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Is a 555 a reliable low frequency oscillator?

Discussion in 'Electronic Basics' started by Joe, Jul 23, 2003.

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  1. Joe

    Joe Guest

    I am trying to build a timer circuit to turn on a relay for 60 seconds
    every 1, 2, or 4 hours. I was planning on using a 555 astable running at
    approximately 1 pulse every 3.5 seconds connecting that output to the cd
    4020 clock input , and then using a dip switch to take the outputs from the
    cd 4020 at the 1024, 2048, or 4096 output for my 1hr, 2hr, or 4hrs (plus or
    minus a few minutes). This signal would trigger a oneshot to turn the relay
    on for 60 seconds. My plan was to also reset the 4020 at this same time
    using the signal from the oneshot.

    I am wondering if the 555 is a good choice at such a low frequency (0.28
    Hz). It can supposedly supply a frequency as low as 0.1Hz according to my
    TTL cookbook.

  2. The 555 is probably plenty stable for this task, assuming you keep all
    power supply spikes away from its reference (bypass pin 5 to pin 1 and
    also tie your timing capacitor very close to pin 1). The reference is
    what is compared to the capacitor voltage to determine time-out, and
    these two need to be clean with respect to pin 1 if they are going to
    be accurately compared.

    I suggest you use a mylar or polypropylene timing capacitor, instead
    of an electrolytic, and use the cmos version of the 555 (E.g.. LMC555)
    to keep the comparator bias currents much lower than the capacitor
    charging current. Its output is CMOS, which is a better match to the
    CMOs counter.

    The 555 has a slight glitch problem, that might double clock your
    counter, occasionally, but you can cure this by putting a 10k resistor
    between the 555 output and the clock input.
  3. Rich Grise

    Rich Grise Guest

    According to National Semi's LMC555 data sheet (you'll have to search
    it out - I don't have the URL handy, I've downloaded the PDF.), you
    can get a 100 sec delay with 10M and 10 uF. In astable mode, the
    chart goes to .1 Hz at 10M and 1 uF.

    So use the CMOS one, and use good, low-leakage caps.

    Good Luck!
  4. Joe

    Joe Guest

    Thanks for the help, I am very grateful. I also appreciate learning about
    the microcontrollers. Thank you for the links. Maybe as my abilities
    improve, I will make the transition to them.

    Anyway, off to the breadboard!

  5. manuka

    manuka Guest

    Forget 555s - longer intervals pretty swampy ( due to electrolytic
    cap. leakages etc )& their standby power is too high anyway- even for
    the CMOS 7555 versions. Instead suggest the UK sourced PICAXE-08 => . The "08" is PIC12F629 based anyway, but has such high
    level commands ( SLEEP, NAP, GOTO, WAIT etc ) that setting up hours
    long repetitive delays is a breeze. Their SLEEP command exploits a low
    power mode drawing just microAmps. Free editor & bubble gum cheap
    chips ( start at a GBP1 pound !)Check also =>
  6. Joe

    Joe Guest

    Thank you for the suggestions. I am just now getting back into this after
    20+ years. I am still getting used to all this new technology. I have
    successfully implemented the project referenced in the original subject line
    using a 555, and also accomplished the same thing using a cd4060. The
    original intent was to generate a frequency and turn a relay on for 60 sec
    (not a long interval one shot) every 1,2 and 4 hrs. The project worked
    nicely both ways. When I need to make the technological leap to
    microcontrollers (maybe with my next project), I will. Someone else posted
    some good links.

    Thank you for your reply,
  7. Bill Bowden

    Bill Bowden Guest

    I found a good tutorial on the PIC microcontrollers at:

    It's mostly assemply language (35 instructions) which you can
    write using IDE software from The IDE will
    generate the hex code to load into the PIC. And there is a free
    programmer schematic at (plus DOS software which will load the hex file into the PIC from the
    PC printer port. It's all free stuff except the micro.
    The PIC16F628 is popular and has one 8 bit I/O port and another
    4 bit I/O port and can be bought on e-bay for $2.68 each.

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