Ok i'll read into this and try to understand it lol
That circuit :-
- Needs a common ground with the arduino.
- Is 9V enough to work for a TENS?
The working circuit is right next to me on a breadboard, and i have a common ground, just forgot to show it in the schematic.
9V is more than enough for a Frog muscle tens (yes, i'm torturing frogs in our lab at med school) even 400mV-1V will work (that's why i asked how to reduce the voltage). Theoretically it is enough for human muscle too, nerve fibers operate at 60mV-90mV and for muscles it's generally a bit higher but varies depending on the muscle.