433MHz sensors and devices are good value

Today’s wireless sensors send their measurements and key presses over Bluetooth, Wifi, Z-wave and Zigbee. They send them to displays and light bulbs, phones and tablets. When you use a home automation platform like Home Assistant (need explained here), you can integrate totally different brands so that they interact in useful ways.

Yesterday’s sensors and gadgets mostly used 433MHz (also called RF ) for wireless communication. The sensors include door contact; PIR motion sensor; temperature or water leak sensor; bell push; wall switch and remote key / button. The gadgets include a socket; relay; chime; siren; door lock solenoid and lamp fitting. The 433MHz frequency is still used – in doorbells or cheap remote controlled sockets or home security systems. I unknowingly had a lot of this technology – for example the RF control for ceiling LED lights (group photo below) uses the 433MHz band. Best of all the signal can travel the length of the house and the batteries inside last for months or a year. A modern wifi device would need to be near a power outlet. The signals probably wouldn’t travel as far and if they did you’d need to pay bigger money for it.

This page is about integrating older stuff with the newer stuff. We upcycle some of my box of gadgets that use a frequency that’s unfashionable. And we buy a few new sensors for crazy, nice prices of £5 to £10. If you wanted a phone alert when the basement was flooding; or you wanted the heating to respond to temperatures in different rooms, here’s a way forward.

see also: 433MHz smarter letterbox
The first thing to become clear about is that 433MHz devices are either signal ‘senders’ or signal ‘receivers’. On the left is a light relay (receiver) and a socket (also a receiver). The other devices send / transmit signals into the ether and a receiver will react if it’s been taught the signal.

how it works (easy) and how it doesn’t work

How it usually works: get yourself a 433MHz button and say, this mains plug-in wireless chime (M2D 300m 433MHz 60 chimes) as shown below. Press the chime button to select a tune; press the set button for a couple of seconds. An LED will probably turn on to show it’s ready to learn a button. Press the white button and the chime will now sound when the doorbell is pressed.

It is common for different 433MHz devices to be compatible with each other. For example, the chime can learn the signal from the door contact sensor above – and you can set a different chime sound should the door be opened. You can also teach the chime the signal from a PIR sensor (main picture) so that anyone crossing its path will cause a suitable chime sound.

Similar set up to above. The Digoo ROSA wireless alarm/siren can also be taught to respond to these sensors. The massive noise this device makes is appropriate for home security and less good for a doorbell!

How it doesn’t work: after experimenting with a lot of devices (and do try everything a few times each) you’ll have a lot of happy surprises as to what works – but also see that not everything 433MHz is compatible with each other. And even if you could pair two devices, you also can’t use say, the doorbell button to send an alert to your phone; or turn on a light or boost the water heating or close the blinds. But you CAN if you get this Sonoff RF Bridge. The bridge reads many kinds of 433MHz signals and let’s me use cheap sensors to trigger events.

Sonoff® RF Bridge wifi and 433MHz

make use of a Sonoff RF bridge

  • This device is a 433MHz receiver that listens for RF signals such as a press on that white doorbell button. Install the app Ewelink on your phone and use it to learn your sensors.
  • The device is also a 433MHz sender such that it can send a signal to say, a RF socket. You can now use the app to set up a time schedule for a device. For example, if my laptop charger was plugged into an RF controlled socket, I’d make the socket turn off a couple of times a day. That way I can’t overcharge a battery and ruin it as I’ve done several times. I have smart socket controlling an electricity-hungry set of lights and they’re on a schedule that only turns them off. A motion sensor turns them on as required.
  • Finally this Sonoff device is indeed a bridge – it receives 433HHz signals and can then send out wifi signals to Sonoff and RF devices. The video below by Csongor Varga will get you started to appreciate the connections or the bridge you’ve now made between different systems.
  • There more fun to be had by connecting Ewelink to IFTTT or Amazon Alexa or Google Home. These topics are well covered on youtube.
With thanks to Csongor Varga for a very useful and never too technical youtube channel

going further with the Sonoff RF bridge – Tasmota & MQTT

I’ve so far described sensing and controlling at the phone app level but you may have devices that you really need to work in the way you want. The price to pay is in the time to use different software on the Sonoff RF bridge and connect it to Home Assistant. It took me a year to get to a point where I was missing doorbells (solved by having multiple alerts) or I wanted to know not just the temperature of the room but to also have a graph of how fast the room heats up and cools down.

You’ve a choice of software to use – there’s openmqttgateway which is a key to having a bridge that responds to a variety of 433MHz, Bluetooth and other senders. The software turns received data into MQTT messages which Home Assistant can process. The software I used for my Sonoff RF bridge is called Tasmota which also turns received RF into MQTT messages for Home Assistant. Tasmota can be installed on many modern devices. I went for it because has a web interface that makes it easy to see what’s being sent.

What follows involves taking apart your bridge. You’ll need some ‘female headers’, Dupont jumper wires, a 3.3v FTDI unit and some flashing software.

optional: how to flash Tasmota on an RF bridge

With thank to alsolh for producing one of the first and few guides to flashing the RF bridge.

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