[time-nuts] Building a mains frequency monitor

[Morris Odell]

Hi all,

This is an interesting thread. I have built many clocks using the mains as a 
reference and have settled on a "belt and braces" approach that works well 
and does not seem to pass noise or transients. I begin with low voltage AC 
from the mains transformer and use it to feed the input diodes in a dual 
optocoupler back to back through a single pole LPF made up of the 
appropriate current limiting resistor and as big a cap as I can get away 
with. On the output side the phototransistors are paralleled and thus 
produce pulses at double the mains frequency. These are used to injection 
lock a free running oscillator made from one stage of a 1489 quad line 
receiver followed by another stage in cascade. The end result in a nice 
train of clean square waves. It's cheap and easy and doesn't take up too 
much PCB real estate.

Morris