[sdiy] Issue with CD4024 Ripple Counter

Roman Sowa modular at go2.pl
Tue Sep 21 10:42:59 CEST 2021

oops, I forgot it has to be inverting buffer
Try now (attached)


W dniu 2021-09-21 o 10:16, Roman Sowa pisze:
> Why rectifier when 3 resistors would be just as good?
> And if you want buffered CLKout, then maybe something like this (attached)?
> Roman
> W dniu 2021-09-20 o 19:18, David G Dixon pisze:
>> 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.
>> Roman
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