I’m looking to add an expression pedal input to my guitar pedal design. I have some question about how to best achieve this.
It’s my understanding that an expression pedal is basically just a passive device with a pot in it attached to a level (at least mine seems to be so). So I was thinking it would be as simple as just adding a TRS jack to my pedal, connecting GND, ring to 3v3A, and tip to one of the ADCs on the Daisy seed. However, I took at a look at the Daisy Petal schematics and I’m seeing there are some op amps involved:
Do the op amps somehow protect the seed from a short though the expression pedal, or am I missing something as to why they are necessary?
Hi,
For the buffer for 3v3 out to the potentiometer I can see the advantage that there will be no load for this reference voltage and that no hum from long cables will be introduced.
I wonder, if a capacitor in parallel to expression_in_ext would make sense too to suppress any noise?
An analog opamp buffer will stabilize the exp’s output current significantly. This happens because the impedance that the MCU sees will always be fixed to the output impedance of the opamp (instead of making it variable based on the exp pot position!). exp pots don’t have a standard value either - I’ve seen them be 10K pots all the way up to 100K pots, again giving us a big reason to standardize the current provided to the MCU using an analog buffer.
The opamp will provide “protection” against overcurrent in this way and improve exp performance since the pot value used is taken out of the equation. It doesn’t really protect from shorts like a fuse would. If the opamp is being powered at a voltage above the rated max of the MCU input, then it will still be capable of frying the MCU with overvoltage (very unlikely).
It will also reduce hum from long cables (like cebersp was saying), but I don’t think this is really a factor in this context because exp voltage isn’t something you listen to, it’s a control voltage where noise isn’t really a huge concern.
