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From: "Helmut Sennewald"
Subject: Re: Need circuit for: click-------click-click-click...
Date: Sat, 14 Sep 2002 00:48:48 +0200
References: <email@example.com> <3D711283.49CAB0F1@texas.net> <firstname.lastname@example.org>
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"Terry Pinnell" schrieb im Newsbeitrag
> Terry Pinnell wrote:
> >John Fields wrote:
> >>Check alt.binaries.schematics.electronic "Typematic" for a 2 chip
> >>solution. If you need a circuit description post a request here
> >>(s.e.d.) and I'll write one up.
> >I was hoping the OP would take you up on your offer, as I've so far
> >been unable to breadboard it (don't have a 123). Nor have my attempts
> >to simulate it yet been successful. When you have time, I'd appreciate
> >at least a timing diagram please, or failing that just a few comments
> >to break my mental logjam.
> I've still not breadboarded or simulated your circuit, but after
> slogging away for a while I reckon I may have now understood it.
> Please advise if timing diagram at
> is correct.
I have checked your circuit with my simulation and can say it is correct.
The only drawback of the circuit is the chance of a too short pause
between key presses. This problem can be solved by using a
quad Schmitt-Nand(74HC132) instead of the 74HC02 . The circuit
would then no more need a SPDT switch. Instead a simple switch would
be sufficient. Making the SPICE model for the 74HC123 was
a hard work. The simulation was done with SwitcherCadIII which
is free SPICE from Linear Technology www.linear.com .
key press _____| |____| |____
___ ___ ___ ___
output _____| |__________| |___| |_| |__________
too short 0
By the way, when simulating my own circuit with the shift
register, I also found a similar problem. Solving all the
problems I would end up with 4 ICs compared to the 2 ICs
which ar needed for John's circuit.
Finally I recommend the circuit presented by John Field.
It only needs two ICs. Instead of the NOR-gates I would go
with Schmitt-NAND gates to avoid the short pulse problem
and it also allows the use of a simple switch.
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