 555 is very sensitive to false triggering. This will help some. If this
    doesn't do it, and you are still getting false triggering when you bring the 1K
    resistor down to 150 ohms or so, you might have to add further input signal

    Good luck
  2. Bill Bowden

    Bill Bowden Guest

    Did you use a termination resistor at the flip-flop input?
    If the trigger signal is from a 555 timer, you can use a fairly
    low value resistor at the flip-flop input, maybe 1K will help.
    The resistor will short out the noise on the long wires.
    If you have access to both sides of the flip-flop, you can use the
    other side to indicate the off condition.

  3. Rubicon

    Rubicon Guest


    I have put together a switch circuit using two tactile switches with
    555 de-bouncers to a positive edge triggered flip-flop controling

    The on-board tactile switches work fine but I have also added pins for
    off-board switches.

    With off-board switches what do I need to do in addition to using
    shielded wires to prevent false triggering? Twisted wires caused the
    switch circuit to flip-flop when a fluorescent light was turned on.

    In addition I have LEDs indicating the flip-flops output ON state but
    how do I indicate the OFF state with LEDs? Formerly I used a double
    pole switch.


  4. 1. Your one-shot is currently about 110 ms; I'd make it a bit longer,
    say 300 ms, by increasing either the Rt or Ct.

    2. You don't show a schematic, but have you included a small cap, say
    10 nF ceramic, at pin 5?

    3. Similarly, have you added a smoothing cap, say 22 - 100 uF
    electrolytic or tantalum on 5V rail, and also for good measure a 0.1uF
    ceramic, both close to 555?
  5. Rubicon

    Rubicon Guest


    Thanks for the reply. The circuit I used was a general 5V-15VDC 555
    de-bouncer circuit and has a 100K resistor from my 5VDC+ to Pin#2 of
    the 555 then via the on-board push-button (tactile) switch to V-. A
    100K resistor goes from V+ to 555 Pin#7/6 and on to a 1uF ceramic cap
    to V-. I chose the cap value via experimentation and I would estimate
    that the circuit is about 95% effective like this as sometimes it
    "misses a beat".

    I will make the alterations you suggested though I've already gone and
    chipped away the stripboard! Blast!


  6. Rubicon

    Rubicon Guest

    No Bill I did not. 1K approx to V- at F/F input.

    Unfortunately the other side if the F/F has an identical 555 trigger

    I can't help but think that it's a waste of a 555 timer here - perhaps
    I should have gone for a transistor/resistor/capacitor circuit


  7. Rubicon

    Rubicon Guest

    O.K. I'll do that.
    Yes I have the 10nF ceramic cap on Pin#5.
    I have 0.1uF ceramic caps on each 555 de-bounce circuits V+, 0.1uF
    ceramic bypass caps next to the 555s and a single 10uF electrolytic
    cap on the V+ for both de-bounce circuits.

    Thanks for your advice. I'll alter the 10uF cap value and add the
    other electrolytic caps as you suggested.


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