Wiring a potentiometer to control a 9V battery powering 2 LEDs in series
Yes, Arduino is a 5V microcontroller, meaning it can directly control 5V circuits only. You might incur in problems also trying to deal with 3.3V circuits, even if Arduino is pretty friendly to 3.3V counterparts.

That does not mean though you cannot control higher voltage circuits, just not directly. Actually if anything above 6V is connected to an Arduino PIN you will blow that pin and potentially others which are logically connected within the MCU, so be aware.

To control higher voltage circuits you have multiple options, in your case you are probably interested into using a BJT transistor, an 2N2222 might do.







simulate this circuit &ndash; Schematic created using CircuitLab

Please note the values of R1 and R2 must be adjusted to the LEDs you are using depending on their Vf (forward voltage drop) and the maximum current you want to provide.