Custom Sensor Component

Warning

While I do try to keep the ESPHome configuration options as stable as possible and back-port them, the ESPHome API is less stable. If something in the APIs needs to be changed in order for something else to work, I will do so.

So, you just set up ESPHome for your ESP32/ESP8266, but sadly ESPHome is missing a sensor integration you’d really like to have 😕. It’s pretty much impossible to support every single sensor, as there are simply too many. That’s why ESPHome has a really simple API for you to create your own custom sensors 🎉

In this guide, we will go through creating a custom sensor component for the BMP180 pressure sensor (we will only do the pressure part, temperature is more or less the same). During this guide, you will learn how to 1. define a custom sensor ESPHome can use 2. go over how to register the sensor so that it will be shown inside Home Assistant and 3. leverage an existing arduino library for the BMP180 with ESPHome.

Note

Since the creation of this guide, the BMP180 has been officially supported by the BMP085 component. The code still applies though.

This guide will require at least a bit of knowledge of C++, so be prepared for that. If you’ve already written code for an Arduino, you have already written C++ code :) (Arduino uses a slightly customized version of C++). If you have any problems, I’m here to help: https://discord.gg/KhAMKrd

Step 1: Custom Sensor Definition

To create your own custom sensor, you just have to create your own C++ class. If you’ve never heard of that before, don’t worry, at the end of this guide you can just copy the example source code and modify it to your needs - learning the intricacies of C++ classes won’t be required.

Before you can create your own custom sensors, let’s first take a look at the basics: How sensors (and components) are structured in the ESPHome ecosystem.

In ESPHome, a sensor is some hardware device (like a BMP180) that periodically sends out numbers, for example a temperature sensor that periodically publishes its temperature state.

Another important abstraction in ESPHome is the concept of a component. In ESPHome, a component is an object with a lifecycle managed by the Application class. What does this mean? Well if you’ve coded in Arduino before you might know the two special methods setup() and loop(). setup() is called one time when the node boots up and loop() is called very often and this is where you can do things like read out sensors etc.

Components have something similar to that: They also have setup() and loop() methods which will be called by the application kind of like the arduino functions.

So, let’s now take a look at some code: This is an example of a custom component class (called MyCustomSensor here):

#include "esphome.h"

using namespace esphome;

class MyCustomSensor : public Component, public sensor::Sensor {
 public:
  void setup() override {
    // This will be called by App.setup()
  }
  void loop() override {
    // This will be called by App.loop()
  }
};

In the first two lines, we’re importing ESPHome so you can use the APIs and telling the compiler that we want to use the esphome “namespace” (you don’t need to know what this is now, it’s basically just there to have a clean, well-structured codebase).

Let’s now also take a closer look at this line, which you might not be too used to when writing Arduino code:

class MyCustomSensor : public Component, public sensor::Sensor {

What this line is essentially saying is that we’re defining our own class that’s called MyCustomSensor which is also a subclass of Component and sensor::Sensor (in the namespace sensor::). As described before, these two “parent” classes have special semantics that we will make use of.

We could go implement our own sensor code now by replacing the contents of setup() and loop(). In setup() we would initialize the sensor and in loop() we would read out the sensor and publish the latest values.

However, there’s a small problem with that approach: loop() gets called very often (about 60 times per second). If we would publish a new state each time that method is called we would quickly make the node unresponsive since the MQTT protocol wasn’t really designed for 60 messages per second.

So this fix this, we will use an alternative class to Component: :cpp:class`PollingComponent`. This class is for situations where you have something that should get called repeatedly with some update interval. In the code above, we can simply replace Component by PollingComponent and loop() by a special method update() which will be called with an interval we can specify.

class MyCustomSensor : public PollingComponent, public sensor::Sensor {
 public:
  // constructor
  MyCustomSensor() : PollingComponent(15000) {}

  void setup() override {
    // This will be called by App.setup()
  }
  void update() override {
    // This will be called every "update_interval" milliseconds.
  }
};

Our code has slightly changed, as explained above we’re now inheriting from PollingComponent instead of just Component. Additionally, we now have a new line: the constructor. You also don’t really need to know much about constructors here, so to simplify let’s just say this is where we “initialize” the custom sensor.

In this constructor we’re telling the compiler that we want PollingComponent to be instantiated with an update interval of 15s, or 15000 milliseconds (ESPHome uses milliseconds internally).

Let’s also now make our sensor actually publish values in the update() method:

// class MyCustomSensor ...
  // ... previous code
  void update() override {
    publish_state(42.0);
  }
};

Every time update is called we will now publish a new value to the frontend. The rest of ESPHome will then take care of processing this value and ultimately publishing it to the outside world (for example using MQTT).

Step 2: Registering the custom sensor

Now we have our Custom Sensor set up, but unfortunately it doesn’t do much right now. Actually … it does nothing because it’s never included nor instantiated. First, create a new file called my_custom_sensor.h in your configuration directory and copy the source code from above into that file.

Then in the YAML config, include that file in the top-level esphome section like this:

esphome:
  # ... [Other options]
  includes:
    - my_custom_sensor.h

Next, create a new custom sensor platform entry like this:

# Example configuration entry
sensor:
- platform: custom
  lambda: |-
    auto my_sensor = new MyCustomSensor();
    App.register_component(my_sensor);
    return {my_sensor};

  sensors:
    name: "My Custom Sensor"

Let’s break this down:

  • First, we specify a lambda that will be used to instantiate our sensor class. This will be called on boot to register our sensor in ESPHome.
  • In this lambda, we’re first creating a new instance of our custom class (new MyCustomSensor()) and then assigning it to a variable called my_sensor. Note: This uses a feature in the C++ standard, auto, to make our lives easier. We could also have written MyCustomSensor *my_sensor = new MyCustomSensor()
  • Next, as our custom class inherits from Component, we need to register it - otherwise ESPHome will not know about it and won’t call our setup() and update methods!
  • Finally, we return the custom sensor - don’t worry about the curly braces {}, we’ll cover that later.
  • After that, we just let ESPHome know about our newly created sensor too using the sensors: block. Additionally, here we’re also assigning the sensor a name.

