> Two photo-sensitive inputs. Input will be sensitive to low audio frequency
> pulses of light, rather than steady light levels. One LED output.
>
> With no input, the output flashes at a "rapid" but visible rate (1KHz?).
>
> If only one input is active, there is no change in the output.
>
> As the input frequency increases, assuming both inputs are activated, the
> output frequency decreases.
>
> When both inputs are receiving "rapid" (1KHz?) pulses, the output is
> completely suppressed.

Oho....

Now that is definitely more interesting.

1KHz is much too fast to see. Maybe more like 20-30Hz max visible as
'blinking'.


So, what you need is something like this:

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

I would probably have interrupt level code maintain the values of (f1,f2)
and control toggling the output based on fout.

Bob Ammerman
RAm Systems

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