ULN2803 8 Darlington Transistor Array IC (DIP-18)

A microcontroller pin can switch a few milliamps, nowhere near enough to pull in a relay coil or drive a solenoid directly, and the ULN2803 exists to bridge that exact gap. Its eight Darlington transistor channels each take a logic-level input and switch a much higher current load on the output side, making it the go-to chip for driving a bank of relays, a stepper motor's coils, or any other moderate-current load straight from a microcontroller's GPIO pins without extra discrete transistors.