It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
Thanks for reading,
bob prohaska
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
Thanks for reading,
bob prohaska
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
Thanks for reading,
bob prohaska
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
On 30/07/2026 12:43 am, bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
Really? I suspect that you are suffering from a misapprehension.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
The first question is, "what do your mean by a 10V AC signal"?
If you mean 10V rms, it is a roughly a +/-14.1V peak to peal sine wave.
The next is, what make you think you can make a passive frequency
divider with diodes, resistors and capacitors?
You can make a frequency doubler - a full bridge rectifier could pull
out a raised half sine wave with about half the amplitude at 200Hz, and
you could double that again to 400 Hz. Schemes that pull out a narrow
pulse once per cycle create a waveform that has all the odd harmonics up
to a limit set by the width of the pulse, and you can filter out the
higher harmonic you want.
Frequency dividers depend on counting zero crossings and sending out a signal when you have counted enough. Sending out a pulse on every eighth transition would give you a 12.5Hz clock. 8x8=64 and 4x9=36, so counting
two batches of 8 edges then one batch of nine would give you a jittery approximation to a 12Hz clock - dead accurate in the long term, but
visibly jittery.
That wouldn't be an kind of passive scheme.
There are better ways of doing that - you could use a phase locked loop
to set up a high frequency oscillator that was locked to some multiple
of your 100Hz reference - say 300Hz - and divide that by 25 to get the
12Hz you want without the jitter.
The usual question at the point is "what are you actually trying to do".
It usually turns out that there is a well-established solution to the problem that you actually need to solve, and that most of the people who post here have known about it for decades.
If anybody cares, the motive was to use a 14 pulse per
revolution signal to drive a tachometer (speedometer,
actually) that wants about 1 pulse per revolution input.
Bill Sloman <bill.sloman@ieee.org> wrote:
On 30/07/2026 12:43 am, bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
Really? I suspect that you are suffering from a misapprehension.
It wouldn't be the first time 8-)
....
Ahh, I see your point. I was misguided......
Counting down requires discarding pulses, which
implies something akin to memory of which pulse
to discard. That takes latches, which need power.
If anybody cares, the motive was to use a 14 pulse per
revolution signal to drive a tachometer (speedometer,
actually) that wants about 1 pulse per revolution input.
Thanks to all who replied, and apologies for the noise.
For some reason I thought that if diodes could double
frequency, the right topology could halve it too....
bob prohaska
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
The first question is, "what do your mean by a 10V AC signal"?
If you mean 10V rms, it is a roughly a +/-14.1V peak to peal sine wave.
The next is, what make you think you can make a passive frequency
divider with diodes, resistors and capacitors?
You can make a frequency doubler - a full bridge rectifier could pull
out a raised half sine wave with about half the amplitude at 200Hz, and
you could double that again to 400 Hz. Schemes that pull out a narrow
pulse once per cycle create a waveform that has all the odd harmonics up
to a limit set by the width of the pulse, and you can filter out the
higher harmonic you want.
Frequency dividers depend on counting zero crossings and sending out a
signal when you have counted enough. Sending out a pulse on every eighth
transition would give you a 12.5Hz clock. 8x8=64 and 4x9=36, so counting
two batches of 8 edges then one batch of nine would give you a jittery
approximation to a 12Hz clock - dead accurate in the long term, but
visibly jittery.
That wouldn't be an kind of passive scheme.
There are better ways of doing that - you could use a phase locked loop
to set up a high frequency oscillator that was locked to some multiple
of your 100Hz reference - say 300Hz - and divide that by 25 to get the
12Hz you want without the jitter.
The usual question at the point is "what are you actually trying to do".
It usually turns out that there is a well-established solution to the
problem that you actually need to solve, and that most of the people who
post here have known about it for decades.
Bill Sloman <bill.sloman@ieee.org> wrote:
On 30/07/2026 12:43 am, bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
Really? I suspect that you are suffering from a misapprehension.
It wouldn't be the first time 8-)
....
Ahh, I see your point. I was misguided......
