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555 & 556 timers

  1. Jun 8, 2008 #1
    I'm planning on using a 556 timer for a 3-way solenoid valve. I need to have both valves closed for a period of 10 seconds, then open the first valve for 20 ms, followed by the opening of the second valve for 20 ms. This is astable. Is it possible to line up the timing using a 556 timer or do I have to use two 555 timers? How do I get the second valve to start its timing sequence 20 ms after the first?
  2. jcsd
  3. Jun 8, 2008 #2


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    Staff: Mentor

    555 timers are not very accurate. It would be better to use a stable time base (crystal oscillator) and divide down your clocks. You could base it on the 74HC4060, for example, and use a 32kHz watch crystal for the time base. Have you considered doing it that way?

  4. Jun 8, 2008 #3
    No, I've never heard of this. I'm new to electronics and was hoping to use the 555 timer because I understood it.
  5. Jun 9, 2008 #4
    A 556 timer is just two 555's in a single package. (I think you can get 4 in a package in a 558). Each are independent. So you would use one to create the 10 sec delay as a one-shot. Then use the output as the trigger for the second to create the 20msec delay. Then use the output of the second to trigger a 3rd for the second valve.

    As discussed above, there are many, many ways to accomplish this. It all depends on what you are trying to "optimize". For example, you could just use one of the 555's as a 20msec clock. Then use other logic to switch the desired pulse to the right valve driver.

    But, if the timing isn't overly critical and package count, etc. aren't critical, 555's are a straightforward way to go, and will interface pretty easily with your valve driver. They have a wide Supply Voltage range, so you can probably power them with the same supply as the valve driver (no need for separate 5Volt supply, etc.).

    For my part, I'm partial to the CD4538 which is a dual timer, even easier to use than the 555, and cheaper.
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