Introduction

This program can read data from cheap 433 MHz wireless temperature and humidity sensors and integrate with home automation systems using MQTT broker. It was designed to work with popular ARM boards like Raspberry Pi, Orange Pi, and many others. Home Assistant MQTT sensor discovery is available.

To make it work you need only one external component – 433 MHz receiver. It has to be connected to one of the I/O pin, the program can decode directly data from sensors without any third party additional components. It supports Nexus sensor protocol which is implemented in many cheap sensors.

Features

Works on popular ARM Linux boards
Only 1 EUR external part required
Supports 5 EUR cheap Temperature and Humidity sensors
Tracks sensor connection quality
Reports sensor online and offline status
Seamless integration with Home Assistant
Easy integration with MQTT home automation systems
Many configuration options

Applications

Home automation
Temperature monitoring
433 MHz to MQTT gateway
433 MHz sensor diagnostics

License

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

Hardware

Supported boards

Any board that can run Linux with block device gpiod I/O driver can be used. That means any board with a modern version of Linux is supported. Specifically:

Orange Pi running Armbian version 5.x. All boards.
Raspberry Pi running Raspbian. All boards.
Any board running Armbian 5.x.

433 MHz receiver

Any of cheap and popular receivers can be used. Make sure it is superheterodyne and supports ASK/OOK modulation. The cost of the receiver is about 1 EUR when ordering directly from China.

Sensors

There are many vendors that produce temperature sensors compatible with Nexus protocol. Here are a few that are confirmed to use Nexus protocol:

Digoo DG-R8H – Temperature and Humidity sensor. Easily available on the internet. Cost 5 EUR.
Sencor SWS 21TS – temperature only.

Nexus protocol

A device sends every minute or so (Sencor every 58 seconds, Digoo every 80 seconds) 12 data frames.

Each data frame consists of 36 bits. There is no checksum.

The meaning of bits:

Bits	8	1	1	2	12	4	8
Meaning	ID	Battery	0	Channel	Temperature	1111	Humidity

ID – unique ID; Sencor generates new ID when the battery is changed, Digoo keeps the same ID all the time.
Battery – low battery indicator; 1 – battery ok, 0 – low battery
Channel – channel number, 0 – first channel, 1 – second and so on.
Temperature – signed temperature in Celsius degrees.
Humidity – humidity

Every bit starts with 500 µs high pulse. The length of the following low state decides if this is 1 (2000 µs) or 0 (1000 µs). There is a 4000 µs long low state period before every 36 bits.

How it works

The program is written in C++. It supports the new block device GPIO subsystem. It uses libgpiod C library for I/O operations. For MQTT communication Mosquitto client libmosquittpp is linked. INI file parsing with the help of https://github.com/jtilly/inih.

One input pin is used to read data from 433 MHz receiver. Any pin can be used for this operation. There is no requirement pin supports events.

The state of the pin is probed to detect valid frames. When 36 bits are read the data is converted to JSON and pushed to MQTT broker. In addition, program tracks when a new transmitter is detected and when the transmitter becomes silent. This information is also pushed to MQTT.

New transmitters can be automatically detected by popular home automation system Home Assistant.

Standard operations

Nexus protocol does not offer any way to check if the frame was received properly. The software accepts data from the sensor only when the minimum 2 valid frames were received. A valid frame is when temperature and humidity values are reasonable and fixed 4 bits are set to 1.

All sensor data is sent as one JSON package in topic nexus433/sensor/XXXX/state when XXXX is unique transmitter ID made of MAC address, ID (first byte) and channel number (second byte) in hex format. For example ae01 means transmitter with id 0xAE and channel 2 (channels are 0 based).

In addition, when the transmitter first sends data program sends online on topic nexus433/sensor/XXXX/connection. If there is no data from the sensor for 90 seconds, the program sends offline to the same topic.

When the program starts and stops it sends online or offline to global connection topic: nexus433/[MAC]/connection.

For newly detected transmitters specially formatted JSON data is sent to homeassistant topic to make Home Assistant automatically discovers the sensor. For one transmitter 4 sensors are created: temperature, humidity, battery, and quality.