Counting down requires discarding pulses, which
implies something akin to memory of which pulse
to discard. That takes latches, which need power.
If anybody cares, the motive was to use a 14 pulse per
revolution signal to drive a tachometer (speedometer,
actually) that wants about 1 pulse per revolution input.
Thanks to all who replied, and apologies for the noise.
For some reason I thought that if diodes could double
frequency, the right topology could halve it too....
bob prohaska
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
The first question is, "what do your mean by a 10V AC signal"?
If you mean 10V rms, it is a roughly a +/-14.1V peak to peal sine wave.
The next is, what make you think you can make a passive frequency
divider with diodes, resistors and capacitors?
You can make a frequency doubler - a full bridge rectifier could pull
out a raised half sine wave with about half the amplitude at 200Hz, and
you could double that again to 400 Hz. Schemes that pull out a narrow
pulse once per cycle create a waveform that has all the odd harmonics up
to a limit set by the width of the pulse, and you can filter out the
higher harmonic you want.
Frequency dividers depend on counting zero crossings and sending out a
signal when you have counted enough. Sending out a pulse on every eighth
transition would give you a 12.5Hz clock. 8x8=64 and 4x9=36, so counting
two batches of 8 edges then one batch of nine would give you a jittery
approximation to a 12Hz clock - dead accurate in the long term, but
visibly jittery.
That wouldn't be an kind of passive scheme.
There are better ways of doing that - you could use a phase locked loop
to set up a high frequency oscillator that was locked to some multiple
of your 100Hz reference - say 300Hz - and divide that by 25 to get the
12Hz you want without the jitter.
The usual question at the point is "what are you actually trying to do".
It usually turns out that there is a well-established solution to the
problem that you actually need to solve, and that most of the people who
post here have known about it for decades.
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
On Wed, 29 Jul 2026 17:20:50 -0000 (UTC), bp@www.zefox.net wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
On 30/07/2026 12:43 am, bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
Really? I suspect that you are suffering from a misapprehension.
It wouldn't be the first time 8-)
Sloman has declared that you are ignorant. Apologize, repent, say 14
Hail Marys, and sin no more.
A barefoot pilgrimage to New Jersey wouldn't hurt either.
On 30/07/2026 3:39 am, john larkin wrote:
A barefoot pilgrimage to New Jersey wouldn't hurt either.
Why New Jersey?
And why barefoot?
Bill Sloman <bill.sloman@ieee.org> wrote:
On 30/07/2026 3:39 am, john larkin wrote:
A barefoot pilgrimage to New Jersey wouldn't hurt either.
Why New Jersey?
It's a standard joke in the USA 8-)
And why barefoot?
For emphasis ;-)
Sorry you didn't get it!
bob prohaska
<bp@www.zefox.net> wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:You can make parametric frequency dividers in a couple of ways that I know of.
On 30/07/2026 12:43 am, bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
Really? I suspect that you are suffering from a misapprehension.
It wouldn't be the first time 8-)
....
Ahh, I see your point. I was misguided......
Counting down requires discarding pulses, which
implies something akin to memory of which pulse
to discard. That takes latches, which need power.
If anybody cares, the motive was to use a 14 pulse per
revolution signal to drive a tachometer (speedometer,
actually) that wants about 1 pulse per revolution input.
Thanks to all who replied, and apologies for the noise.
For some reason I thought that if diodes could double
frequency, the right topology could halve it too....
bob prohaska
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers. >>>> If there are better search terms please point them out!
The first question is, "what do your mean by a 10V AC signal"?
If you mean 10V rms, it is a roughly a +/-14.1V peak to peal sine wave.
The next is, what make you think you can make a passive frequency
divider with diodes, resistors and capacitors?
You can make a frequency doubler - a full bridge rectifier could pull
out a raised half sine wave with about half the amplitude at 200Hz, and
you could double that again to 400 Hz. Schemes that pull out a narrow
pulse once per cycle create a waveform that has all the odd harmonics up >>> to a limit set by the width of the pulse, and you can filter out the
higher harmonic you want.
