How to Wire a Relay for Wireless Control of Your Caravan Accessories and Appliances

If you don't know how to wire a relay correctly, it's always best to seek professional help. Of course, the device is simple to set up, especially for someone who has worked with switches. However, you must remember that incorrect wiring can prevent your load from powering up, trigger a short circuit, or both.

For van projects where you need to control items like Starlink, a water pump, water heater, fridge or electric step, general-purpose power relays are usually the best choice. All of our options in this category include a built-in RF module and come with a remote controller for wireless operation.

In this quick guide, we'll walk you through how to correctly wire a wireless power relay for remote switching of your caravan, campervan, RV or motorhome accessories and appliances. Four examples of such relays are:

• 6 – 36V Wireless Relay Switch with Remote Control
• 6 – 36V DC Leaded Wireless Relay with Remote Control
• 12V DC Wireless Relay Switch with Remote Control
• 9 – 265V Wireless Relay Switch with Remote Control

As we'll cover in this article, relays aren't wired exactly the same. While they appear similar, some are plug-and-play while others require a little terminal bypassing.

SPST vs SPDT Wireless Relays: What's the Difference?

Like most electrical devices, the way a power relay works varies depending on its underlying technology and circuit configuration. The relays we're looking at today are electromechanical relays available in SPST and SPDT configurations.

An SPST (Single Pole Single Throw) relay is the simplest relay configuration. It features a single set of contacts — NO and COM — that close when the relay is actuated. You can therefore only use it to connect or disconnect a single circuit.

An SPDT (Single Pole Double Throw) relay is also relatively simple but has a more advanced configuration. It features a single set of contacts — NC, COM and NO — and is capable of controlling two different circuit paths.

The majority of our wireless relay switches currently in stock are SPST designs, but we hope to add more SPDT versions soon. An SPST relay has four terminals in total, whereas an SPDT version has five terminals.

Quick notes:

• COM — abbreviation for the "common" shared terminal point
• NO — abbreviation for the "Normally Open" terminal
• NC — abbreviation for the "Normally Closed" terminal

How Do You Wire a Wireless Relay for a Caravan Accessory or Appliance?

Generally, how a wireless relay works in your project depends on its wiring schematic. But when it comes to SPST and SPDT relays, the wiring is almost the same.

When you remove either of the two wireless relay modules from its plastic housing, you'll notice it is a fairly simple component. To wire it up for use, you only need the four or five terminal points along the edge of the unit. Depending on the power relay you bought, these terminal points may be either screw terminals or integrated leads.

How to Wire an SPST Wireless Relay with Pre-wired Secondary Terminals

This is a modern SPST relay that has the V+ input pre-wired to the V+ output using copper pathways on the back of the circuit board. This powers both the relay and the load.

The relay also has its V– input wire running to the COM pin, and the NO pin connected to the V– output terminal to create the "decisive" ground path. Overall, the manufacturer has done this to simplify the wiring process. When installing, you'll just need to:

1. Connect the positive wire of your power source to the V+ input terminal of the relay.
2. Connect the negative wire of your power source to the V– input terminal.
3. Hook up the positive wire of the accessory/appliance you want to control to the V+ output terminal. Then, connect the negative wire of the accessory/appliance to the V– output terminal.

As soon as power flows through the relay, the status LED will illuminate. You can then use the integrated "Mode" button and the provided remote control fob to configure the relay to your desired settings.

How Does the SPST Wireless Relay Work?

Once configuration is complete, you can effortlessly control your accessory or appliance with the remote. When you turn on the relay switch:

• The supplied low-current power energises the coil on the primary side and activates its electromagnetic rod.
• The created "magnet" attracts the pivoted armature, which then moves the movable contact (link to COM) on the secondary side of the relay.
• When moved, the movable contact (COM) connects with the fixed (NO) terminal contact. This completes the electrical circuit, allowing ground current to flow out of the NO pin, through the V– output, and into the negative wire of your load.

With the ground path now complete, the current in your load starts to flow and your device turns on.

How to Wire an SPST Wireless Relay without Pre-wired Secondary Terminals

If you're working with the older bare-bone electromechanical relay, the setup described above won't work. The primary and secondary sides of the unit are electrically isolated. Therefore, after connecting your input power, you must manually bridge certain terminals to power your load.

To wire this type of relay, you'll need to:

1. Connect the positive wire from your power source and the positive wire from your accessory/appliance to the V+ input terminal on the relay.
2. Connect the negative wire from your accessory/appliance to the NO terminal of the relay.
3. Hook up the negative wire from your power source to the V– input terminal. Then, use a short jumper wire to connect the V– input terminal to the COM terminal.

When you supply power to the relay, its status LED will light up. Then, you can use the integrated "Mode" button and the supplied remote control fob to configure your preferred settings.

Once the wiring and configuration are complete, this relay will function the same way as the pre-wired model. When power flows into the relay:

• It energises the coil and activates the electromagnetic armature.
• The resulting magnetic field attracts the pivoted armature, moving the movable contact (on COM).
• The movable contact then connects with the fixed NO terminal. This completes the circuit, allowing current to flow and turn on your device.

