A light controlled from two points is a two-way circuit; from three or more, there is an intermediate switch in the middle as well. They are among the most common arrangements in any home, and also where most people get stuck when automating: the setup works, but the physical switch stops behaving as it should.
The cause is almost never the wiring. It is a setting.
Before anything else
Switch off at the board and verify the absence of voltage with a multimeter. If you are not electrically trained, this is a job for an electrician.
And one check beforehand that decides whether the job is possible at all: you need a neutral in the back box. Plenty of older boxes carry only live and switched live. With no neutral, the device cannot be powered there and you need to rethink where it goes.
Identifying the conductors
- Live — brown, black or grey.
- Neutral — blue.
- The cable going to the light.
- The return from the switches — here the colour is whatever the previous installer had to hand, so identify it by measuring, not by colour.
The wiring
- Live to terminal L.
- Neutral to terminal N.
- The cable to the light to output O.
- The return from the switches to input SW.
The heart of the job is that last line. The wall switches stop feeding the light and become nothing more than a signal to the device. What switches the light is the relay; the switches on the wall only tell it somebody has pressed.
Understanding that explains everything that follows.
The setting that solves it
With the power back on, configure the device and go to the input type. For a two-way circuit you want edge switch mode.
The reason is how a two-way switch behaves. Unlike a push button, it does not spring back: it stays where you left it. If the device is set to read «contact closed = on», half the time the switch will end up the opposite way round from the light — down turns it on, up turns it off.
In edge mode the device does not look at what position the switch is in, but at whether it has just changed. Every change toggles the light, whichever switch it came from. Which is exactly how an ordinary two-way circuit behaves.
With an intermediate switch in the middle the principle is the same: it reverses the path and produces a change of state, which is precisely what the device is watching for.
When the installation has several
A single two-way circuit takes two minutes to configure. A whole home — hallway, stairs, living room, bedroom — is six or seven devices, all wanting the same setting and all wanting a network and an address.
That is where the SIT-IoT app provisions them as a batch and assigns the static IP in the same pass, instead of going one at a time through each access point with the phone in one hand and the ladder in the other.
The static IP matters particularly here because of where these devices end up: inside the back box, behind the switch. If one loses the network because the router handed out a different address, getting it back means opening the wall. Fixing it at provisioning time costs nothing.
What happens afterwards
A device buried in a wall has an obvious problem: checking it means building work. And they are exactly the ones that drop off most quietly, because a back box behind a metal faceplate is one of the worst places you can put a Wi-Fi antenna.
In the console, each device’s last seen tells you which one has dropped off without touching a screw. In a home with six or seven buried devices, that list is the difference between knowing and finding out when the client rings to say «one of the lights doesn’t work from the phone».
The physical switch, meanwhile, carries on working. That is the point of this arrangement and it is worth explaining to the client: if the device loses the network, the light still switches from the wall. What is lost is remote control, not the light.
If something is wrong
- The light works opposite to the switch: it is not in edge mode. The classic symptom.
- It works from the app but not from the wall: check the switch return on the SW input.
- It will not power up: almost always a missing neutral in the box.
- It loses the network when you close the box: coverage. Test it with the faceplate fitted, not with the box open.
The app and the console work with Gen2, Gen3 and Gen4. The app is on Google Play: SIT-IoT Smart Installer Tool.