Shelly 2.5 relay with wiring inside yellow flushbox on white wall

Why Smart Relays?

You might ask: What is a Smart Relay?

A relay is an electrically operated switch that can control the flow of electricity in a circuit. A Smart Relay is a relay that can be added to an existing electrical circuit to allow remote wireless control of anything else connected to that circuit. This means you are able to control just about any device, appliance or electrical fitting by adding a Smart Relay in to the same circuit.

A Smart Relay allows you to use your existing switches and power points without the need to change them. For example, if you have solar electricity at your home you can optimize devices such as heated towel rails or dryers to turn on when the sun is out. Or you could have your garage door open automatically when you get near your home. Some relays like the Shelly 2.5 are designed for roller blinds so you can set up a sunrise/sunset routine where your blinds open  and close automatically depending on the time of day. And/or use voice control with Alexa or Google Home.

How do relays work?

There are several options when it comes to how smart relays work and which one you choose will depend on the use case.

Does your switch have a neutral wire? Some older houses have the neutral only at the device such as a light and only run the 2 wires to the switch.
If you look behind your switch and there are only 2 wires then you don’t have a neutral and the Shelly 1L is your best option as a relay. The Shelly 1L has a maximum load 4.1amps at 230V for a total of 922 watts and requires at least a 20W load to run the relay and wifi otherwise you will need to add in an additional Shelly bypass to keep it running.
If you require dimming the Shelly Dimmer 2 can dim up to 230W of LEDs or Halogen Bulbs and doesn’t require a neutral either. With the Shelly Dimmer 2 you can run 2 circuits at 0.1-1.1amps. The Dimmer2 requires at least a 10W load to run the relay and wifi otherwise like the Shelly 1L you will need to add in an additional Shelly bypass to keep it running.
If you do have a neutral the next question is how many circuits do you want to run and how many amps?
If you have 2 switches and each circuit is under 10amps (like a typical lighting circuit) the Shelly 2.5 can operate 2 switches independently.
For a larger load like a resistive heater then you should probably use Shelly 1PM as it can handle up to 16amps (3.5kW at 240V) and will give you the added benefit of monitoring energy consumption which can be displayed through the Shelly Cloud app.

The Shelly 1 is a dry contact relay and can also control 12V and 24-60V as well as 240V. The dry contact feature allows you to power the relay via a different voltage to the one you are controlling.

To summarise: 
No neutral and under 4A load – Shelly 1L
No neutral and dimming 0.1-1.1amps - Shelly Dimmer 2
Neutral, 2 Circuits, 10A (roller shutter as well) – Shelly 2.5
Neutral, up to 16A – Shelly 1PM
Neutral, Dry Contacts, 12VDC – Shelly 1

Shelly relays are small enough to be hidden away behind a fitting or at another point within the circuit which makes for a sleek and unobtrusive addition. If you have Google Assistant and/or Amazon Alexa you can control these devices with your voice too.

Why do installers love Smart Relays so much?

Smart Relays are extremely versatile devices that can solve a load of headaches for professional installers and qualified electricians. When paired with a wider home automation system, an installer can set up a wireless ‘magic button’ from any Smart Dimmer that triggers a Smart Relay. This means that, between the switch and the light fitting or device, there is no need for additional wiring.

Some great examples of this would be using a relay to control a bathroom extractor fan, or using a relay for your garden lights which can be controlled from any Smart Dimmer around your entire home. 

Getting Started

Installing and configuring your Smart Relay can be a little more complicated than Smart Sockets or Switches, so we always recommend getting in touch with a professional to help you out.

If you have unique light switches such as colour matched or antique switches just add a Smart Relay and you won't have to replace them.

Here is a video from Jimmy James showing installation and setup of a Shelly Dimmer. The set up he uses is the preferred method for adding the device to your network although he uses Home Assistant which isn't necessary.


 This article has been adapted from an original blog post from Lightwave dated 04/07/2021


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