Hal, The resistor H bridge with a capacitor in between sounds like a good idea. Yes - I'm starting with a wall wart. About 12 VAC. Maybe a couple of back to back diodes (zeners?) across the capacitor to make sure the voltage stays in the input range of the op amp/comparator. I like the LT1122 for speed. It is not too pricey. But there is always the ubiquitous LM111 family. I do get the ECL biasing trick you pointed out.
Chris, Experimenting is good. Except that to do a valid test I'd have to run the experiments in parallel. I'm not just using this for triac triggering. I want to take measurements in real time. i.e. a PC running an FFT will not get me what I want. I may ultimately have to create a test set that generates 60.000 Hz and injects a noise signal ( +/- .1V pulses at 100 KHz say) that can be turned off. The 60 Hz need not be extremely accurate. Just stable. My counter does time triggering so I can do averaging if necessary without the counter swallowing pulses. Simon Engineering is the art of making what you want from what you can get at a profit. On Sunday, February 9, 2014 8:38 PM, Hal Murray <hmur...@megapathdsl.net> wrote: >msimon6...@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. > > > > > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.