Here are some photos showing the construction of the peg matrix from my School Rhythmer drum machine project: Basically, it involves micro-switches, wires and diodes arranged in columns and rows, creating an 8 x 5 matrix The columns carry the trigger signals from the step sequencer - there are only eight steps - while the rows carry the trigger signals to each of the five drum sounds. When a peg is inserted, it closes the associated micro-switch so that the particular drum sound is triggered at that step. The diodes isolate each switch from the matrix. Without these, any trigger signal would dissipate throughout the whole matrix triggering every drum sound that has a peg inserted along its row, regardless of the current step. The rest is 'mechanical'. I used micro-switches as a cost effective solution. An alternative might be 3.5mm jack plugs and sockets, such as the switching type that make/break a circuit when a plug is inserted. A further idea would be to use tact switches w...