[sigstoat]

> 50Hz is within the range of most LFO circuits (Low Frequency Oscillators) of the kind used for modular synthesisers and guitar effect pedals.

i don't think those are designed for long-term frequency stability, either. not at the <0.01% level needed for a clock. rest of your comment is on track, but the original low-voltage low-power 50Hz signal needs to come from something that was designed for low long-term drift.

[seabass-labrax]

Good point, and thinking about this further makes me realise just how stable the frequency of mains power is! Also, in the museum at the Greenwich Royal Observatory, I saw an exhibit of a clock which was synchronised with regular pulses in the mains electricity, which I believe were in addition to the usual 50Hz AC. However, I don't know if the power grid still has such a pulse in Britain, or whether there was/is a similar system in the USA.

[dgacmu]

I love that this discussion is rapidly headed to sticking an XO on the circuit suppling current and frequency to control the old digital clock that lacks an XO. Hackery at its finest.