What a dingus! - CD4024BE

ive got one of these in my modular which is a key to almost every pathc i make.

i was just hunting about for schematics cos im about to stick one into my 80s yamaha pc100 keyboard to divide down the main clock. its all internal so i dont need as much buffering to the outside which is nice.

i dont need reset either but i just want to know: what the hell is up with that crazy reset circuitry?


I really can’t explain that sorry, but surely someone else here :slight_smile:

If i remember correctly the logic high levels are dependent on the supply voltage.

My guess is the reset circuitry is there to accommodate for gate voltages lower than Vdd.
It’s just two cascaded inverting stages, biased by the the other transistor.

Also the rise time will be faster with higher input levels

I have no idea what’s up with that bias circuit, I haven’t seen anything like it before.

Here’s another 4024 based divider whose reset input is conditioned by two transistor stages, but without any such bias circuit.


(Does it really need four diodes for reset though?)

Never mind, I went back and looked at Rönnberg’s page (Niklas R�nnberg's homepage - Audio DIY) and it says

It’s original design was made by Mathias Herrmann for his inspiring Fonitronik. However, his design gives inverted outputs, which I changed to normalized outputs. Also I added a start-up reset pulse, to ensure that all outputs are low when the modular is started.

So the “bias circuit” is actually that startup pulse: When power is applied, the center left transistor turns on and kicks the reset input. The rest is a pair of inverting buffers to condition the input pulses, as said.

Fonitronik’s design: http://www.modular.fonik.de/pdf/SimpleDividerSCH.pdf


Oh wow that’s pretty cool, i didn’t think of it but it makes sense because it’s an npn that would go low when the capacitor on the base charges.

In fact, it’s so clever it might be worth adding to a lot of other circuits :slight_smile:

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Nice. I also get that and like tge benefit in some circumstances. I tried to look it up but it was down when I went there…

Rönnberg’s site looks to be permanently dead; above I linked to an Internet Archive copy.

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