source= http://www.piclist.com/piclist/2003/11/24/174650a.txt?

Bob Ammerman defined it better with: <BLOCKQUOTE>

fmax = maximum output frequency

loop forever:
    f1 = get frequency of first input
    if (f1 > fmax) then f1 = fmax
    f2 = get frequency of second input
    if (f2 > fmax) then f2 = fmax
    fout = fmax - min(f1,f2)
end loop

</BLOCKQUOTE>

IF I wanted to use a PIC for this, this is about what I would have written.

However, I find it hard to believe that the cost of the photo detectors,
LED, PIC, crystal or resonator, etc... is going to be less than the cost of
photo transistors, LED, a few caps, diodes and resistors and I can't believe
at all that this can't be done in a purely analog circuit. It violates KISS
to use a PIC, for this doesn't it? Yes, I know there is KISS for design and
KISS for production and I could have had it designed, prototyped, debugged,
and off to the board house long ago if I did a KISS design using a PIC,
etc...

But I want to make some number of these and enable non-PIC people to cobble
them together from Radio Shack parts, so I'm really hoping for an analog
design.

The AND part is easy... the photo transistors just get hooked in series. And
the invert is easy... the LED is just powered from the difference between a
cap and Vcc so when the cap is charged, the LED doesn't light. The cap is
charged through a diode by the pulses from the photo transistors and
discharged via a large value resistor.

What I don't know is how to bias the phototransistors so that they only
conduct when the light level /changes/ i.e. they need to constantly re-bias
themselves so that changes in the light level produce a brief burst of
current flow. So how does one bias a phototransistor? There is no darn base!

Also, how do I keep the LED from coming on until the cap is discharged to
the point that the LED can really get up a bright pulse before it charges up
the cap again?

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