A relay is an electrically operated switch. A small control current through an electromagnet coil moves a mechanical armature, opening or closing a high-power circuit. This provides complete electrical isolation between the control (3.3–5V MCU) and load (up to 250V AC / 10A) circuits.
| Coil voltage | 5V DC |
| Coil current | ~70 mA |
| Max load (AC) | 250V / 10A |
| Max load (DC) | 30V / 10A |
| Switch time | ~10 ms |
| Contacts | NO · NC · COM |
The MCU drives a transistor (often via a ULN2003 Darlington array) that saturates and allows current through the relay coil. The magnetic field pulls the armature, changing the contact position. A flyback diode across the coil suppresses the inductive spike when the field collapses.
Mechanical wear limits cycle life (~100k operations). Slow switching (~10ms) — not suitable for PWM. Coil draws ~70mA continuously when energised. Contact arcing reduces lifespan on inductive loads — use snubber circuits.