Frequency dividers depend on counting zero crossings and sending out a
signal when you have counted enough. Sending out a pulse on every eighth >>> transition would give you a 12.5Hz clock. 8x8=64 and 4x9=36, so counting >>> two batches of 8 edges then one batch of nine would give you a jittery
approximation to a 12Hz clock - dead accurate in the long term, but
visibly jittery.
That wouldn't be an kind of passive scheme.
There are better ways of doing that - you could use a phase locked loop
to set up a high frequency oscillator that was locked to some multiple
of your 100Hz reference - say 300Hz - and divide that by 25 to get the
12Hz you want without the jitter.
The usual question at the point is "what are you actually trying to do". >>> It usually turns out that there is a well-established solution to the
problem that you actually need to solve, and that most of the people who >>> post here have known about it for decades.
Varactors can do it, especially if you forward-bias them on the positive peaks. You?d need a honking big one for 100 Hz.
Alternatively it can be done with nonlinear magnetics. The 20 Hz telephone ring signal was originally generated by a divide-by-3 mag amp circuit
running from 60 Hz mains.
I second the flea power CMOS suggestion.
joegwinn@comcast.netwrote:
On Wed, 29 Jul 2026 14:43:53 -0000 (UTC), bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
Thanks for reading,
bob prohaska
Are a few discrete transistors OK? What I've done to divide by a
small integer is to charge a film capacitor with a constant amount of
charge per input pulse, plus a comparitor that dumps the charge when
the foltage indicating that N-1 pulses have been exceeded.
Joe
joegwinn@comcast.netwrote:
On Wed, 29 Jul 2026 14:43:53 -0000 (UTC), bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
Thanks for reading,
bob prohaska
Are a few discrete transistors OK? What I've done to divide by a
small integer is to charge a film capacitor with a constant amount of
charge per input pulse, plus a comparitor that dumps the charge when
the foltage indicating that N-1 pulses have been exceeded.
Joe
I like that!
You could use a single UJT (Unijunction transistor) where your pulses charge the C
and it will generate an output pulse if the C voltage gets high enough :-)
I still use UJTs in some projects, ebay has those..
On Wed, 29 Jul 2026 14:43:53 -0000 (UTC), bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers.
If there are better search terms please point them out!
Thanks for reading,
bob prohaska
I think it's (barely) posible with a varicap.
I posted a circut here once but someone disputed that it actually
works. But it had inductors too. That wasn't resolved.
On 30/07/2026 3:46 am, Phil Hobbs wrote:
<bp@www.zefox.net> wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:You can make parametric frequency dividers in a couple of ways that I
On 30/07/2026 12:43 am, bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
Really? I suspect that you are suffering from a misapprehension.
It wouldn't be the first time 8-)
....
Ahh, I see your point. I was misguided......
Counting down requires discarding pulses, which
implies something akin to memory of which pulse
to discard. That takes latches, which need power.
If anybody cares, the motive was to use a 14 pulse per
revolution signal to drive a tachometer (speedometer,
actually) that wants about 1 pulse per revolution input.
Thanks to all who replied, and apologies for the noise.
For some reason I thought that if diodes could double
frequency, the right topology could halve it too....
bob prohaska
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers. >>>>> If there are better search terms please point them out!
The first question is, "what do your mean by a 10V AC signal"?
If you mean 10V rms, it is a roughly a +/-14.1V peak to peal sine wave. >>>>
The next is, what make you think you can make a passive frequency
divider with diodes, resistors and capacitors?
You can make a frequency doubler - a full bridge rectifier could pull
out a raised half sine wave with about half the amplitude at 200Hz, and >>>> you could double that again to 400 Hz. Schemes that pull out a narrow
pulse once per cycle create a waveform that has all the odd
harmonics up
to a limit set by the width of the pulse, and you can filter out the
higher harmonic you want.
Frequency dividers depend on counting zero crossings and sending out a >>>> signal when you have counted enough. Sending out a pulse on every
eighth
transition would give you a 12.5Hz clock. 8x8=64 and 4x9=36, so
counting
two batches of 8 edges then one batch of nine would give you aÿ jittery >>>> approximation to a 12Hz clock - dead accurate in the long term, but
visibly jittery.
