DPDT relay

A bare 5 V double-pole double-throw signal relay, the small sealed kind: two changeover contacts on one arm, and a coil with nothing in front of it.

This is the relay itself rather than a module, so there is no transistor, no diode and no LED on it, and driving it is the lesson. Two poles on one arm are what lets one coil reverse a motor.

Pins

Pin	What it does
COIL1, COIL2	The coil, either way around.
COM1, NO1, NC1	Pole 1: the arm, normally open, normally closed.
COM2, NO2, NC2	Pole 2.

Properties

coil_volts: the rated coil voltage, 5 V. coil_ohms: its resistance, 125 ohms, which is 40 mA at 5 V. must_operate and must_release: the fractions of the rated voltage at which it pulls in and lets go, 0.75 and 0.1. pull_ms and release_ms: 5 and 4 ms.

Driving it

40 mA is more than a board pin should give, so the coil goes between 5 V and the collector of a transistor, with a diode across the coil (band to 5 V). Pin high through 1 k to the base and the coil pulls in.

Reversing a motor

Wire the motor between COM1 and COM2. Put the supply on NC1 and NO2, and ground on NO1 and NC2. Released, COM1 is on the supply and COM2 on ground; pulled in, it is the other way around.

What the model gets right

The coil is a real 125 ohm load, so a resistor in series or a weak supply leaves it with less than it needs. It pulls in at 75 percent of its rating and does not let go until it falls under 10 percent, so once in it holds on at a voltage that would never have pulled it in. Both poles move together, and neither contact is made while the arm travels.

What it does not model

The coil's inductance, which is what the diode is for: here the current stops instantly and nothing breaks without the diode, so the circuit is right to have one and the simulator cannot show why. Contact bounce, contact ratings, and the change of coil resistance with temperature.

See it in action

Relays reverse a motor uses one to reverse a motor from a single pin. Open it at /templates.