5V Single-Channel Relay Module

A relay is an electromechanical switch that allows a low-power circuit to control a completely separate high-power circuit. It contains a small electromagnet coil. When a small current energizes the coil, the magnetic field physically pulls a set of mechanical switch contacts closed (or open).

Relays provide total electrical isolation (galvanic isolation) between the control circuit and the load. This allows a delicate 5V microcontroller pin to safely turn on a 120V AC appliance without any direct electrical connection between the two systems.

The active-LOW trigger convention on most of these modules is a direct consequence of how their onboard driver is wired, not an arbitrary design choice. The module's small NPN driver transistor has its base biased through a resistor connected to the module's own 5V supply rail; grounding the input pin (pulling it LOW) is what completes that base-bias path and turns the transistor on, which in turn energizes the relay coil. A microcontroller sending a HIGH signal instead leaves that base path un-grounded, so the transistor — and the relay — stays off. This is a subtle but common source of confusion for anyone assuming relay-on always means signal-high.

The module exists in the first place because a bare relay coil (see the sibling entry in this category) draws considerably more current than a microcontroller pin can safely supply directly, and switching that inductive coil without a flyback diode risks a voltage spike damaging whatever is driving it. This module packages the transistor driver and flyback diode (and, on many boards, an optocoupler for additional electrical isolation between the low-voltage control side and the relay/load side) onto one small breakout board specifically so a beginner doesn't have to build that driver circuit from discrete parts — the tradeoff being less visibility into, and control over, exactly how that driver stage is built compared to wiring a bare relay by hand.