[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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