Relay Modules

What Is a Relay Module and How Does It Work?

A relay module is an electronic switch that allows a low-voltage microcontroller like Arduino or ESP32 to control high-voltage devices. The microcontroller sends a 3.3V or 5V signal to the relay. The relay closes an internal mechanical switch that completes a separate high-voltage circuit — up to 250V AC or 30V DC at 10A.

Each relay module has three output terminals. COM (common) connects to your power source. NO (normally open) means the circuit is off until the relay activates. NC (normally closed) means the circuit is on until the relay activates. Most projects use the NO terminal to turn devices on when triggered.

Relay modules include optocoupler isolation between the microcontroller side and the high-voltage side. This protects your Arduino or ESP32 board from voltage spikes. An LED indicator on each channel shows when the relay is active.

What Types of Relay Modules Does Kunkune Stock?

Kunkune stocks relay modules in three operating voltages and multiple channel configurations. Choose based on your microcontroller’s output voltage and how many devices you need to control.

All relay modules support high or low level triggering via a jumper setting. This makes them compatible with both 5V Arduino boards and 3.3V ESP32 boards.

How Do I Connect a Relay Module to Arduino?

A relay module connects to Arduino with 3 wires: VCC to 5V, GND to GND, and IN to a digital pin. Upload a sketch that sets the digital pin HIGH or LOW to switch the relay on and off. No special library is needed.

On the high-voltage side, connect your mains device through the COM and NO terminals. When the Arduino sets the pin HIGH, the relay closes and the device turns on. When the pin goes LOW, the relay opens and the device turns off.

For multi-channel modules, each channel has its own IN pin. A 4-channel relay module uses 4 digital pins on your Arduino — one per channel. Each channel switches independently.

Safety note: relay modules can switch 250V AC mains voltage. If you are switching mains-powered devices, ensure all high-voltage wiring is done by a qualified person and housed in a proper enclosure. Low-voltage DC switching (LED strips, pumps, fans) does not require special qualifications.

For a complete smart home project using relays, read our smart home build guide.

Do Relay Modules Work with ESP32 and Raspberry Pi?

Yes. All relay modules in the Kunkune range work with Arduino, ESP32, ESP8266, and Raspberry Pi boards. The wiring is the same: VCC, GND, and a digital signal pin.

ESP32 and ESP8266 run at 3.3V logic. Most Kunkune relay modules support both high and low level triggering, so they activate correctly from a 3.3V signal. Check the jumper setting on the module — set it to low level trigger for ESP32 boards.

Using a relay with an ESP32 board adds WiFi control. You can switch lights, pumps, and heaters remotely from a phone, a web browser, or a home automation system like Home Assistant.

The ESP-01S WiFi Relay module is a standalone solution. It combines an ESP8266 WiFi module with a single relay on one board. No Arduino or separate microcontroller is needed. Program it once and it runs independently.

How Do I Choose the Right Relay Module?

Start with two questions: what voltage does your microcontroller output, and how many devices do you need to control?

Arduino Uno, Nano, Mega — Use 5V relay modules. These boards output 5V on digital pins. A 1-channel module controls one device. A 4-channel module controls four devices independently.

ESP32, ESP8266, Raspberry Pi — Use 5V relay modules with low level trigger. These boards output 3.3V on digital pins. The low level trigger activates the relay from a 3.3V signal. Set the jumper to LOW.

12V or 24V systems — Use 12V or 24V relay modules for automotive, industrial, or PLC applications where the control signal is 12V or 24V rather than 3.3V or 5V.

Timer applications — The NE555 timer relay and digital cycle timer modules switch loads on a set schedule without needing a microcontroller. Adjust the delay with an onboard dial.

All relay modules at Kunkune are priced from £0.90. Dispatched within 1 working day. Delivery in 1–2 days. Free shipping on orders over £25.

What Can I Build with Relay Modules?

Relay modules are used in every project that needs to switch a device on or off from a microcontroller. They are the bridge between your code and the real world.

Smart home lighting — An ESP32 with a 4-channel relay controls four light circuits from a phone app or Home Assistant dashboard. Add a PIR motion sensor for automatic lights that turn on when you enter a room.

Automatic plant watering — A soil moisture sensor triggers a relay that turns on a water pump. The Arduino reads the sensor, decides if the soil is dry, and activates the pump through the relay. See our plant watering project guide.

Pump and heater control — A temperature sensor and relay module create a thermostat. The Arduino reads the temperature and switches a heater or pump on or off to maintain a set range. Useful for aquariums, greenhouses, and fermentation.

Garage door opener — A 1-channel relay connected to an ESP32 simulates a button press on your garage door motor. Control it over WiFi from your phone.

For microcontroller boards to pair with these relays, browse our Arduino boards or ESP32 boards. For the relay shield that stacks directly onto an Arduino Uno, see our Arduino shields range.