Cascading binary counter ICs

AFAICR: you can pre load a 7490 to divide by 3, which is basic to the chronology.

There is now a dual BCD counter, something like 74290 - which should be available as HC or HCT.

Not too sure if you still get all the pre load pins, but you can gate for premature reset anyway.

The underlying logic is what it is - you can construct it with a couple of dividers and some glue logic.

No problem with the ripple counters themselves. Though depending on your application a problem may be faced with the propogation delays of the many stages causing glitches to appear at the decoded output.

The delay between gates changing state after the clock pulse means that there will be periods (of a length in the order of microseconds) where the outputs match the required count in error, because each one changes before the next.

If the output is to an LED or similar, this won't matter, but if it is to another logic input, it may be advisable to use an RC filter on the output of the AND gate decoding the counter state, along with a schmitt buffer on the filter output.

If you wanted to count to around 9hrs and 27hrs, you could use a single 4521 on the 8Hz output of the first 4020 to output

9hrs on its Q18 output, and 27hrs with both Q18 and Q19 HIGH. It might be possible to do 8 and 24hrs with an input clock higher than 8Hz. 64Hz might work, AND Q18, Q19 and Q20 for 8hrs... Q18, Q20 and Q22 for 24hrs? You'd better check for yourself.

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Very helpful, CN Kev. No CD4521 in stock but I've ordered a few for future exploration. You gave me one good tip which was taking 8Hz from the first CD4020 to clock a CD4521. I suddenly realized that I could pick a higher frequency off the first CD4020 to clock a second CD4020 so its outputs Q1 to Q14 have periods that can be decoded to give me 8rs and 24 hrs.Some quick scratching with pencil and paper suggests that it can be done. That will reduce my CD4020's to two. I need to have a 1Hz clock tick too but that can be achieved by dividing the 2Hz at Q14 of the first CD4020 with half of a dual flipflop. That increases the chip-count by one again but I'm not constrained much by PCB space or power budget. I note your cautions about glitches caused by the propagation delay. Maybe I'll be able to see any glitches with my Rigol DSO and if there are any I can follow your suggestion of RC filter and Schmitt buffer. My "design", if you can call it that, features a hex inverter chip that will use one gate to take the low at the 24 hour mark and apply a high to all the reset pins of the counters. I guess I can select a Schmitt type hex inverter. I could use two of the five spare inverter gates to make non inverting Schmitt buffers if required. Much thanks. Peter

I'd have suggested a CD4060 for the first counter, ans they can connect directly to the crystal.

all the from-scratch CMOS clock designs I've encountered used divide by five and divide by three stages in addition to the divide by two stages. (some used divide by 6 and divide by 10 stages)

to gwt an 8-hour periodic signal from 2hZ you need to count to 57600

which is 256 * 225

225 is 128 + 64 + 32 + 1

So, and output 9 snd 14 of the second 4020 with outputs 1 and 2 of the third and feed that back to the reset pins of those two counters.

output 2 of the third of will have a high to low transition after 8 hours from reset (it spends most of its time low)

invert that* and use it to clock a divide by 3 counter to get the 24 hour count.

(*) or use a counter with falling edge triggering

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No, because they are ripple copunters and only count upwards, and changes (carries) propogate leftwards (by place value) they never will: all the glitch states are lower than the correct count. ie "missing ones" and so will never trigger AND logic early

synchronous counters on the other hand can lead to glitches depending on which wire has the most capacitance etc...

say you're looking for the 225 (11100001 binary) you only need to look for the ones, the zeros can be don't care because the first match for that pattern (111xxxx1) comes at count 225

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I don't suppose that this helps:

(Bigclive talking logic and timers for lights...)

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Shaun. 

"Humans will have advanced a long, long way when religious belief has a cozy  
little classification in the DSM*." 
David Melville (in r.a.s.f1) 
(*Diagnostic and Statistical Manual of Mental Disorders)

You should be able to cascade them indefinitely, even to the point where the pulse length exceeds the MTBF of the device. Just don't forget to include enough power decoupling.

Sylvia.

Yes that helped. Only published yesterday too. Thanks! Big Clive is, I think, quite a clever bloke but he talks to people like me in language I can understand. Without actually talking down.

So given enough 4020s I could trigger a doomsday device 5000 years from now? Just have to leave a large enough bank deposit to pay generations unborn to replace the solar panel every 30 years and the battery every three years. For 5000 years. Assuming the present financial system, the thermonuclear doomsday package and humanity last that long. Puts my little project in perspective.

Quite right. The warning I was thinking of was only for the case of inverted counter outputs.

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The 4040 is active HIGH and sets all the outputs LOW, so wouldn't this cause the reset signal to be locked on? To simply reset as soon as 24hrs is reached, you should be able to connect the HIGH-going output of the

24hrs gate straight to the counter reset inputs.

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Sort of like a digital version of this:

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Yes, except that I suspect the MTBF will get you.

Sylvia

I'm pleased that it helped.

Yes, agreed. Big Clive is one of the few youtube channels that I subscribe to that I've enabled notifications for so that I see his videos the day he uploads them. I watch them all, even if they're not something I'm presently interested in as he often goes off on tangents and gives good information. :-)

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Shaun. 

"Humans will have advanced a long, long way when religious belief has a cozy  
little classification in the DSM*." 
David Melville (in r.a.s.f1) 
(*Diagnostic and Statistical Manual of Mental Disorders)

That's pretty cool. It would easilly have enough power to break the concrete although the gears will shear their teeth long before that happens. It's a shame he didn't go up in gear size / strength further away from the motor.

In fact it would be interesting to make a (less reduction) gear train of increasingly-strong materials with the first gears being made of light plastic or similar and the final one lifting or perhaps cutting through an anvil...

--
Shaun. 

"Humans will have advanced a long, long way when religious belief has a cozy  
little classification in the DSM*." 
David Melville (in r.a.s.f1) 
(*Diagnostic and Statistical Manual of Mental Disorders)

the first worm gear will be worn smooth before the last gear tuns a degree.

using lightweight materials would only make that worse.

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I was talking about a shorter gear run with less overall reduction (and perhaps using a smidgeon of high MoO2 open-gear grease).

Yep. Unless they were harder lightweight materials (think PVD titanium dioxide over magnesium alloy) and also 'lighter' in size and scale early in the train. (Also it was 'thinking out loud' in the spirit of the 'art' mentioned above.)

--
Shaun. 

"Humans will have advanced a long, long way when religious belief has a cozy  
little classification in the DSM*." 
David Melville (in r.a.s.f1) 
(*Diagnostic and Statistical Manual of Mental Disorders)