I'd like to add two things:

On Sat, 10 Mar 2018 20:46:16 -0800
"Tom Van Baak" <t...@leapsecond.com> wrote:

> 1) Safety. I usually use a low voltage step-down transformer. This gives 
> isolation and safety. Anything from 3 VAC to 24 VAC is fine.

Please, do not just add a transformer. Make it also impossible to touch
any wire comming from mains or anything that could potentially have
mains voltage on it. In europe, the minimum isolation distance between mains 
and anything on the low voltage side is at least 5mm (IIRC). I recommend
to use at least this much distance in your designs as well, even if a much
smaller distance would be enough to prevent arcing in most cases (the reason
for this large gap is to ensure that dust and humidity do not cause an
isolation fault under normal, household conditions).

> 2) Trigger. There are dozens of schematics on the web for capturing the zero-
> crossing of a low-voltage sine wave. You can easily go overboard on this. Or 
> just keep it simple and feed the signal through a resistor directly into a 
> microprocessor input. The internal clamping diodes do their thing. A Schmitt 
> trigger input is helpful but not necessary depending on how your software 
> makes the measurement.

Please do not use the internal clamping diodes of an IC as the main way
to clamp a voltage. These diodes are ment for protection against ESD events.
They are not designed to constantly dissipate energy. If you do it anyways,
the diodes will fail after a while... and slowly at that.
If you need to clamp voltage use two Schottky diodes (BAT54, MBR0502,...)
to Vcc and GND. Also ensure that the current drawn from Vcc at all times
exceedes the current injected into Vcc through the clamping diodes, otherwise
you will have an uncontrolled raise in power supply voltage and might exceed
the rating of your circuit.

                        Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson
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