[sdiy] tap tempo circuit

[tuj at fuse.net]

Ok, so I've exhausted google looking for how this is done.  The only thing I could find was someone who had done it using a PIC, which I don't really want to get into.

Anybody know how to build a tap-tempo circuit?  I know lots of gear these days has this type of feature.  Not sure if it just times the interval between the first two presses, or if its more complicated.  Most software versions I've seen do a least-squares method across the intervals to determine the tempo.  

I suspect this can be done with a phase-locked-loop somehow, but I'm at a loss as to exactly how.  Thanks.
-tuj