Christopher said:
Thanks for the very useful advice. I'm looking for something simple
which just works reliably, and this seems to fit the bill. If I've done
my calculations right a potential divider consisting of a 100k ohm
resistor and 22 nF capacitor
An R and a cap are a filter not a PD.
gives a maximum voltage of around 10 V
between the Schmitt trigger input and logic ground (for a 50 Hz signal).
And the schmitt is running of how many volts ? 5 ? The job of the PD is to reduce
that 10V to a voltage within the supply rails of the schmitt.
Actually, yout 12V ac winding will produce about 17V pk-pk so a PD of just
greater than 3:1 should be used. Make that 100k into 330k and add a 100k from
schmitt input to ground in parallel with the 22n ( this won't alter the RC time
constant very much ).
Are the 100 k ohm and 22 nF standard values,
Yes.
or did you and John
calculate them specially?
No.
Should I connect another resistor between
logic ground and the "bottom" node of the bridge rectifier?
No. Not required.
Graham