Replace a Single Switch With a Double Without a Second Cable

Have you ever had two electrical appliances on one switch that you would like to control separately without dealing with running a second cable for the second appliance to connect it to a dual switch, breaking of walls and plastering of new ones?

A typical example for this is a bathroom switch which controls the fan and the lamps, since they are connected together and always run simultaneously.

Luckily, we live in 21st century and products which can simulate a double-switch even in case we have a single wire exist.

To complicate matters further, we do not have a neutral cable behind the switch, e.g. a scheme as the one above exist.

This is a typical example of something I encountered in my apartment.

To achieve a double switch with one cable we need:

• 1 Shelly 1L that will stand behind the switch and does not need a neutral wire to work.
• 2 additional Shelly that will control the fan and lamps. In this case, I chose 1PM, but there is no problem to use any other Shelly (for example, Shelly Bulb / Vintage / Duo for a light bulb and Shelly 1 for the fan) or to control another appliance with Shelly Plug / PlugS
• *In case there is a neutral wire behind the switch, we can also use Shelly i3, but in the particular one, I will use Shelly 1L, because it is the only Shelly with a relay that does not require neutral wire.
• *If the total consumption of electrical appliances is below 20W, we also need Shelly Bypass.

Shelly 1L, behind the key will act as an intermediary between the key itself, which will be connected only to it (Sx, SW1, SW2) and the other two Shelly.

The Shelly 1L's output will always be on, thus powering the other two Shelly 1PMs. If the total consumption of the appliances that Shelly 1L supplies is below 20W, it is necessary to add a Bypass.

More about connecting Shelly 1L can be found here: https://shelly.cloud/documents/user_guide/shelly_1l.pdf

The two Shelly 1PMs, in turn, will turn electrical appliances on and off.

Shelly 1L will command the other Shelly with Device to Device Communication, which you can read more about here: https://shelly.cloud/documents/developers/ddd_communication.pdf

We connect Sx from Shelly 1L to L on the key, SW1 to first key, and SW2 to the other one.

Shelly 1L is small enough to fit behind the key, and in case you encounter any difficulty, you can use the ready-made Wall Switch for Shelly: https://shelly.cloud/wall-switch-for-smart-relays/

I will first set up Shelly 1L and the other two Shelly. We connect them to WiFi and update the firmware using the Shelly Cloud App.

Only Shelly 1L needs additional settings for the purpose:

• Settings -> Power on default mode -> On (Configure Shelly relay to turn ON, when it has power) - because the relay must always be on to power the other two Shelly
• Settings -> Button 1 Type -> Detached (switch is separated from the relays)
• Settings -> Button 2 Type -> Detached (switch is separated from the relays)

To turn the relays on and off via DDD Communication we need to know their IP addresses.

It is good to set them static so that the communication does not break down. We can achieve this from the router if there is a DHCP reservation option, or directly from Shelly App from the menu Internet / Security -> Wifi Mode - Client -> Set static IP address

Set the commands in Shelly 1L from the I/O URL Actions menu of the tabs:

• "Button 1 Siwtched ON URL" - http://{first_shelly_ip}/relay/0?turn=on
• "Button 2 Siwtched ON URL" - http://{second_shelly_ip}/relay/0?turn=on
• "Button 1 Switched OFF URL" - http://{first_shelly_ip}/relay/0?turn=off
• "Button 2 Switched OFF URL" - http://{second_shelly_ip}/relay/0?turn=off

We can also use http://{shelly_ip}/relay/0?turn=toggle, this way the status of the relay will change every time the key is pressed, even if it is changed from another place.

In this way we successfully have a double switch, through one cable, without breaking and running any additional cables.