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School Rhythmer Peg Matrix

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 whereby inserting a peg keeps the switch depressed. Of course, foregoing the whole peg idea and simply using toggle switches would make everything much easier but this project was intended to capture the 'charm' of the original Kawai machine.

Since the micro-switches are activated by levers, I found it worked best to align them perpendicular to the surface of the matrix (as can hopefully be seen in the photo above). This way, when a peg is inserted, it gently but firmly closes the switch. To achieve this, the switches needed to be aligned correctly behind the holes for the pegs. Fortunately, the micro-switches, having mounting holes through their base, could be slid onto rails (aluminium pipe in this case) and their alignment easily adjusted. Again, the photo above hopefully shows this. Furthermore, some support is needed to keep the pegs straight when inserted since the sprung levers of the micro-switches will push them away. This was achieved quite easily with strips of wood positioned to counter the force of the levers: the pegs are simply held between the lever and wooden strip.


On the face of the peg matrix I used small rivet/eyelets (the kind of thing used in leathercraft) which serve to both support the pegs and enhance the aesthetics. The pegs themselves are short lengths of 3.5mm aluminium pipe with painted wooded handles. Almost any material would be fine since the pegs themselves are not making electrical contact. The important thing here was getting the diameter of the eyelets and pegs right so that they can be inserted and removed smoothly, while remaining firmly in place when inserted.

OK, that's all for now. I'll add more details later if any questions arise.



 

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