In reality a transistor is generally added to the circuit to provide enough power to control the relay, while a diode ... than sufficient in this case. Most NPN-type BJTs will work fine though.

Speaking of the Pinecil, it uses an unorthodox driving circuit – it has an NPN transistor, but its base driven through a capacitor, so that only the AC component of the driving signal gets through.

This transistor switching circuit uses an npn transistor, a voltage divider (consisting of a \(5 kΩ \) resistor and thermistor) and a \(5 V\) power supply. Use the information in the table to ...