[time-nuts] Line Frequency
Hal Murray
hmurray at megapathdsl.net
Sun Feb 9 15:38:25 EST 2014
msimon6808 at yahoo.com said:
> Does any one have a circuit (tested - operational) for monitoring line
> frequency? I'd like something that checks zero crossing so that it is
> relatively insensitive to line voltage variations.
I'm assuming you are starting with an AC wall wart and 2 resistors to divide
the voltage down to something within range.
The easy to understand way is to use 2 more resistors to bias your input pin
at the switching threshold and a cap to connect the middle of both pairs.
The circuit would look like a H with 4 resistors on the vertical bars of the
H and a cap on the horizontal bar. Top left of H connected to AC in, top
right to +V, bottom left and bottom right to ground, and center right to
input pin.
You can do it with 3 resistors. Replace the lower of the 2 resistor setup
with a pair, one to ground and the other to +V. Adjust the size of those
resistors so the parallel resistance is the same as the one you are replacing
and the middle voltage is the switching point. Mumble. There is a word for
this that I can't remember. It's used for things like terminating ECL input
signals.
If you are going in to a RS-232 port, you can probably get a useful +V from
one of the modem control signals. I forget the polarity. You may have to
hack your software to set it to the right polarity.
--
These are my opinions. I hate spam.
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