5 V SPDT Relay

The relay part models a 5 V single-pole double-throw (SPDT) electromechanical relay. A coil magnetically actuates a contact arm that switches the COM terminal between NO (normally-open) and NC (normally-closed). The coil and the contacts are completely isolated, so a low-power microcontroller pin can switch a high-voltage or high-current load it could never touch directly.

Pinout

Coil polarity does not matter — the relay responds to the magnitude of the coil voltage, so coil_a and coil_b are interchangeable. The COM/NO/NC contacts are electrically isolated from the coil.

Default coil parameters

These can be overridden via the Inspector properties panel. With 5 V across a 70 Ω coil the coil draws roughly 71 mA. The recommended supply range is 3.0–12.0 V (5.0 V nominal).

Coil vs contacts

The relay is two separate circuits in one package:

• Coil side (coil_a, coil_b) — the control input. Energising the coil creates the magnetic field that throws the contact arm.
• Contact side (com, no, nc) — the switched output. This is galvanically isolated from the coil, which is the whole point of a relay: your 5 V logic never shares a connection with the load it is switching.

Pull-in and drop-out

Contact switching uses hysteresis on the coil voltage:

• Pull-in: when |V_coil| ≥ vPull (default 3.5 V) the relay energises and COM connects to NO.
• Drop-out: when |V_coil| < vDrop (default 1.5 V) the relay de-energises and COM falls back to NC.

The gap between the two thresholds keeps the contact from chattering near the switching point.

Coil model and the flyback diode

The coil is modelled as a series resistor-inductor (R + L). Because an inductor resists sudden changes in current, switching the coil off produces a large reverse voltage spike — the inductive kick — that can destroy the transistor or pin driving it.

Always add a flyback diode anti-parallel across the coil (cathode to the positive coil terminal). A common choice is a 1N4001. It clamps the spike by giving the collapsing coil current a path to circulate.

de:volt raises a missing-flyback diagnostic if you drive the coil without one, so wire the diode in from the start.

Driving the coil

A microcontroller pin cannot source the ~71 mA the coil needs, so switch it with a transistor:

• NPN collector to coil_a
• NPN emitter to GND
• coil_b to +5 V
• Base to the microcontroller pin through a resistor
• Flyback diode anti-parallel across the coil

Contact ratings

The contacts are rated for substantial loads — typically 10 A / 250 VAC or 10 A / 30 VDC for this class of relay — which is why a relay is the standard way to switch mains lamps, motors, and other loads from logic-level signals.

Example circuit

Arduino D7 ─── 1 kΩ ─── B  [NPN]  C ─── coil_a ─┐
                              E             │  ╪ 1N4001 (flyback)
                             GND   coil_b ──┴─── +5 V

Load switching:  +V_load ─── COM      NO ─── Load ─── GND

Energise the coil and COM swings from NC to NO, powering the load. Drop the coil and the flyback diode absorbs the kick while COM returns to NC.