Making a dumb light switch smart

A few months back, when planning the rewiring of our house, we decided not to include any switched 5A sockets; you know the kind of arrangement where a switch on the wall can turn on some floor lights and lamps in a room? If you want light, but something subdued, it gives you an alternative to turning on the “big light”

We had this arrangement in our previous house and we used it all the time, but for some reason, we opted to do without in our new house. Really don’t know why.

After living in the house for a few weeks it became painfully obvious that we had made a mistake. Yes, we can turn the lamps on individually, but it’s just so damn convenient having a switch!

I have WiFi smart plugs, which I could use, but these just make matters worse as you usually need your phone to turn on and off the lights and aren’t great for guests.

I began thinking of how I might make a dump light switch turn into a smart light switch.

I had a flash of inspiration, when I began thinking about the Aqara light switches I have (in boxes now) and how they had a battery powered model. They are just a push button switch with a Zigbee module. I wondered how I might break one of those apart and wire it in to the my MK switch. I did some googling and I found a YouTube video which showed exactly that – the only downside was that they were using a push switch!

Then I had the light bulb moment; Contact sensors! I did more googling and sure enough, loads of people had just connected standard Zigbee contact sensors into an existing light switches. It was the perfect arrangements. Contact sensors have simple magnetic reed switches, which send the appropriate of open or closed. Swap the reed switch for a mechanical switch and the operation should be exactly the same!

An Aqara contact sensor, opened up

I opened up one of the Aqara contact sensors I had, and inside, I found a small little battery powered module. On the reverse, the reed switch was visible. Please forgive the photo. I didn’t realise it was that blurred.

The reed switch is operated by an external magnet and indicates if the contact is open or closed
I removed the reed switch and added a wire to each of the contact plates

I removed the reed switch and put two wires onto the contacts. After pairing the device with Home Assistant, I was delighted to see that touching the wires put the sensor into a closed position. The next step was putting into a standard switch.

Put the contact sensor into a real switch

That was easy. The switch now successfully opened and closed the contact sensor. I originally used the normally open P1/COM connectors on the switch, but Home Assistant equates “open” with on, so I ended up using P2/COM, so when the switch is Off, the contact is closed.

With the switch working as expected, I created a simple automation in Node-Red to turn a smart plug on and off. Nothing fancy. When the contact is open, turn on the light and when closed turn off the light.

My thoughts turned to actually then installing the light switch and whether or not the zigbee signal would get through the metal backing box of the switch.

Please don’t do anything with electricity without first isolating at the fuse board. If you’re not comfortable doing this, please don’t. Electricity can kill.

I started by opening up the exiting one gang switch and the wiring was as expected. One switched live and

The existing single gang switch
The two gang switch in place
It works!!