Looks like a cool project! I've built lots of custom MIDI controllers and guitar DSP boards over the years. As far as voltages go, yeah, as I'm sure you know 5V is not safe for the Teensy 4.x.
The Teensy can take 5V in on it's VIN pin so that's nice. But if the FCB1010 circuits are designed to operate at 5V, you need to do some work. Here's my comments (you've likely already figured this stuff out but it might be helpful for readers looking at something similar).
- LEDs - these circuits likely have an 0/5V signal into an LED and current limit resistor. If you hook this up to a Teensy 3.3V output you will reduce the current and hence the brightness of the LED. However if the stock micro feeds a driver circuit like a BJT, you may not have to do anything at all. Let's assume the 5V micro output directly supplied the current to the LED circuit. You will need to replace the stock resistor with a lower valued one such that when driving by 3.3V, the current is the same as it was when driven by 5V. Instead of desoldering and replacing the old resistor, you can lower the overall resistance by connecting one in parallel with the original such that the parallel combination achieves the desired final value. This is may be easier then desoldering.
- Switches - You didn't specify in your post but I'm assuming these are pulled up to 5V, and grounded when the switch is pressed. The simple solution would be to disconnect the 5V on to the switch circuit and connect 3.3V from the Teensy instead. However that's probably not the most practical. By adding another resistor in parallel with the switch, you can create a voltage divider with the pullup such that you get 3.3V instead of 5.
Original circuit: VAL is where the switch state is measured
Code:
5V --- Rp --- SW --- GND
|
VAL
New Circuit:
Code:
5V --- Rp --- SW --- GND
|
|---VAL
Rd
|
GND
- Expression inputs: the FCB1010 circuits are likely designed to go from 0V to 5V (approximately). This needs to be reduced to 0 to 3.3V. This one may be a little harder to correct as expression pedals have very different circuits and the solution must be tailored to that. Attempting to put a resistor in series or parallel with parts of the circuit may reduce the voltage to the correct range but create a non-linear behavior. Since there are very few expression pedals, you may want to simply feed the output of each expression circuit into a BJT voltage follower circuit, then use an appropriate divider on the BJT output to simply reduce the range to the 3.3V target without altering the linearity of the response.
Much of this based on speculation of the various circuits. If you have a link to a schematic for the FCB1010 it will be easier for people to offer suggestions if you want feedback on your circuit alteration ideas.