>How about a piezo disc with a rubber pad on
>top that can be tapped?
I had been thinking in terms of an old crystal microphone. I used to have a
couple which were about 1.5" dia, and had soem largish holes in the front
shield. Glue a suitable plastic standoff onto that and then a small circular
sheet of Perspex on that as the "drum face", and even with fairly light taps
I suspect you would get enough output that you would need clamping diodes to
clip the level to prevent damage feeding straight into a pin.

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