Battery value is either 100 (normal) or 0 (low). These values are compatible with Home Assistant battery class. They can be changed in the configuration file.

Quality is the percentage of received frames. 100% is when all 12 frames are received.

Configuration

Command line options

Short	Long	Description
	`--verbose`	Enable verbose mode. More information is printed on the screen.
	`--daemon`	Run in daemon mode. Program works in background without a console. When verbose mode is on, it will prints messages.
`-g`	`--config`	Path to configuration file. Configuration options from file got higher priority than command line options. Default config file is `/etc/nexus433.ini`.
`-c`	`--chip`	GPIO chip device name, by default `/dev/gpiochip0`.
`-n`	`--pin`	Pin number to use from specified GPIO chip, default 1. This is not physical pin number but number assigned by block device GPIO. See discussion below for more info.
`-a`	`--address`	MQTT broker host, default `127.0.0.1`.
`-p`	`--port`	MQTT port number, default `1883`.
`-h`	`--help`	Displays help message.

Port number and GPIO chip name

Pin numbers are assigned by block GPIO device driver. To see available lines you can use gpioinfo utility from the libgpiod library.

Raspberry Pi

Only one GPIO device is available /dev/gpiochip0. Pin numbers are the same as GPIO line number, for example, physical pin 11 is GPIO17. Pin number for that line is 17.

Orange Pi

Boards based on Allwinner H3 chip got 2 I/O devices:

/dev/gpiochip0 with 223 I/O lines
/dev/gpiochip1 with 32 I/O lines

Port PA lines are assigned to 0–7, PBB to 8–15 an so on. PL port is the first assigned to chip number 2.

Configuration file

Thanks to configuration file you can alter the way program behave. The file is a typical INI file divided into sections. Every section got its own configuration keys.

By default, the program tries to open /etc/nexus433.ini file. This location can be changed using -g/--config command line option.

Strings must be entered without " characters. Boolean values accepted: true/false, yes/no, 1/0. Comments must begin with ;.

`[transmitter]`

Key	Type	Default	Description
`silent_timeout`	number	90	When no data is received during this time in seconds, transmitters are treated as offline.
`minimum_frames`	number	2	Minimum number of data frames to accept transmission as valid.
`battery_normal`	string	100	String sent in JSON MQTT message when the battery level is normal.
`battery_low`	string	0	String sent in JSON MQTT message when the battery level is low.
`discovery`	bool	false	Enable or disable Home Assistant MQTT sensor discovery.
`discovery_prefix`	string	homeassistant	Home Assistant MQTT discovery topic.

`[receiver]`

Key	Type	Default	Description
`chip`	string	/dev/gpiochip0	GPIO device
`pin`	number	1	I/O pin number
`resolution_us`	number	1	decoder resolution in microseconds. Lower is better but higher system load.
`tolerance_us`	number	300	± tolerance of pulse length in microseconds.
`internal_led`	string		LED device name from `/sys/class/leds` used to indicate new readings; disabled by default.

`[mqtt]`

Key	Type	Default	Description
`host`	string	127.0.0.1	MQTT host name.
`port`	string	1883	MQTT port number.
`user`	string		MQTT user name.
`clientid`	string	nexus433	MQTT client id. If more than one instance of the program is connected to the same MQTT broker, use a different ID for each client.
`password`	string		MQTT password.
`cafile`	string		Path to a file containing the PEM encoded trusted CA certificate files.
`capath`	string		Path to a directory containing the PEM encoded trusted CA certificate files.
`certfile`	string		Path to a file containing the PEM encoded certificate file.
`keyfile`	string		Path to a file containing the PEM encoded private key. If encrypted, the password must be passed in `keypass`.
`keypass`	string		Password to encrypted `keyfile`.

SSL/TLS support

To enable SSL/TLS encrypted connection to MQTT broker, specify CA certificate and optionally client certificate.

For CA certificate, one of two options must be set: capath or cafile. For capath to work correctly, the certificates files must have .pem as the extension and you must run openssl rehash <path to capath> each time you add or remove a certificate.

If capath or cafile is specified without any other SSL/TLS option the client will verify Mosquitto server but the server will be unable to verify the client. The connection will be encrypted.

To specify a client certificate set certfile, keyfile and optionally keypass.

