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


This is actually a digital application :-)

I'd say that the cheapest solution to this, all things considered, is liable
to be the cheapest available microcontroller that has internal power on
reset and internal clock.

You can define acceptable

- minimum & maximum pulse widths,
- overlap durations
- recovery times between pulses (which can be near instantaneous)
- more

Some of this can be quite hard to do with low component count using analog
parts.
Using a microcontroller you need one IC and two optical sensors. Close to
the ideal zero part design.

A PIC part MAY be cheap enough, but the newish Philips flash parts will give
them a very good run for their money (which BTW is not an accident).

Future Active have the 89LPC901FD at $US0.67 in 1s.if I read their pricing
correctly.

hard to beat with resistors, caps, IC, room, ... .

Any PIC from Digikey seems to be > $US1 in 1s


        Russell McMahon

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