[sdiy] Rectifier for logic level pulse?

[David G. Dixon]

> Another solution would be to use a simple RC-delay and an XOR-gate.
> First use an XOR-gate just for buffering purposes. Then feed one of the
> inputs directly and another from a RC-lowpass filter... <snip>

Ray Wilson does this in his matrix keyboard scanning circuit.  I've built a
modified version of this circuit, and can attest that the triggers work
perfectly.  Here's the link:

http://www.musicfromouterspace.com/analogsynth/KEYENCODERPCB/KBDMATRIX022005
.html

See page 2 of the schematic.