ℹ️ Please note that the traffic to MQTT broker would be encrypted but temperate and humidity data from sensors is available for everyone as radio transmission is not encrypted in any way.

`[ignore]`

List of transmitter IDs (2 bytes, ID and channel number) to be ignored. Data from ignored transmitters are not sent to MQTT.

Key must be 2 byte ID and the value is true if the transmitter is ignored, otherwise false. For example to ignore transmitter 0xAE on channel 1 add: ae00=true.

`[substitute]`

This section allows changing the original ID of the transmitter to another number. This may be useful when the sensors are broken or the battery was changed and you do not want to reconfigure existing infrastructure.

Key must be an ID of the transmitted and the value new ID. For example, to change 0xAE channel 1 to 0x78 channel 2 use: ae00=7801.

`[temperature]`

Name of the temperature sensor reported to Home Assistant discovery mechanism for the specified transmitter. Key must be 2 bytes ID, the value is the name, for example: ae00=Kitchen Temperature If not specified the default value of Temperature Id:XX ch Y will be reported.

`[humidity]`

Name of the humidity sensor reported to Home Assistant discovery mechanism for the specified transmitter. Key must be 2 bytes ID, the value is the name, for example: ae00=Kitchen Humidity If not specified the default value of Humidity Id:XX ch Y will be reported.

`[battery]`

Name of the battery sensor reported to Home Assistant discovery mechanism for the specified transmitter. Key must be 2 bytes ID, the value is the name, for example: ae00=Kitchen Sensor Battery If not specified, the default value of Battery Id:XX ch Y will be reported.

`[quality]`

Name of the quality sensor reported to Home Assistant discovery mechanism for the specified transmitter. Key must be 2 bytes ID, the value is the name, for example: ae00=Kitchen Sensor Connection Quality If not specified the default value of Quality Id:XX ch Y will be reported.

Installation using Debian package

Download appropriate package. Check board name and platform. armhf means 32 bit ARM platform (not RPI4).

Verify package signature

This step is optional but highly recommended. First download signing key from key server (only once). You need gpg installed (sudo apt install gpg).

gpg --keyserver pgp.mit.edu --recv-key 9E4441F6F3B07930

Now install tool to vefiy debian packages.

sudo apt install dpkg-sig

and finally verify signature

dpkg-sig --verify nexus433_1.0.2-raspberrypi_armhf.deb

You should see GOODSIG if the signature is valid (the rest of the numbers ay differ). That means the package was not altered and was created by nexus433 maintainer.

Processing nexus433_1.0.2-raspberrypi_armhf.deb...
GOODSIG _gpgbuilder 513D72E653874FC357849F759E4441F6F3B07930 1613422321

Now install package

sudo apt install ./nexus433_1.0.2-raspberrypi_armhf.deb

After installation, change name of nexus433.ini.example to nexus.ini and edit options.

Manual build and installation

Build system is based on cmake.

Install cmake

sudo apt install -y cmake

Install mosquitto C++ library

sudo apt install -y libmosquittopp-dev

Install libgpiod C library

sudo apt install -y libgpiod-dev

Clone git repository:

git clone https://github.com/aquaticus/nexus433

Build

First call cmake, you do it once.

mkdir release
cd release
cmake ../nexus433 -DCMAKE_BUILD_TYPE=RELEASE

Now build

make -j

and install

sudo make install

If default build configuration is used this copy:

nexus433 to /usr/local/bin
nexus433.ini.example to /etc
nexus433.service to /etc/systemd/system/

You must change the name of nexus433.ini.example to nexus.ini and fine tune the options after install.

Build options

Every time you change something in the source code you must call make (no need to execute cmake).

The build may be optionally modified by passing arguments to cmake. For example to change the default name of the configuration file, call cmake ../nexus433 -DINSTALL_INI_FILENAME=my.ini.

