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555 Timer Help

  1. Oct 7, 2006 #1
    I need to make a timing circuit that will count 30 seconds and give a high output. I was going to use a monostable 555 with 90k and 10uf as my values. Using the formula I am getting a time of 1 sec. I use the program "circuitmaker" to make the circuit (as shown in the pic) but the output it shows this making is not a nice square wave that I can use to make a d-flip flop count. If anyone has made a counting circuit that is simaliar can you please send me a pic or advice on how to revise my circuit so that it will work. Also would a ripple counter be better for the counting? Thanks

    Attached Files:

  2. jcsd
  3. Oct 8, 2006 #2
  4. Oct 8, 2006 #3


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    Science Advisor
    Homework Helper

    Just about any pulse that swings below Vin low and and above Vin high for the logic family will triger a d-flip flop.

    As Daivid notes the accuracy of a 555 at 1sec intervals is low.

    Ripple counters are generally better (cheaper) for counting.

    Usually, if you want accurate timing then you start with a crystal oscillator and divide down.
  5. Oct 8, 2006 #4
    Do you guys have any sugestions on a crystal oscillator that I can use. I have never used one.
  6. Oct 8, 2006 #5
    Exactly what are you trying to do? How accurate does it need to be? I have used cmos 555 timers and tantalum capacitors timing way out to almost 10 minutes. They've been in use over 10 years. The accuracy doesn't need to be down to the microsecond, but within 10 to 20 seconds over a wide temperature range. One thing you want to watch is discharging a large capacitor into the 555 with the discharge pin (7). Use a series resistor to limit the current into that pin. Does this project always have access to 120 VAC power? No need for a clock when you have the 60 hertz timebase already.
  7. Oct 16, 2006 #6


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    Staff: Mentor

    I'd suggest that you just use a 74CH4060 chip. It's a combination oscillator plus 14-bit counter, so it's very handy for generating time bases. You should be able to google 74HC4060 application notes to get more info.
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