How to Wire an SPDT Wireless Relay with a Pre-wired COM Terminal

As mentioned earlier, SPDT (5-pin) power relays are the most common type for both general-purpose and automotive applications. Although it is a 5-pin relay, the extra terminal does not significantly change the wiring process — especially if you understand the roles of the COM, NC and NO terminals.

For a modern mechanical relay with the COM terminal permanently bonded to V+ on the PCB, you can easily wire it as follows:

1. Connect the positive wire from your power source to the V+ input terminal on the relay.
2. Hook up the negative wire from your power source to the V– input terminal.
3. Add the positive wire from your accessory/appliance to either the NO or NC terminal: use the NO terminal if you want the load to turn on only when the relay is energised; use the NC terminal if you want the load to be on only when the relay is de-energised.
4. Connect the negative wire from the accessory/appliance to the power source ground or the V– input terminal.

When power flows through the relay, the status LED will illuminate. Then, you just use the relay's "Mode" button and the supplied remote control fob to complete the configuration.

How to Wire an SPDT Wireless Relay without a Pre-wired COM Terminal

In the case of an older, bare-bones electromechanical relay, you can wire it as follows:

1. Connect the positive wire of your power source to the V+ input terminal of the relay.
2. Hook up the negative wires from the power source and your load to the V– terminal of the relay.
3. Take a small wire and bridge the V+ to COM — connect one end to the V+ terminal and the other to the COM terminal.
4. Connect the positive wire of your load to either the NO or NC terminal. The "NO" terminal will only power the load when the relay is on, whereas the "NC" terminal keeps the load powered only when the relay is off.

When you switch on the power supply to the relay, its status LED will illuminate. You can then use the integrated "Mode" button and the supplied remote control fob to complete the configuration.

Should I Choose an SPST or SPDT Wireless Relay?

Both SPST and SPDT relays are available here at Campervan Builders. Each has its ideal applications, with the SPST version being the best choice if you need a relay that you can set up without technical know-how. You simply connect your power source lines and load connections, just like with most devices.

On the other hand, an SPDT power relay is a great choice if you need to control two different circuits. Of course, you can still use it with a single load while leaving the other terminal open for future use.

When operating a single accessory or appliance with an SPDT relay, it is sensible to connect its positive wire to the NO terminal. Otherwise, using the NC terminal will only power off your load when the relay is on. This means your setup will consume power in both states — when the relay is off (switch side) and on (coil side).

In some cases, though, you might want to use both the NO and NC terminals. A few examples include when:

• You want to build an automatic or remote-controlled isolator switch for your solar battery setup.
• You are building an automatic or remote-controlled switch to change the power source of high-load appliances (such as a fridge) from the alternator to the leisure battery, or vice versa.
• You want to create an automatic or remote-controlled switch to safely retract an electric step.

Can I Wire a Wireless Relay and Load on Different Power Sources?

Whether you want to remotely switch your Starlink, water pump, water heater or battery charger on and off, that's how to wire a wireless relay. It's quite simple across all four relay versions, but the SPST relay with pre-wired secondary terminals is the simplest of all.

As we conclude, it's important to note that all four wiring methods we've covered involve the relay and load using the same power source. If you plan to set up your system with different power sources, the wiring becomes even easier. For example, if you're using a bare-bones SPDT wireless relay without pre-wired secondary terminals, you simply:

1. Connect the positive wire from the first power source to the relay's V+ terminal. Then, connect the negative wire to the V– terminal.
2. Connect the positive wire from the load to the positive terminal of the second power source. Then, connect the negative wire from the load to the relay's COM terminal.
3. Connect the negative wire from the second power source to either the NC or NO terminal. If you use the NC terminal, the load will remain on when the relay switch is off. If you use the NO terminal, the load will turn on when the relay is activated.
4. Use the relay's "Mode" button and remote control to configure your desired settings.

Frequently Asked Questions

What's the difference between an SPST and an SPDT relay?

An SPST (Single Pole Single Throw) relay has four terminals and a single set of contacts (NO and COM), so it can only connect or disconnect one circuit. An SPDT (Single Pole Double Throw) relay has five terminals and contacts for NC, COM and NO, allowing it to control two different circuit paths.

What do COM, NO and NC mean on a relay?

COM is the "common" shared terminal point, NO is the "Normally Open" terminal that powers the load only when the relay is energised, and NC is the "Normally Closed" terminal that keeps the load powered only when the relay is off.

What can I control with a wireless relay in my caravan?

Wireless power relays are ideal for remotely switching accessories and appliances such as Starlink, a water pump, water heater, fridge, battery charger or electric step — all from the supplied remote control fob.

Do I need an electrician to wire a wireless relay?

The wiring is simple if you've worked with switches before, but incorrect wiring can prevent your load from powering up or trigger a short circuit. If you're not confident, it's always best to seek professional help.

Ready to add remote switching to your setup? Browse the full range of 12V wireless relays for DIY campervans and caravans at Campervan Builders.