List of useful build parameters:

Name	Description
`INSTALL_INI_DIR`	Directory where the configuration file is stored, default `/etc`
`INSTALL_INI_FILENAME`	Configuration file name, default `nexus433.ini`
`GPIOD_DEFAULT_DEVICE`	Default gpiod device name where 433MHz receiver is connected, default `/dev/gpiochip0`
`GPIOD_DEFAULT_PIN`	Default pin number name where 433MHz receiver is connected, default `1`
`CMAKE_BUILD_TYPE`	Build type: `DEBUG`, `RELEASE`, `RELWITHDEBINFO`, `MINSIZEREL`
`CMAKE_INSTALL_PREFIX`	Install prefix, default `/usr/local`
`ENABLE_FAKE_TRANSMITTER`	Enable simulated transmitters. Usefull for development.

By default build script detects the board type and sets GPIOD_DEFAULT_PIN. This selects pin when no configuration is available or pin was not set by -p/--pin option.

Currently, Raspberry Pi and Orange Pi boards are recognized. Actually, these parameters can be easily overridden in the configuration INI file, so no problem when the board is unrecognized. When the project is cross compiled it may be useful to force board type by passing to cmake -DBOARD=RASPBERRYPI or -DBOARD=ORANGEPI. Even when the target board is set, the executable is still portable as the only board specific parameter is default pin number.

Debugging

To debug the application, first generate DEBUG configuration and then compile

mkdir debug
cd debug
cmake ../nexus433 -DCMAKE_BUILD_TYPE=DEBUG
make

The above compiles sources (with DEBUG macro defined) and generates debug information.

It may be also useful to generate project files for Eclipse (if you use Eclipse for development):

cmake ../nexus433 -G"Eclipse CDT4 - Unix Makefiles"

Quickstart

433 MHz receiver got typically 3 pins: VIN, GND, and DATA. For Raspberry Pi and boards with a compatible connector like Orange Pi connect VIN to pin #1 (3.3V), GND to pin #6 (GND) and data to pin #11 (GPIO17).

433 MHz receiver	Raspberry Pi	Orange Pi
`VIN`	`1` (+3.3V)	`1` (+3.3V)
`GND`	`6` (GND)	`6` (GND)
`DATA`	`11` (GPIO17)	`11` (PA1)

ℹ️ Note image below shows VIN connected to 5V not 3.3V.

MQTT Broker

Make sure you got any MQTT broker up and running. If not please install Mosquitto. The examples below assume that MQTT broker is installed on the same machine. If not use -a command line option to specify MQTT address and -p for a port number if different than default 1883.

First run

Get your sensor close to the receiver and remove the battery.

Run program in verbose mode and specify pin number.

Raspberry Pi	Orange Pi
`sudo nexus433 --verbose -n 17`	`sudo nexus433 --verbose -n 1`

Insert battery – that way the sensor sends immediately data, you do not need to wait (remember sensor sends data about once per minute). You should see new readings on the screen:

Loading configuration from /etc/nexus433.ini
Reading data from the 433MHz receiver on /dev/gpiochip0 pin 17.
Decoder resolution: 1 µs; tolerance: 300 µs
Connected to MQTT broker.
New transmitter 0x5c channel 2
0x5c910ff38 Id:0x5c Channel:2 Temperature: 27.1°C Humidity: 56% Battery:1 Frames:12 (100%)

The last line is the actual data from the sensor. What is interesting that all 12 data frames were received. That means reception is very good. The next step is to monitor MQTT data. Install mosquitto client:

sudo apt install mosquitto-clients

and use mosquitto_sub to subscribe to interesting topics.

mosquitto_sub -v -t "nexus433/#"

Of course, add -h and -P options if needed to specify MQTT host and port. You should see:

nexus433/sensor/5c01/state { "temperature": 27.3, "humidity": 56, "battery": "100", "quality": 100 }

Now lets see what other data is sent to MQTT broker. Stop both nexus433 and mosquitto_sub by pressing Control-C.

Run mosquitto-sub first and subscribe to an additional topic:

mosquitto_sub -v -t "nexus433/#" -t "homeassistant/#"

Now run:

Raspberry Pi	Orange Pi
`sudo nexus433 --verbose -n 17`	`sudo nexus433 --verbose -n 1`

and wait for the reading (you can use battery trick to limit waiting time). When you receive the first data packets check mosquitto_sub output. This time it logged more lines:

nexus433/connection offline
nexus433/connection online
nexus433/sensor/5c01/connection online
nexus433/sensor/5c01/state { "temperature": 27.4, "humidity": 56, "battery": "100", "quality": 100 }

Note that there are 2 connection topics. When the program starts it publishes online message on nexus433/connection. When stops for some reason or connection to MQTT is lost offline. Because offline is published with retain flag you get this message every time subscribed to this topic. Thanks to this feature when the program is down any client receives this information.

