Modding analog pedals for digitally controllable pots+switches

[nashville]

Hello,

I'm curious if anyone here has modded analog stompboxes with digitally controlled pots and switches.

My favorite pedals are simple 4 knob 9V stompboxes with only an on-off switch. I've seen Chase Bliss and a few other brands with digitally controllable analog circuits and if it's possible, I'd love to have some of my favorite circuits rehoused, possibly all together in a larger enclosure that could accept MIDI messages. If the the knobs could be digital encoders circled with LEDs (like a Kemper has) that control a non-mechanical potentiometer chip then I could reach down and tweak knobs or load settings from an external MIDI signal and the knob positions would always be visible and accurate to the current settings.

Is this something that's currently possible? How would you suggest I go about it?

[potul]

This is a recurring topic in this area of the forum.

My learnings so far:

-There is no "universal solution" that you can use in any pedal you want. You will always have to taylor the solution to your circuit.
-There are multiple ways to do this, ie: Digipots, JFETs, Opticouplers. Ideal solution will depend on your needs
-You will need a mC to control the whole thing (PIC, Arduino or similar). This is the "easy part" for me.
-In most applications, you may need 2 synchronized digipots to replace one physical pot.
-You may need to modify the original circuit to adapt to the solution you want to use.
-When you factor in everything you need (multiple digipots, some sort of inputs devices like an encoder, leds rings...) you realize you need a mC with some serious I/O capabilities. Not a small project.


Some posts I found just by searching "digipot"

https://www.diystompboxes.com/smfforum/index.php?topic=117493.msg1091211#msg1091211
https://www.diystompboxes.com/smfforum/index.php?topic=120359.msg1127212#msg1127212
https://www.diystompboxes.com/smfforum/index.php?topic=105202.msg946686#msg946686
https://www.diystompboxes.com/smfforum/index.php?topic=42789.msg309544#msg309544

[ElectricDruid]

+1 agree with Potul

Well, except for one small detail - I'd say you don't need 2 synchronised digipots. You can get digipots with three legs like a normal pot. For example:

https://www.microchip.com/wwwproducts/en/MCP41100

The problems are the voltage and the current handling - these are 5V devices. There are high voltage versions, but they're only available in little tiny SMD packages - you may or may not think that's a good thing.

Another limitation is the range of values. The digipots I posted above only come in 10K, 50K and 100K values. If you want anything else...well, tough. You have to redesign the circuit to suit those values.

If you wanted LED rings and encoders, you'd need a lot of IO, like Potul said. At that point, I'd be looking at parallel-to-serial and serial-to-parallel chips to avoid the need for large numbers of pins (74HC165 and 74HC495 are numbers that come to mind, but there are a couple of options, IIRC).

Still, despite all these quirks, programmable pedals with digipots is something that I'd still like to do at some point. It's just such an appealing idea that it's hard not to! 

Tom

[potul]

Ahhh, good to know there are 3 legged digipots. I was not aware!

Tom is bringing a very good point on the available values.

[Ice-9]

A lot of complexity can be reduced if you were to use I2C digi-pots they are available with non volatile memory as well. Again the main design considerations are the supply voltage which is usually 5v and the available resistance values.