Circuit 12.1 (power)
The base circuit with a power supply, reference voltage generator, several configuration jumpers, as well as connectors for the Arduino and further circuits like Circuit 10 (electrodes).
You can change the values for R3, R4, and all capacitors by ~1 order of magnitude without too much impact. R3 needs to be equal to R4. R5 may need to be adapted to whichever LED and supply voltage you use, see e.g. this LED resistor calculator.
To supply 3.3V instead of 5V, change the value of R2 from 110kΩ to 180kΩ. This will also change Vref from 2.5V to 1.65V.
Do not connect all three pads of jumper JP9.