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555 timer circuit

Discussion in 'Electronic Basics' started by John Winstead, May 13, 2005.

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  1. I'm trying to build a 555 or 556 timing circuit that creates a 5 second
    pulse to activate a small reed relay once every 5 or 6 minutes. Can anyone
    help me with that?
    Thank You.
  2. Here's a circuit to do it.

    A simple 555 astable cannot give a duty cycle of under 50%. Your
    requirement is for about 2% (1/60). However, a standard astable with
    any of the combinations below gives an output that is approximately 5s
    low and 5 mins high. You then simply invert that, or (easier) use a
    reed relay which gives the required action, as shown in my schematic.
    IOW, instead of using normally open contacts, use normally closed or
    vice-versa as appropriate to your intended application.

    You didn't specify voltage supply. I've assumed 12V, but the identical
    circuit is good for 5 - 15V.

    If you want reasonable precision, ensure Ct is a good quality, low
    leakage cap. Use a pot or preset in series with one or both timing
    resistors to adjust High and Low times by trial and error if you wish.

    Ct R(Upper) R(Lower)
    --- ------ --------
    82uF 5.2M 88k
    47uF 10.0M 154k
    100uF 4.7M 72k
    220uF 2.2M 33k
    330uF 1.3M 22k
    470uF 1.0M 15k
    1000uF 470k 8k
  3. John Fields

    John Fields Guest

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