> I've considered a manual "tap box

Got feet ?

> It's not ideal when DJing as you'd need to re-tap as you adjust
> the speed of the music. Also, getting down to 0.1 beat accuracy
> might be a bit harder

Is this for mostly Drum'n'Bass / Techno type music ? That should
be pretty easy to filter down to a beat. If this was for personal use
I think I'd characterise the track beforehand with a particular filter.
With two BPM units you could synchronise the two tracks. Then
you'd not have to worry about the absolute value of the count.

Punters are amazed not by an ultra-accurate LED display, they are
by seamless music

> Any ideas on debouncing a switch? I can't have too much lag
> on it (ie a large RC) or it'll skew the BPM measurement...right?

For something long-lasting and bounce-free I'd look at either
a magnet and Schmitt trigger Hall switch or a blade and opto-
interrupter. Anything that doesn't have mechanical contacts

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