Time Delay Circuit

Discussion in 'The Projects Forum' started by burrik, Jul 28, 2011.

  1. burrik

    Thread Starter New Member

    Jul 28, 2011
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    Hello All,

    I've read several posts but I can't seem to find what I'm looking for...

    I currently have a circuit which outputs a 60 Hz pulse wave at a low duty cycle (pulse width is approximately 50us) using a 555 timer. I would like to wire this output to two seperate circuits that will allow me to maintain its characteristics and delay it at different times (both delays would be on the order of microseconds). I would like to accomplish this using two additional 555 timers for the time delays if possible.

    Please help! Any input is greatly appreciated! Thanks!
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Do you happen to know what the shape of the wave form is?
     
  3. burrik

    Thread Starter New Member

    Jul 28, 2011
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    It's a square wave.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    So in other words, your pulse repetition time is 16.667mS, with a pulse of 50uS.

    Would you care to post that circuit so that we have a baseline to start from?

    .PNG format image files are preferred, as they are compact, need no software besides a browser to view, and are not "lossy" like .jpg images are. Use the "Go Advanced" button on the bottom, then "Manage Attachments" to upload your image.

    Well, if you're going the 555 route, it seems that you will actually need two one-shot timers to follow the output of your original timer.

    The 1st one shot would be the delay from your original. The 2nd one shot would output the 50uS pulse.

    Bill Bowden put up an example of a delayed pulse timer on his website:
    http://www.bowdenshobbycircuits.info/page9.htm
    Scroll all the way down; 2nd to the last on that page - or just search for "Generating a Delayed Pulse Using The 555 Timer".

    The pushbutton switch on the left could be replace by a transistor or MOSFET driven by the output of your existing timer circuit.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    I don't know how much or how little you'll want to delay the start of the pulse, but if you want to be able to have very small delays, you won't have much of an adjustment range, as that'll make the size of the 1st stage timing cap rather critically small.
     
  6. burrik

    Thread Starter New Member

    Jul 28, 2011
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    SgtWookie, thank you for your help. Bill Bowden's example looks like it might work but I wouldn't be able to use a switch to trigger the delay. My delay will have to be fairly precise. The idea I've been working with consists of an astable 555 that will output at the frequency of my desired delay (pulse width of 3us) and trigger a monostable 555 that will have a very long pulse width. This will enter an AND gate along with the original 60 Hz output I am trying to delay which would hopefully delay it and then output it for the duration of the "very long pulse width." Not sure if that would work/make sense, but here's a picture of what I ultimately want to do. Hopefully it's understandable.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Well, I've come up with some mods of Bill Bowden's circuit which look like they have a chance of working - in simulation, anyway.

    Have a look at the attached.

    I haven't shown larger aluminum electrolytic caps nearby, but since your timing is so particular, you'll really need to use care in ensuring that you have proper bypassing of all IC's power/ground pins with 0.1uF/100nF caps.

    The 555 timer causes a "glitch" on the power supply buses when it toggles states; it shorts the two together for an instant. Unless you have enough bypass caps near to it, that will upset other circuitry in the neighborhood.

    Both of the delayed 555s will be very susceptible to false triggering due to noise, and these delayed 555s may cause problems with the original circuit you have unless it has good bypass caps, too.

    This is one of those "prime candidates" for a microcontroller. An 8-pin PIC12F683 running at 20MHz would handle delays with not much effort.
     
  8. burrik

    Thread Starter New Member

    Jul 28, 2011
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    Yikes. A little more difficult than a thought. I'll see what I can do with this but it looks like a microcontroller might be the way to go. Thanks for your help. It is much appreciated.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Also, you'd need two of the circuits I posted, but with R3 adjusted to suit the delay required. If the delayed pulsewidth is not correct, you can adjust R6 or R7.

    Except for component values, it's much the same as Bill Bowden's circuit. I had to add pull-up resistors on the outputs of the 555 timers to help their rise times and maximum Vout, R8 and Q1 were necessary to connect your original 555 circuit as the trigger. Everything else was "tuning" to get the delays and pulse widths close to what they needed to be.

    There may be easier ways to do this. I didn't know what you need in the way of output source/sink current or voltage, either.
     
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