This full schematic is a bit cluttered. The Simplified Analog Input Schematic is easier to read and understand, and it has a detailed description of how this circuitry works.

schematic diagram

Other Schematics:

Main CPU Section Schematic
Simplified Analog Input Schematic
Power Supply Schematic
MIDI Port Schematic
Front Panel Schematic
Wiring Schematic - not on-line yet

or skip to the firmware.

The MIDI Drum Machine, Paul Stoffregen and Rod Seely.
Designed and constructed Fall, 1991. Project status: Complete.
http://www.pjrc.com/tech/midi-drums/sch-ana.html
Last updated: February 24, 2005
-- These drum-machine web pages are still under construction --
Questions, Comments?? <paul@pjrc.com>