Now all that’s left to do is upload the code and let it run :)

If you have Home Assistant MQTT discovery setup, it will even automatically show up in the frontend 🎉

https://d33wubrfki0l68.cloudfront.net/45e2783c6563e03efed1fe2cb1259bb76402ff1d/09747/_images/custom-ui.png

Step 3: BMP180 support

Let’s finally make this custom sensor useful by adding the BMP180 aspect into it! Sure, printing 42 is a nice number but it won’t help with home automation :D

A great feature of ESPHome is that you don’t need to code everything yourself. You can use any existing arduino library to do the work for you! Now for this example we’ll use the Adafruit BMP085 Library library to implement support for the BMP085 sensor. But you can find other libraries too on the platformio library index

First we’ll need to add the library to our project dependencies. To do so, put Adafruit BMP085 Library in your global libraries:

esphome:
  includes:
    - my_custom_sensor.h
  libraries:
    - "Adafruit BMP085 Library"

Next, include the library at the top of your custom sensor file you created previously:

#include "esphome.h"
#include "Adafruit_BMP085.h"

using namespace esphome;

// ...

Then update the sensor for BMP180 support:

// ...

class MyCustomSensor : public PollingComponent, public sensor::Sensor {
 public:
  Adafruit_BMP085 bmp;

  MyCustomSensor() : PollingComponent(15000) { }

  void setup() override {
    bmp.begin();
  }

  void update() override {
    int pressure = bmp.readPressure(); // library returns value in in Pa, which equals 1/100 hPa
    publish_state(pressure / 100.0); // convert to hPa
  }
};

// ...

There’s not too much going on there. First, we define the variable bmp of type Adafruit_BMP085 inside our class as a class member. This is the object the adafruit library exposes and through which we will communicate with the sensor.

In our custom setup() function we’re initializing the library (using .begin()) and in update() we’re reading the pressure and publishing it using publish_state.

For ESPHome we can use the previous YAML. So now if you upload the firmware, you’ll see the sensor reporting actual pressure values! Hooray 🎉!

Step 4: Additional Overrides

There’s a slight problem with our code: It does print the values fine, but if you look in Home Assistant you’ll see a) the value has no unit attached to it and b) the value will be rounded to the next integer. This is because ESPHome doesn’t know these infos, it’s only passed a floating point value after all.

We could fix that in our custom sensor class (by overriding the unit_of_measurement and accuracy_decimals methods), but here we have the full power of ESPHome, so let’s use that:

# Example configuration entry
sensor:
- platform: custom
  lambda: |-
    auto my_sensor = new MyCustomSensor();
    App.register_component(my_sensor);
    return {my_sensor};

  sensors:
    name: "My Custom Sensor"
    unit_of_measurement: hPa
    accuracy_decimals: 2

Bonus: Sensors With Multiple Output Values

The Sensor class doesn’t fit every use-case. Sometimes, (as with the BMP180), a sensor can expose multiple values (temperature and pressure, for example).

Doing so in ESPHome is a bit more difficult. Basically, we will have to change our sensor model to have a component that reads out the values and then multiple sensors that represent the individual sensor measurements.

Let’s look at what that could look like in code:

class MyCustomSensor : public PollingComponent {
 public:
  Adafruit_BMP085 bmp;
  sensor::Sensor *temperature_sensor = new sensor::Sensor();
  sensor::Sensor *pressure_sensor = new sensor::Sensor();

  MyCustomSensor() : PollingComponent(15000) { }

  void setup() override {
    bmp.begin();
  }

  void update() override {
    // This is the actual sensor reading logic.
    float temperature = bmp.readTemperature();
    temperature_sensor->publish_state(temperature);

    int pressure = bmp.readPressure();
    pressure_sensor->publish_state(pressure / 100.0);
  }
};

The code here has changed a bit:

  • Because the values are no longer published by our custom class, MyCustomSensor no longer inherits from Sensor.
  • The class has two new members: temperature_sensor and pressure_sensor. These will be used to publish the values.
  • In our update() method we’re now reading out the temperature and pressure. These values are then published with the temperature and pressure sensor instances we declared before.

Our YAML configuration needs an update too:

# Example configuration entry
sensor:
- platform: custom
  lambda: |-
    auto my_sensor = new MyCustomSensor();
    App.register_component(my_sensor);
    return {my_sensor->temperature_sensor, my_sensor->pressure_sensor};

  sensors:
  - name: "My Custom Temperature Sensor"
    unit_of_measurement: °C
    accuracy_decimals: 1
  - name: "My Custom Pressure Sensor"
    unit_of_measurement: hPa
    accuracy_decimals: 2

In lambda the return statement has changed: Because we have two sensors now we must tell ESPHome about both of them. We do this by returning them as an array of values in the curly braces.

sensors: has also changed a bit: Now that we have multiple sensors, each of them needs an entry here.

Note that the number of arguments you put in the curly braces must match the number of sensors you define in the YAML sensors: block - and they must be in the same order.

Configuration variables:

  • lambda (Required, lambda): The lambda to run for instantiating the sensor(s).
  • sensors (Required, list): A list of sensors to initialize. The length here must equal the number of items in the return statement of the lambda.