Live MIDI Interrupter
I had a fairly crude monophonic interrupter built for big-coil operation, but I wanted to improve on its design. It had no software control over pulse width so every note had the same width, and of course polyphonics is more fun.
One critical design requirement was that I need hardware protection against the controller accidentally going CW, even if the microprocessor somehow screws up. This is relatively easy by using a 555 as a one-shot on each rising edge, then AND gating that to the actual signal so that if an abnormally high duty cycle (or more likely a duty cycle of 100%) is attempted, the one-shot will turn off at a pre-set time (determined by trimpot R4). It is worth pointing out that an inverter could be used to trigger the 555, which requires a downward-going edge to trigger. I chose to use a second microprocessor output since it saves on components (and I wanted as many gates buffering the output power as possible).
I also wanted to have a fixed oscillator in the interrupter so that I can still use it as an easy optical oscillator for bench testing coil controllers without the need to setup a MIDI device. I threw in another 555 for this. A SPDT switch on SV1 allows for selecting between MIDI or this fixed oscillator.
Another goal was to have the controller programmable over the MIDI interface to control parameters such as pulse lengths specific to note (allowing low notes to be on for a longer bang than higher notes), MIDI channel, and multipliers to control pulse width during polyphonic playback. This can all be programmed over SYSEX, a communication means within the MIDI protocol that is designed for flashing MIDI hardware with firmware or updates.
I put together a quick python script to allow for changing parameters, although I may make a GUI at some point.
This project was also the first time I’ve tried 0.4mm isolation on ground planes, which allows for ground planes to pass between pins of a DIP socket. It etched fine using the laminator for toner transfer. The etch precision is getting so good that I need to look into solder masks.