MIDIfying old organ drum unit

Other Hardware Audio
1

I have an old organ drum unit called Godwin Drummaker 32
It has standard drum patterns like waltz and bossanova.
It also has a bass/chord function, that I would like to control with MIDI

On the back it has a 20 pin connector, to attach a 13 key pedalboard and a 25 key piano keyboard.

After opening the unit I have established that 12 pins are the scale C-B, and 3 pins (13,14,15) are octaves.
So connecting pin 13 with each of the 12, plays a deep scale, connecting pin 14 higher scale etc.

All the pins are "hot" in relation to Ground. The 12 pins between 0 and 4V, and 13,14,15 around 1,5V

I don't have the keyboard and pedalboard, but imagine it is a mechanical switch between these pins.

My idea was to connect these pins in a matrix with an arduino, but I am not sure how to.

First I thought I could use transistors as switches, so the arduino pins would switch on /off the connections.
But the transistors would add voltage to the line, and maybe do some damage?
Other ideas where Darlingtons or MOSFET's, but I guess they do the same.

I guess 15 relays could work, but it seems such a overkill.

Any suggestions anyone.

2

A transistor or MOSFET can be tricky if you don't really know what's going on or which way the current is flowing.

The safest way of automating a switch is with a relay, which is an electrically-isolated electrically-controlled switch.

I'm not sure if a matrix is the best solution. You won't have enough current to drive a relay matrix directly from the Arduino, so if you don't have enough outputs, some sort of multiplex/shift register approach might be better.