Relay module (SRD-05VDC style)

The single-channel relay module every kit ships: a coil, a spring, and a metal arm with a contact on the end.

Energize the coil and the arm is pulled across with a click; let go and the spring takes it back. The point of the part in a classroom is that the two sides share nothing at all: a 5 V board pin switches something that has nothing to do with 5 V.

Pins

Pin	What it does
VCC	Coil supply.
GND	Coil return.
IN	Control input; a logic level against half the supply.
COM	The arm.
NO	Normally open: where COM moves to when energized.
NC	Normally closed: where COM rests when not energized.

Properties

trigger: high (the default here, matching a bare coil and transistor) or low (the cheap blue boards are nearly all active-low, because their opto-coupler is wired to the rail). pull_ms / release_ms: 10 and 5 ms, the SRD-05VDC datasheet numbers.

What the model gets right

A relay is slow, and the timing is the part people get wrong: 10 ms to pull in and 5 ms to drop out, matching the datasheet. During that travel neither contact is made (the arm is genuinely in the air, which is "break before make"), so a sketch toggling a relay at a kilohertz does nothing at all here either, exactly as it does nothing on the bench. Below 4 V across VCC/GND the coil cannot pull the arm over at all, whatever IN says, which is the failure everyone meets running a relay off a board's regulator with a motor on the same rail.

What it does not model

Coil current (about 70 mA, which matters for the transistor driving it), contact bounce on the way in, the arc across an inductive load, and the flyback diode, which is already on the module, and is why nothing here needs one added.

The coil is not a coil: VCC/GND are read only to decide whether the supply is above 4 V, so the control side draws nothing at all and there is no inductance, no pull-in current, no hold current and no way to find out that a board's regulator cannot run one. IN is a logic level against half the supply rather than the opto-coupler's LED.

pull_ms and release_ms are exact and identical on every operation, where a real relay's travel time varies with coil voltage and temperature and gets longer as it ages. The contacts are ideal metal with no rating, no resistance drift and no welding.

Common mistakes

Assuming trigger is active-high because the pin is called IN. The cheap blue boards are nearly all active-low, and a tutorial written for the other kind will leave the relay backwards until trigger is set to match.

See it in action

Keypad lock switches a relay from a correct code. Open it at /templates.

For a module that switches backwards, clicks when the board starts or does nothing on a 3.3 V board, see Arduino relay module: active LOW and startup clicks.