MIDI organ pedals

Touch-sensitive music keyboards detect the force applied to each key by measuring the time delay between when the first contact closes, and when the second contact closes.

The harder you hit a key, the faster the key will move. As the key comes down, it presses two blister-like bits of carbon-coated rubbery plastic onto the two pads on the board, but one blister touches first. I think it's the back one that touches first.

The microcontroller at the other end of the ribbon cable scans the contacts continuously. When two contacts on the same key close in order, it measures the time between the closures and uses this to calculate the key velocity.

If you want your organ pedals to be touch-sensitive you will have to duplicate this behaviour, using two switches on each pedal. This could be a problem because standard switches (such as microswitches) suffer from contact bounce (Google it) and this could cause the microcontroller to misread them, causing unreliable behaviour. You will also need to get the delay between contact closures into the right range so that the microcontroller reports a reasonable range of touch velocities

If you don't want velocity sensitivity for the pedals, you may be able to use a single contact of each key. It depends how the code in the scanning microcontroller deals with abnormal conditions. One keyboard I had would respond to the second contact closing without the first one closing by reporting maximum velocity; if yours works like that, you could go that way. You can test how the microcontroller behaves by manually pressing a carbon-coated blister onto each contact of a key in turn, and seeing whether it gives any consistent and useful result. I hope you can get something useful with a single contact closure; if not, you might be forced to use two switches per pedal, or a two-pole switch that can be connected across both sets of blister contacts to make it appear that they both closed at the same time.

You may have some trouble soldering to the blister contact pads (and you may want to avoid spoiling them, too), but you can trace the tracks from the pads to the nearest diode or connector pin and solder to them instead.

Each switch in your pedal assembly needs to be wired across the appropriate pair of blister contacts on the board. You don't need to use two separate wires for each switch; you can take advantage of the commoning that is already present on the circuit board to reduce the number of wires.

You also need to minimise the capacitance of the connections to the switches, because they are being scanned constantly at a fairly fast rate. This means keeping wires as short as possible, and using good quality switches with good electrical isolation.

Ideally, it would be best if you could manually transfer the movement of the pedals onto the existing plastic blisters, and keep the switch contacts on the board.

Alternatively, you could build a complete replacement board - duplicate the connector and diodes onto your own circuit board, so you wouldn't need to modify the original board at all - just plug the ribbon cable into yours.