Analog design challenge: Build a NAND gate that is sensitive to /pulses/
(not level) of light from the cheapest possible components.

Actually, it's more of a plea for help than a challenge because I don't have
a good design... I suck at analog... digital and software I can do just
fine, and I have been educated and have some small ability at analog
electronics, but somehow it just doesn't... click... in my head.

Anyway, I really want to make a set of visible light (LED) NAND gate Lego
blocks for my kids to learn from. And I don't want them to be light level
sensitive, so I think that reacting to pulses (or their absence) would be
best. Anyway, it makes it easier for inputs like motor shafts and optical
interrupters, and easier for the kids to see. The common "is that LED on?"
question is much less common when the LED is expected to flash or not.

I imagine that two phototransistors and an AGC circuit that turns on the LED
when the input detects no changes in light level. The light from the LED
will then cause an oscillation. But if there is already pulsing light at the
sources, the LED will not have a chance to get to the point of wanting to
turn on.

Actually, the best thing would be for the /frequency/ of the input pulses to
result in an (inversely) corresponding frequency of output. Coupling the
output of such a thing to a motor, with one input coming from the shaft, and
the other coming from a variable frequency source, makes a regulated motor
control.

net lists or schematics or any sort of description of an actual circuit
would be very appreciated.

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