When a new sensor is detected online is published on sensor specific topic. The format is nexus433/sensor/XXXX/connection, where XXXX is 2 bytes sensor ID. When a sensor does not send any data during 90 seconds (this can be configured) on the very same topic offline is published.

LED

It is possible to use one of the built-in LEDs to indicate new packets. When configured, every time valid data is received LED will be on for 500 ms.

To check available LEDs use: ls /sys/class/leds. Depending on your device number and names may differ. Here is the result for Orange Pi PC board:

orangepi:green:pwr   
orangepi:red:status

To use red LED modify nexus433.ini configuration file:

[receiver]
internal_led=orangepi:red:status

Start as a service

make install automatically installs service configuration files. To start the service:

sudo service nexus433 start

To stop

sudo service nexus433 stop

To view service status:

sudo service nexus433 status

To run the service every time the system starts:

sudo systemctl enable nexus433

Home Assistant integration

Home Assistant is a popular open source home automation system. It can automatically discover and configure sensors.

To make this feature working you must first enable MQTT support and enable discovery feature. Make sure you add the following lines to your configuration.yaml:

mqtt:
  discovery: true

For more information read this article: https://www.home-assistant.io/docs/mqtt/discovery/

Nexus433 by default support MQTT sensor discovery. To disable it, modify nexus433.ini

[transmitter]
discovery=no

Discovered devices can be shown in MQTT integration tab in Configuration settings. There will be always one device Nexus433 433MHz gateway. It provides just one sensor (disabled by default) Number of active devices.

When 433 MHz sensor is detected a new device will be added Temperatue Sensor Id:XX ch Y (XX–Sensor Id, Y–channel). This device provides 4 sensors:

Temperature
Humidity
Battery
Quality

In addition there is one sensor available Number of active 433 MHz transmitters that indicates active temperature sensors.

Adding sensors manually to Home Assistant

When automatic device discovery is disabled, sensor can be added manually.

To add a new sensor, you must perform exactly the same steps as adding any other MQTT sensor. See https://www.home-assistant.io/components/sensor.mqtt/ for more information.

To create a new humidity sensor add the following lines to your configuration.yaml (it is assumed the sensor id is 5c01 and MAC address of network card is 09815B5331AA):

- platform: mqtt
  state_topic: "nexus433/sensor/09815B5331AA_5c01/state"
  name: "Bedroom Humidity"
  expire_after: 90
  unit_of_measurement: '%'
  device_class: humidity
  availability_topic: "nexus433/sensor/09815B5331AA_4c01/connection"
  value_template: "{{ value_json.humidity }}"

Reception problems

If you got problems receiving proper data from sensors you can do one the following:

Check if data is properly received from close range, if not check batter or test with another sensor.
Move the antenna to a different position.
Reduce resolution_us parameter to 0 and increase tolerance_us.
Use longer antenna. Just attach a piece of wire. The length of the wire is related to wavelength, you can use: 69cm, 34cm, 17cm, 8cm, 4cm wires.
If you got more than one sensor make sure they transmit data in different time. If two sensors collide remove the battery and insert again.

CPU usage

Linux is not a real time operating system. You never know how much time scheduler assigns to the application.

If the system is not very busy, the application got enough time to process entire transmission more or less in real time. If the system is heavily loaded the program can easily miss transmitted bits.

For now the only solution (but unfortunately not guaranteed 100% success) is to force the system to assign more time for the application:

Increase the priority of the app; see nice command
Change resolution to 0 (CPU usage will raise). In addition increase tolerance (but big values can also degrade significantly quality).
Run the application on a separate system that is either dedicated to 433MHz reception or does not run too many other apps (but that makes nexus433 quite obsolete because you can use cheaper dedicated board just for 433MHz).

To definitely solve the problem, a kernel device driver is needed and entire signal processing should be made there.