Are you trying to build a expresion pedal on your own?
Why not just get the Mission Engineering SP-H9? It has a built-in footswitch which can be assigned to Toggle Tuner. You don’t need to connect to an extra auxiliary footswitch.
Thanks for your reply. Yes, I’m trying to built an expression/footswitch pedal on my own because I don’t need that bulky wah-wah-style pedal and don’t need to operate the expression control like I do on a wah pedal.
Another problem of the built-in switch of SP-H9 is that you should end up setting the pedal to the fully toe-down position whenever you activate the tuner. I will mostly use the expression control to set the rate of the modulation effect on-the-fly. So, again, a wah-style pedal is not ideal.