PWM Electronic Relay Switch

What is a Relay? Relays are electromechanical switches that open and close circuits by receiving electrical control signals from outside sources. For RC related applications, the control signal is PWM provided from a radio receiver, flight controller or device like a servo tester. Relays are particularly useful as they provide electrical isolation between the signal generator i.e. your radio receiver and the device being controlled. This can protect the receiver from potential damage due to voltage spikes or electrical faults in the controlled circuit.

RC hobbyists often use relays to control high-power devices such as smoke generators, lights, rocket launch systems, or other electronic components that require more current or voltage than what the RC receiver or controller can directly handle. Aside from power applications, relays can be used to switch between multiple devices or circuits with a single RC channel. This can be useful for tasks like selecting different lighting configurations, switching between different motors; or triggering auxiliary functions like deploying landing gear, activating camera systems, or releasing payloads in drone or robotics applications.

How to use a relay 

A single relay module typically has 6 electrical connections; three connectors for the control circuit and three for the switching circuit.

For the control side there is a PWM signal input connection (S), and power input (+ / -). You would connect your radio receiver to this side of the relay module.

On the switched circuit side of the module, you’ll see the three switched connection points marked NO (Normally Open), COM (Common), and NC (Normally Closed). The relay can be triggered to switch between the NO and NC circuits. The NO circuit will by default be turned off and when triggered will be turned on, while the NC circuit works the opposite and is on by default and turns off when triggered. The Common contact is the switching point of the circuit.

When you want to control power to a device using a relay module, you will have to disrupt its electronic circuit. Any DC device contains two wires that create a circuit. To connect your device to the relay, intersect one of the wires and connect the two open ends to the relay switch. One wire should be connected to the COM terminal and the other to either the NO or the NC switch. Whether you connect to NO or NC depends on whether you want the device switch on or off when the relay isn’t being triggered. For example, say we want to control a light and have the lamp be turned off until we switch it on. We would need to connect the second wire to the NO terminal in this case.