Intelligent Pedal Relay Bypass

A cheap but powerful relay bypass board for guitar effect pedals. In addition to being able to do basic true bypass functionality, it also has the following capabilities:

• Quick tap to switch between engaged and bypass, or hold the foot switch to temporarily toggle engage/bypass until you release
• Hold when powering on to toggle auto-on/auto-off
• Remebers state when powered off
• Small footprint (28.4 mm x 20.3 mm) that fits neatly under an SPST switch
• Neatly labeled and arranged input/output pads that line up neatly with PCBs from vendors like PedalPCB

This board is designed to use an inexpensive, reliable latching relay that's available from LCSC/JLCPCB.  All of the components except for the microcontroller and electrolytic capacitor are pre-selected for assembled by JLCPCB, with every component except for the relay being basic components. For my most recent order, the total cost per unit, including the SPST foot switch, microcontroller, and assembled PCB, was less than $4. That's basically the same price as a 3PDT foot switch.

Here are the components that you will need to purchase separately if you get the board assembled by JLCPCB:

• 100µF electrolytic capacitor (I recommend getting one rated for at least 25v to ensure a long lifetime)
• SPST normally open momentary switch
• ATTINY13A microcontroller (ATTINY45 and ATTINY85 should work as well, but they are more expensive and their additional features are not used in this circuit

You can write the programming for the microcontrollers yourself or you can grab this ready-to-use code from Github. You can also buy pre-programmed microcontrollers from MAS Effects.