[sdiy] Multivibrator IC questions...

[john mahoney]

One way to get a variable length pulse is to patch an envelope into a comparator. The envelope should have the shortest possible attack, unless you want a delay. (Hint, hint! Trigger delays are cool, too!) The comparator output will be your variable length pulse, either "off" or "on."

The envelope can come from an ADSR, AD, AR, or a simple one shot monostable. A multivibrator is one way (?) to make a one shot, but it seems to me that you still need a comparator (made with an op amp or something like an LM311).


Another way to do this is with a circuit that does something like this:
    Main ingredients: flip-flop, capacitor, a pot, and a comparator.
    When triggered, discharge the timing cap and SET the flip-flop.
    While the flip-flop is SET, send a HIGH signal to the output of the module -- that's your output pulse.
    Use that same HIGH signal to charge a cap through a variable resistance (the pot).
    When the cap reaches the comparator's threshold, the flip-flop is RESET and the pulse goes off.
It's similar to part of a sawtooth core oscillator. Discharging the timing cap is like SYNCing the oscillator.


By the way, that chip (74VHC123) can't take 12V or 15V power. There must be a CMOS chip that'll do what you want. (If only I could name one... ;-)
--
john <--(still working on the Soddy voting stuff)



  ----- Original Message ----- 
  From: James R. Coplin 
  To: 'Synth-DIY' 
  Sent: Wednesday, February 02, 2005 2:46 PM
  Subject: [sdiy] Multivibrator IC questions...


  I'm looking into making a circuit that takes a trigger or gate as input and outputs a modified pulse length.  I was looking at some multivibrator ICs like:

   

  http://www.fairchildsemi.com/ds/74/74VHC123A.pdf

   

  My problem is that I often seem to be stuck with pulses, triggers, clocks etc. that I need to transform into shorter or longer bits for control of envelopes, sequencers, etc.  The multivibrator seemed the easiest approach to have a user assignable gate length based on a triggered input.  The pulse time in seconds is PW=Rx*Cx where Rx is ohms and Cx is farads.  There is no limit on Cx but Rx is limited to 1k.  So I thought by using a 1k push-pull pot, you could select two different caps and have a short and long time control. 

   

  So, there are a kazillion of these chips out there.  Is there really any difference or would the above one work out fine?  They come two to a chip so for a couple of ICs I would get 4 independent processing thingees.  Are there other approaches I should be considering, pitfalls etc.? Thanks!

   

  James R. Coplin

  ***************

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  simply tell them i've gone out the window

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  expected back any time soon.

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