[Trent M. Wyatt]’s CPUVolt library provides a fast way to measure voltage using no external components, and no I/O pin. It only applies to certain microcontrollers, but he provides example Arduino ...

Measuring a voltage is pretty easy: just place your multimeter’s probes across the relevant pins and read the value. Probing currents is a bit trickier, since you need to open up the circuit and place ...