Megadrum32 Small
Atmega8/Atmega32 based MIDI trigger, schematics and source included –

MegaDrum is a simple to construct yet very powerful MIDI drum trigger. This is a DIY project and uses widely available and very inexpensive components. MegaDrum gives you flexibility in what modules you want to build and what components to use.

MegaDrum MIDI Trigger – [via] Link.