[sdiy] Issue with CD4024 Ripple Counter

David G Dixon dixon at mail.ubc.ca
Mon Sep 20 19:18:39 CEST 2021

Hi Team,

Thanks for all of the responses.  I really do appreciate it!

So, this little "project" has followed my normal modus operandi -- I design
something quickly, test it, it doesn't work, I lash out, ask for help, get
all angry -- then, at about 1:00 am, I sit down and have a fresh look and
try to get it right.

Lastnight I did just that.  You will find the results of my sober second
thought attached.  I decided that the problem is a combination of stupid
stuff.  So, the first thing I did was to disconnect the 4024 CLOCK pin from
everything downstream.  All that stuff can be driven in parallel from the
comparator, which has very low output impedance so won't "feel" any of it.
Then, I turned my attention to the comparator.  I halved the hysteresis
resistor from 1M to 499k and added a speed-up capacitor (between + and Out
-- I've not worked with comparators all that much, so I had done this wrong
before).  Simulation suggested that the 100pF capacitor would sharpen and
accelerate the transitions to a nearly perfect straight line with a
transition time of about 1 microsecond, so the comparator should now be
perfectly conditioned to drive the CLOCK.

Next, I turned my attention to the gate output circuit.  This one was really
stupid.  I realized that all I needed was an inverting half-wave rectifier
at about 50% gain to convert the 10V comparator output to a 5V gate output.
In simulation, this works perfectly, and I've built enough rectifiers to
know that it is pretty bulletproof.

I also changed the layout of the offending PCB lastnight, so this morning
I'm going to print, transfer, etch, drill, tin, stuff, and test the new
board.  It's about 10:15 now -- I should have it done by 12:00, and I'll let
y'all know how it worked.

-----Original Message-----
From: Roman Sowa [mailto:modular at go2.pl] 
Sent: Monday, September 20, 2021 1:34 AM
To: synth-diy at synth-diy.org; dixon at mail.ubc.ca
Subject: Re: [sdiy] Issue with CD4024 Ripple Counter

[CAUTION: Non-UBC Email]

The 100k resistor after the diode does not do much in here. The comparator
output load is determined in 80% by 10k to zener and then 10k to inverter,
so you may just remove it.
Next, since comparator output goes to about 10V, then the zener also does
not do anything much, because 2x10k divider gives about 5V at zeners
cathode. So if you replace the zener and 2x 10k with just one 2k resistor
the "clockout" will work the same.
But why 4024 responds to positive edge at all? My only idea at this moment
would be oscillations of opamp used as comparator caused by parasitics
transmitting output fast jump to inverting input, that was already pointed
by someone. Nothing obvious here.

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