I've been studying brain waves for a while and I want to do a project like this for educational purposes, manufactured TENS devices do not have an "audio input" they are programmed with an operating pattern, it would be necessary a circuit that "receives" the analog signal from the output sound from my phone to the board input control. I did a lot of tiring research and I didn't find any similar circuit that worked like this, about converting audio into light pulses it would be something like a step up dc-dc amplifier which in this case the trigger would be the amplitude of the audio signal