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Comment by pthreads | Hacker News Reader

I am just curious is it not be possible to use a crystal oscillator and hence not be dependent on the power line for timing? Are they not doing it just on principle i.e using only transistors, resistors, diodes and capacitors? I vaguely remember building an AM transmitter in high school using a crystal oscillator and transistors but no ICs.

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You could use a crystal oscillator, but you'd need a bunch of circuitry to divide the frequency down to 1 Hz. The lowest-frequency crystal I could find is 20kHz. In other words, it's a lot easier to start with 60Hz than 20kHz when you're using individual transistors.

It's not that bad.. typically you use a 32.768kHz ("watch") crystal then divide by 15 to get one second. You can do it with two chips (a bunch of solutions, one is two 7468).

To nit-pick, of course the division is not by 15 (that'd give 2,184.5333... Hz) but by 2 to the power of 15, i.e. 32,768. Typically implemented by a 15-bit counter.

And how many transistors does that translate to?

You need 15 triggers to go from 32768 Hz to 1 Hz. Existing circuit uses 4 triggers to divide 60 Hz by 10 and another 3 to divide by 6. You can repurpose those so you only need 8 extra triggers. Two transistors each means 16.

You can also save 6 transistors from existing dividers because all divisions are by 2. So net total is 10 extra transistors for division, plus at least one in the oscillator itself.

Probably 30, but you dont need existing transistors needed to divide 60hz down to 1hz, so the change in parts count might be smaller.

Edit: Previously said 30, but it looks like its 60. Each JK flip flop takes 4 transistors.

Edit: Back to 30, the bistable circuit in the clock uses two transistors each.

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