433MHz (aka 'RF') sensors and devices are good value

How to make inexpensive, ‘old’ technology, 433MHz devices work with today’s smart devices

see also: 433MHz smarter letterbox

RF sensors and receivers

If you’ve used any of the following examples you probably own some of the ‘old’ technology discussed on this page. I unknowingly had a lot of it – a RF remote control for my ceiling LED lights (picture below) used the 433MHz RF band.

  • A wireless ‘RF’ doorbell sends a radio signal to a chime.
  • A door contact sends a signal to a burglar alarm.
  • A motion sensor send a signal to a light.
  • A water leak sensors ends a signal to a siren
  • A wall switch or remote control sends a signal to a socket, or a relay or a door lock.
  • A push switch sends a signal to a garage door

types of wireless

Sensors that tell you say, whether a door is open can signal whether they are on / off using Bluetooth, Wifi, Z-wave or Zigbee. Yesterday’s sensors and gadgets mostly use 433MHz (also called RF ) for wireless communication.

The 433MHz frequency is still used in remote controlled sockets and home security systems. The 433MHz signal can travel the length of the house and the battery in say, a doorbell push will last for months or a year. These switches are inexpensive (a few £ each). In comparison, a switch that works over wifi needs to be plugged into a power outlet and its signals probably don’t travel far.

making 433MHz smarter

433MHz RF systems unfortunately aren’t part of smart home automation platforms such as Google or Alexa. For example, if you wanted an alert on your phone when the basement was flooding; or you wanted the heating to respond to temperatures in various rooms, you’d need something proprietary. The way forward is to use a bridge e.g a Sonoff RF bridge – to connect them. This bridge connects your RF devices to home automation platforms like Google Home or Home Assistant (described here). Your RF devices can now talk to the home automation system. Hence projects such as this 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.

pairing is how 433MHz works in practice

Get yourself a 433MHz push button and the plug-in wireless chime (M2D 300m 433MHz 60 chimes) as shown below. Often they don’t need to be the same brand. Press the chime button to select a tune; press the set button for a couple of seconds. An LED will 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. 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 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 with what. You’ll see that not every 433MHz device is compatible with everything – for example the HomeEasy RF system is its own system. A garage door remote need to send a secure code to the lock (called a ‘rolling code’).

If you also wanted a doorbell button to send an alert to your phone or computer; or turn on a light or boost the water heating or close the blinds, the next step is to integrate the RF system with the wifi system of a home automation platform. So get the Sonoff RF Bridge. The bridge reads many kinds of 433MHz signals so you can use its 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 above. Install the app Ewelink on your phone and use it to ‘learn’ your sensor’s code.
  • The bridge can also send a 433MHz signal to say, a RF socket. You can use the app to set up a time schedule for a socket. 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 sockets controlling electricity-hungry lights and they’re on a schedule that turns them off every two hours. A motion sensor turns them on as required.
  • This Sonoff RF bridge is indeed a bridge – it receives 433MHz signals and it can send out RF signals to RF devices and inform you via wifi to your phone. The video below by Csongor Varga illustrates 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.
With thanks to Csongor Varga for a very useful and never too technical youtube channel

going further with the Sonoff RF bridge – Tasmota & MQTT

Even better news: since 2020, a Home Assistant custom component allows you to use the Sonoff RF bridge without the need to open it and flash it with new firmware.

I’ve so far mentioned sensing and sending RF with the bridge and phone app. You may have devices that you need to work in the way you want – for example, I want to know not just the temperature of a room but also to see a graph of how fast the room heats up and cools down. For this I installed different software on the Sonoff RF bridge and connected it to Home Assistant.

There is a choice of software to use on the RF bridge – there’s openmqttgateway which provides 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.

Scarily, 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 thanks to alsolh for producing one of the first and few guides to flashing the RF bridge.

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