[sdiy] Issue with CD4024 Ripple Counter

John Richetta jrichetta at earthlink.net
Mon Sep 20 02:07:28 CEST 2021


Some suggestions:

1. This circuit has a bit of complexity; simplify, to help narrow down the source of trouble. I’d suggest disconnecting the 10k resistor that feeds the circuit following the cd4024 clock output, and then perhaps also fully disconnecting the circuitry that follows from power (though my guess is that if this circuit is partly to blame, merely disconnecting it from the 4024 clock will improve the situation). The thinking is that any oscillatory behavior in this circuit could well feed backward through the 10k resistor sufficiently to affect clocking of the cd4024.

2. The power supply-derived reference voltage for your comparator has no bypassing, and certainly should.

3. I won’t lecture on using op amps as voltage comparators (I assume you’ve read the app notes about this), but will say that the circuit should undoubtedly have some hysteresis. As it is operated open loop, it will be maximally susceptible to noise (such as the unfiltered supply noise mentioned in point 2) during transitions. Lack of hysteresis is a common cause of problems with voltage comparator circuits. Occasionally, excessive hysteresis can be undesirable, but I doubt there will be any issue with it in this application (the main likely effect being ultra small clock edge delay).

I strongly doubt that problems as basic as you are having are due to manufacturer differences. A quick check of a 1979 RCA manual shows that their cd4024 versions also have hysteresis. The edges you are feeding the cd4024 may have spurious oscillations of a large enough magnitude to generate additional clocking when going through the transition zone (those extra transitions will often swing up and down a few or a dozen times, and so will clock any input, regardless of edge polarity).

HTH, -jar

> On Sep 19, 2021, at 01:29, David G Dixon via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> The ASR clock circuit is attached to this email.
> 
> The two switches are how I am simulating the ON-OFF-(ON) switch.  When
> switch "1" is open, then the switch is in the OFF position.  When switch "1"
> is closed, then switch "0" represents the momentary switch.  This switch
> sends either 0V or 12V through a 1k resistor to the 2.2n debouncing cap.
> The signal generator represents a signal plugged into the Clock input jack
> (such as an LFO).  This comes in through a 10k resistor.  In this way, the
> switch always has priority.  They are both fed through 10k into the
> inverting input of the opamp comparator with a 100k resistor to ground to
> provide a current path.  The threshold of the comparator is set to a little
> less than 2V.  Hence, when the LFO signal rises above this threshold, or
> when the momentary switch is pressed, the comparator output is negative.
> The diode changes this to about 0V.  When the clock signal is less than
> about 2V or the momentary switch is not pressed, then the comparator output
> is positive.
> 
> The output of the diode goes directly to the 4024 Clock pin.  This will be
> either about 10V or about 0V, and the transitions will be about 2
> microseconds.  This is dropped to ground through a 100k resistor to provide
> a current path.  This then goes through a 10k resistor to a 5.1V zener to
> fix the level at 5V, then into a unity gain inverter with a +5V level shift
> (-12V through 24k) to give a 5V gate signal for the Gate output jack.  This
> signal will have the same polarity as the incoming clock signal, even though
> the 4024 advances on the negative edge.  This output can be used to gate an
> envelope for a filter, or whatever.
> 
> If you see any obvious mistakes, I'd love to know about it.  As I said, this
> circuit works perfectly with MC14024B, but CD4024B clocks on the wrong edge.
> 
> 
> 
> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of m
> brandenberg
> Sent: Saturday, September 18, 2021 12:04 PM
> To: 'synth-diy mailing list'
> Subject: Re: [sdiy] Issue with CD4024 Ripple Counter
> 
> [CAUTION: Non-UBC Email]
> 
>> On Sat, 18 Sep 2021, David G Dixon via Synth-diy wrote:
>> 
>> Also, I would remind you that I set up a test rig on breadboard, with 
>> similar circuitry (an opamp comparator into a diode), and the 4024s 
>> either didn't work or looked real bad (lots of hash) even though the 
>> driving waveform is very clean.
> 
> Sorry to belabor the point but the 'one diode' mention has made me twitch
> twice now.  Is that one diode feeding into a passively pulled up (or down)
> input or one diode into a floating node that might be drifting around
> metastability?
> 
> 
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