Knockback is a set of self-playing biscuit-tin drums, made for the Bristol Hackspace exhibit at BV Studios Open Day in October 2011.

It uses piezo sensors to detect taps on the tins, DC motors with strikers make of nylon tube to tap on the inside of the tins lids, and an Arduino to run it all.
The first tap sets it off with a sparse pattern of beats, and any beats which are supplied by the audience are added to the pattern,
Beats age out over 15 seconds.

The piezo transducers have a 1M parallel resistor, and are wired between ground and an analogue input on the Arduino.

Motor drive is via a ULN2065B, which is rated at 1.5A per channel. Each motor has 3 ohm series resistor to keep the current down, but still draws most of 1.5A on each beat. Hence the giant 5V supply you can see here.

The Arduino code is pretty simple (, and could really use more work to make for a more engaging experience.

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