[sdiy] Multivibrator IC questions...

[Roy J. Tellason]

On Wednesday 02 February 2005 02:46 pm, James R. Coplin wrote:
> 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

This is the first I've head of "74Vxxxx" parts...

> 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!

I don't know if they change the design of these from one series to another,  
but I remember being told some years ago that the '123 (I think he was 
referring to either the original TTL parts or the LS ones) had severe 
problems with timing stability,  and that the '121 was a much better choice.

Why not use a 555 or similar for this application?  You're not limited to 
TTL-type power supply voltages then,  and there's a bazillion different 
circuits out there on the web already.