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555 timer circuit

Discussion in 'Electronic Basics' started by [email protected], Dec 29, 2004.

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  1. Guest

    Hello,I made a simple astable 555 timer circuit the same as the one on
    the 555 web site,but im having trouble working out the values of the
    two resistors and the capacitor so it will turn on for 3 minuts and off
    for 5 minuts.Could someone please point me in the right direction as i
    found the data sheet a bit confusing.thankyou in advance
  2. You seem to be asking for a 3 / (3 + 5) = 37.5% duty cycle. Most
    555 example circuits, etc do not use such low duty cycle, but this is
    possible. Also, most examples do not show such low frequencies, but I
    have managed this with the 555, and you can ultimately find examples.
    I am very sure you can do exactly what you are wanting with a 555.

    I found good and bad ways to accomplish the task you are now attempting.
    There are two books by two different aithors: Walter Jung and another by
    Howard M. Berlin. I own the latter book and it shows some simple circuits to
    accomplish this. The basic concept in the circuit I am most familiar with manages
    this using a couple of diodes in parellel, but in opposing polarities,
    that connect to the timing cap. There are also two separate potentiometers,
    either both or only one in series with each diode. The diodes and pots are
    connected between pins 6 and 7. The circuit allows independent control over
    the charge and discharge intervals. If you can find the Berlin book "The
    555 timer applications sourcebook with experiments", have a look at
    the chapter on the astable configuration, and particularly figures 3-8
    and 3-9. These allow control over the frequency AND duty cycle. I suspect
    you can achieve exactly what you are wanting from those examples.

  3. Brian Lyons

    Brian Lyons Guest

    Try the following values:

    R1 = 430K
    R2 = 630K
    C = 1000uf

    T1= about 300 sec
    T2 = about 180 sec

    Total = about 8 mins (because of component tolerances, it is difficult
    to obtain precisian with long durations)

    Use single transistor to invert logic levels for T1 & T2

    A good 555 tutorial may be found at

  4. Byron A Jeff

    Byron A Jeff Guest

    Frankly a 555 is a bad choice for such long delays. The precise timing
    depends on the values of the caps and resistors of the circuit and at
    200-300 second delays, the tolerances will be off by tens of seconds
    every cycle.

    A better solution is to count smaller more presice cycles. If you have wall
    power available, an AC wall wart, along with a zener diode and a resistor
    makes for a nearly dead accurate 60HZ reference that can easily be counted.

    My belief is that microcontrollers are the 555 of the 21st century. A
    single microcontroller like a PIC 12F629 could easily handle your
    requirements with or without the 60 HZ reference.

  5. Bill Bowden

    Bill Bowden Guest

    There is a 555 calculator on my site that will work out
    times based on components entered. But in operation
    the times will be a litte longer due to the capacitor
    leakage. Probably get about 10% additional time for 3 minutes

  6. Paul

    Paul Guest

    Thankyou for all your help,i`ll try all your ideas and see how i go,it
    doesn`t worry me if it`s not real precise as im only using it to turn
    on a bilge pump on a boat.I guess i`ll have to put a smoothing
    capacitor in there somewhere to.cheers,

  7. Hmmmm,

    Maybe this, together with one or more Johnson counters (4017) so that a
    higher frequency cap can be used? Also, maybe still higher values of
    R1 and R2 could be used (4.3 and 6.3 MOhm) with a 100 uF cap?

  8. John Fields

    John Fields Guest

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