That wouldn't be an kind of passive scheme.
There are better ways of doing that - you could use a phase locked loop >>>> to set up a high frequency oscillator that was locked to some multiple >>>> of your 100Hz reference - say 300Hz - and divide that by 25 to get the >>>> 12Hz you want without the jitter.
The usual question at the point is "what are you actually trying to
do".
It usually turns out that there is a well-established solution to the
problem that you actually need to solve, and that most of the people
who
post here have known about it for decades.
know
of.
Varactors can do it, especially if you forward-bias them on the positive
peaks.ÿ You?d need a honking big one for 100 Hz.
Alternatively it can be done with nonlinear magnetics. The 20 Hz
telephone
ring signal was originally generated by a divide-by-3 mag amp circuit
running from 60 Hz mains.
I second the flea power CMOS suggestion.
It's a good one. We had a thread - many years ago - about a 5V 100kHz
low power oscillator that drew less than 10uA. It's a pretty silly idea.
We ended up with two solutions, one using an LC oscillator which stored
most of the charge needed to get the switching transistor from on to off
and the other which just used two 5GHz transistors because their collector-emitter capacitance is really low - about 0.2pF. Both came out
at about 3uA.
CMOS transistors tend to come in at about 10pF drain to source,and you'd need four of the them to divide down from 14 Hz to 1 Hz.
10pF over 10V is 100 pC or about 14nAÿ at "14"Hz. The next stage uses 7nA, the next 3.5nA and the last one about 2nA for a total of about 30nA.
That's about 1 ampere.hour per year or a Lithium iron phosphate 9V PP3
cell (typically 1200mA.hour). They last for about ten years in storage.
If you built your own logic with discrete transistor chosen for low collector/emitter or drain/source capacitance, and ran it at a low
voltage you could do better.
joegwinn@comcast.netwrote:
On Wed, 29 Jul 2026 14:43:53 -0000 (UTC), bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers. >>>If there are better search terms please point them out!
Thanks for reading,
bob prohaska
Are a few discrete transistors OK? What I've done to divide by a
small integer is to charge a film capacitor with a constant amount of >>charge per input pulse, plus a comparitor that dumps the charge when
the voltage indicating that N-1 pulses have been exceeded.
Joe
I like that!
You could use a single UJT (Unijunction transistor) where your pulses charge the C
and it will generate an output pulse if the C voltage gets high enough :-)
I still use UJTs in some projects, ebay has those..
That could work. I also have some left over from the 1970s to 1980s.
piglet <erichpwagner@hotmail.com>wrote:
Jan Panteltje <alien@comet.invalid> wrote:
joegwinn@comcast.netwrote:
On Wed, 29 Jul 2026 14:43:53 -0000 (UTC), bp@www.zefox.net wrote:
It's possible to accurately divide the frequency of an ac signal
by powers of two using just diodes, resistors and capacitors.
I'd like to divide a roughly 100 Hz 10 volt AC signal down to
around 12 hz, so it'd take a three stage cascade.
Is anyone aware of a circuit design example for this sort of
thing? Only a volt or so is needed at the output, to operate
a tachometer input. I know it'll be lossy, but am unsure just
how lossy. It seems likely someone's answered this question
already.
Web searches for "passive frequency divider" return a multitude
of hits, but they're either active circuits or loudspeaker crossovers. >>>> If there are better search terms please point them out!
Thanks for reading,
bob prohaska
Are a few discrete transistors OK? What I've done to divide by a
small integer is to charge a film capacitor with a constant amount of
charge per input pulse, plus a comparitor that dumps the charge when
the foltage indicating that N-1 pulses have been exceeded.
Joe
I like that!
You could use a single UJT (Unijunction transistor) where your pulses charge the C
and it will generate an output pulse if the C voltage gets high enough :-) >>
I still use UJTs in some projects, ebay has those..
Yep, that actually was how some frequency dividers worked 60-70 years ago!
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