Arduino Relay Driver — Switching the Mechanical World

An Arduino can't switch a heavy load directly, but a 2N2222 can drive a small relay that can. Add a flyback diode, ship it.

Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$5.80. 7-part bill of materials with live supplier pricing. A relay driver that lets a 5 V Arduino digital pin (sourcing 20 mA) switch a 5 V coil relay (drawing ~70 mA), which in turn switches a separate load circuit at any voltage and current within the relay's contact rating — typically 1 A at…