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A long hallway with a light switch on the right-hand wall and a second switch further down the same wall.

Add a second light switch without running new cable

How it works

A Shelly i4 Gen3 sits behind the second switch. It doesn't switch anything itself: it reads the switch and, each time you flip it, sends a toggle command over your Wi-Fi to the Shelly that runs the light.

That's a Shelly 1 Gen4 behind the light's existing switch. The existing switch keeps working through the relay's own input, so both switches change the light, whichever was used last.

  1. i4 Gen3 01

    Your second switch

    Flip the switch and the i4 behind it sends the light a command over Wi-Fi.

  2. Shelly app main controls for a relay named Hallway light, switched on, with the Turn off button. 02

    Your phone

    Set up the two switches once, in the app.

  3. 03

    Your light

    Changes from either switch, every time.

What it looks like on your phone

  • Main controls. The light shows on or off here, whichever switch you used last.

  • Activity log. Every time the light changes, it's listed with the time.

  • The second switch. Two actions on the i4 change the light each time it's flipped, on or off.

Step by step

How you'll set it up

Add the device to Wi-Fi from your phone, or straight from a browser.

Download Shelly Smart Control on the App Store Get Shelly Smart Control on Google Play

  1. Fit the i4 behind the second switch

    Live goes to L and neutral to N. The switch takes live on one side and connects to SW1 on the other.

    Wire map i4 Gen3
    • Active (supply) L
    • Neutral N
    • Second switch SW1
    Circuit overview
    LN230 V~ 50 Hz1Second switchSW4SW3SW2SW1LLNShelly i4 Gen3
    1Second switch
    Active (supply)Neutral / 0 VSwitch signal / dry contact

    In Australia, fixed 230 V wiring is licensed-electrician work. Use these diagrams to plan the job and brief your electrician.

    Showing the whole circuit

    Shelly i4 Gen3

    100%
    LN230 V~ 50 Hz1Second switchSW4SW3SW2SW1LLNShelly i4 Gen3
    1Second switch

    Select a terminal to follow its wiring.

  2. Fit the Shelly behind the light's switch

    Mains goes to L and N, a short bridge from L feeds I, and O carries the switched active to the light. The light's existing switch lands on SW, so it keeps working.

    Wire map 1 Gen4
    • Active (supply) L
    • Neutral N
    • Bridge from L I
    • Light or appliance O
    • Wall switch SW
    Circuit overview
    LN230 V~ 50 Hz1Light / appliance2Wall switchOISWLNShelly 1 Gen4
    1Light / appliance2Wall switch
    Active (supply)Neutral / 0 VSwitched loadSwitch signal / dry contact

    In Australia, fixed 230 V wiring is licensed-electrician work. Use these diagrams to plan the job and brief your electrician.

    Showing the whole circuit

    Shelly 1 Gen4

    100%
    LN230 V~ 50 Hz1Light / appliance2Wall switchOISWLNShelly 1 Gen4
    1Light / appliance2Wall switch

    Select a terminal to follow its wiring.

  3. Add both to the Shelly app

    Add the i4 and the relay in the Shelly app and put them in the same room. Name the relay after the light (Hallway light) and the i4 after where it is (Hallway second switch).

    Learn how to add a device to the Shelly app

  4. Set the relay's switch to Edge

    Open the relay, then Settings (the cog at the foot of the menu) and Input/output settings. Under Set relay type, choose Set Shelly device to be "Edge" switch. Under Set relay power on default, choose Restore the last mode it was in, then tap Save. Edge makes every flip of the light's own switch change the light, so it never falls out of step with the second switch. In the web interface, open the relay's output, then Input/Output settings. Click Edge and Restore last known state of output/relay, then click Save.

  5. Toggle the light when the switch turns on

    Open the i4, then Actions, and tap Add action. Name it Switch turned on and tap Next twice. Under When, tap Input on, then tap Next. Tap Add action from another device, pick the relay and tap Next, choose Toggle Output and tap Save, then Save again. In the web interface, open Actions and click Create action. Name it, set Event to Input toggled on, and under Then do click Add URL and enter `http://` followed by the relay's IP address and `/rpc/switch.toggle?id=0`. Then click Save.

  6. Toggle the light when the switch turns off

    Add a second action the same way, named Switch turned off. Under When, tap Input off, then tap Next, and give it the same Toggle Output to the relay. In the web interface, create it with the Event Input toggled off and the same URL under Then do, then click Save.

  7. Check both actions

    Actions now lists Switch turned on and Switch turned off, both switched on. Flip the second switch and the light changes. The web interface's Actions page shows the same two.

Frequently asked questions

Should I use the Shelly app or Home Assistant?

If the light is on a Shelly and you don't run Home Assistant, use the Shelly app: the i4 sends the command straight to the relay, with no hub. If Home Assistant already runs your home, or the light is something else, such as Hue bulbs, use Home Assistant.

Does the switch box need a neutral?

Yes. The i4 is powered from live and neutral, so the box for the second switch needs both.

Does the light's original switch still work?

Yes. It stays wired to the relay's SW input, and with the input set to Edge every flip changes the light. Neither switch has an on or off position any more: each one just changes the light.

What happens if the internet drops out?

The second switch keeps working. The i4 sends its command straight to the relay over your home Wi-Fi, so it doesn't go through Shelly's cloud. It does need the Wi-Fi itself: if your home network is down, only the light's own switch works.

Can I use a push button instead of a rocker?

Yes. Set the i4's input to Button, and use one action on Button is pushed that toggles the light.

What about a third switch?

Fit another i4 at the third spot and give it the same two actions. Each i4 talks to the light on its own.

Does it work while Home Assistant is down?

No. The automation runs in Home Assistant, so the second switch does nothing until it's back. The light's own switch keeps working. If you'd rather the two switches didn't depend on it, link them in the Shelly app instead.

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