Here’s a great optical tremolo designed by CultureJam. Schematic can be found here. D1 is the external rate indicator and could be used as the on/off indicator as well. Just use part one pole of your 3PDT to break/connect one of the leads of D1. D2 and the LDR should be isolated together (heat shrink tubing works great). Tayda LDRs and either orange or red LEDs seem to work well in this circuit. A NSL32-SR3 optocoupler works well too.
