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one shot timer MC14541BCP retriggers

Discussion in 'Electronic Design' started by Bernhard Kuemel, Jun 18, 2005.

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  1. Hi sed!

    When the timer switch for our hallway failed I built a new one. It's
    in service for maybe a year now but it has one bug: When the timer
    switches off the lights it retriggers sometimes (maybe 40% chance)
    and runs for another period. The light just is off only for about
    0.1 s so the timer does turn off the relay.

    I added a capacitor paralell to the pull down. Don't remember how
    bad it was before. What should I do? A bigger cap? I don't know its
    current value.

    Here's the circuit diagram:

    A photo of the timer:

    The MC14541BCP data sheet:

    Thank you, Bernhard
  2. Ross Herbert

    Ross Herbert Guest

    For starters, you should have, say, a 10uF electrolytic by-passed with
    a 10nF ceramic mounted close to pin 14 and going to gnd. These values
    are not critical so use common sense.
  3. Ban

    Ban Guest

    Tie pin 5 to gnd
  4. That seems to solve the problem. I put the caps in parallel. Is the
    ceramic cap to reduce oscillations that would occur with the
    electrolytic when the turning off of the relay causes a voltage step?
    I left it at a 100k pulldown to be able to turn AR on/off. If the
    problem shows up again, I'll tie it to gnd directly.

    Thanks, Bernhard
  5. Ross Herbert

    Ross Herbert Guest

    The electrolytic provides a low impedance voltage supply/reservoir
    directly to the IC and the ceramic by-passes high frequency artefacts
    present on the power supply to ground so that they do not affect IC
    operation. Noise can be generated by almost any source of switching
    or rapid voltage transition either common to the circuit around the IC
    or even external to the circuit on the ac mains supply.
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