In this theory-based episode, we take a quick look at the evolution of the relay, how it changed telegraphy, and how to actuate a relay safely from a low-current source like a microcontroller. The method I typically use to select the proper base resistor for a given BJT transistor is presented, as well as why you always see a freewheel protection diode across the relay coil; the resulting waveform with, and without protection diode are viewed on the oscilloscope.
