/ Scala

How to build a remote temperature sensor with ReactivePI

Working with hardware on any platform involves working with a low-level programming language
and a huge amount of constants that have weird values. That's at least how I remember building my first
hardware/software interface.

Today this isn't much better. I love the raspberry PI and the fact that I can run Java applications on it.
But what I absolutely dislike is the fact that I still have to write C programs in order to access the hardware interfaces.
If you want to use the I2C bus on your Raspberry PI you have two choices: Use the python programming language or resort
to C and use the appropriate system calls.

I think we can do better. I'm a fan of the Scala and Java programming languages. I really don't want to go back to an environment
where I have to think hard about memory allocation and pointers. So why not build a library that enables me to write my
IoT application in Scala or Java? I took on this challenge and made a lbirary that makes it easier to talk
to things like I2C on your Raspberry PI.

In this post I will show you how I made a temperature monitor with ReactivePI, Scala and Akka to demonstrate
that using a higher level programming language with proper GC support makes life better when building an IoT application.

What can you do once you have the drivers?

With ReactivePI you can do some pretty cool things on your Raspberry PI.
There's a ton of sensors available for a small amount of money that connect to I2C. Almost all of them
are very well documented. ReactivePI is very generic in the sense that it only knows about sending and receiving bytes.
This means that it is also very flexible. Most sensors come with a sample that require you to send and receive specific
sequences of bytes from a sensor in C. Translate this to Scala or Java with the use of Reactive PI and you should have
the same results in about the same time or less.

One of the things I built using ReactivePI is a small weather station. One of the components in the weather station
is a temperature sensor. I used the Adafruit MCP9808 sensor which you can buy from their website for just under $5.

The application I've created is a small Scala application that runs two actors: The temperature monitor which is going
to periodically ask for the temperature by sending a message to the temperature sensor. The temperature sensor will
read the temperature from the Adafruit MCP9808 sensor and send a message back to monitor with the temperature.

To create the temperature monitor I've written an Actor class that looks like this:

package nl.fizzylogic.reactivepi.samples.temperaturemonitor

import akka.actor.{ActorLogging, Props, ActorRef, Actor}

import scala.concurrent.ExecutionContext

object TemperatureMonitor {
  case object GetCurrentTemperature

  def props(sensor: ActorRef) = Props(new TemperatureMonitor(sensor))

class TemperatureMonitor(sensor: ActorRef) extends Actor with ActorLogging {

  import TemperatureMonitor._
  import scala.concurrent.duration._

  implicit val ec:ExecutionContext = context.system.dispatcher

  context.system.scheduler.schedule(0 seconds,
    5 seconds,self, GetCurrentTemperature)

  def receive = {
    case GetCurrentTemperature => loadTemperatureData()
    case TemperatureSensor.SensorData(temperature) =>

  private def loadTemperatureData() = {
    sensor ! TemperatureSensor.GetActualTemperature

  private def displayTemperatureData(temperature:Int) = {
    log.info(s"The current temperature is $temperature")

When the monitor is asked for the temperature it will send a message to the connected
sensor asking it to measure the current temperature. When the sensor has the data
available it will send a message back to the monitor.

I skipped the code for reporting the temperature and instead log the value.
If you haven't done anything special to your log settings the log statements will be printed on the terminal.

When the monitor actor is created it will automatically start sending itself temperature requests
every 5 seconds. This is done by the call to the context.system.scheduler.schedule method.

With the temperature monitor set up it's time to write the logic needed for the temperature sensor.
The temperature sensor code is similar to the monitor code, but doesn't include the scheduling code.

package nl.fizzylogic.reactivepi.samples.temperaturemonitor

import akka.actor.{Actor, ActorLogging, ActorRef, Props}
import nl.fizzylogic.reactivepi.I2C
import nl.fizzylogic.reactivepi.i2c.Convert

object TemperatureSensor {

  case object GetActualTemperature

  case class SensorData(temperature: Int)

  def props: Props = Props[TemperatureSensor]

class TemperatureSensor extends Actor with ActorLogging {

  import TemperatureSensor._

  // The adafruit MCP9808 sensor is configured to work on address 0x18.
  // You can change this on the device itself by pulling one or more address pins to 3.3V
  val sensor = I2C(1).device(0x18)

  // The ambient temperature register can be read to retrieve the current temperature.
  val MCP9808_REG_AMBIENT_TEMP: Byte = 0x05

  var originalSender: ActorRef = null

  def receive = idle

  def waitingForSensorData: Receive = {
    case I2C.Data(buffer) => sendSensorData(buffer)

  def idle: Receive = {
    case GetActualTemperature => loadTemperatureData()

  private def loadTemperatureData() = {
    log.info("Received request for the temperature")


    originalSender = sender
    sensor ! I2C.Read(MCP9808_REG_AMBIENT_TEMP, 2)

  private def sendSensorData(buffer: Array[Byte]) = {
    originalSender ! SensorData(translateTemperature(buffer))

  private def translateTemperature(buffer: Array[Byte]): Int = {
    val wordData = Convert.wordToInt16(buffer)
    val temperature = Math.round((wordData & 0x0FFF) / 16.0).asInstanceOf[Int]

    if ((wordData & 0x1000) != 0x00) {
      temperature - 256
    } else {

The temperature sensor accepts a GetActualTemperature message. When it receives this message
it will load the temperature data from the MCP9808 sensor (Which is connected to the I2C bus).

To read data from the temperature sensor device the TemperatureSensor actor opens the first I2C bus
and asks to communicate to the device connected on address 0x18 (The default address for the sensor).

Every time the sensor is asked for data we read 2 bytes and translate them into the actual temperature.
The actual temperature is then send back to the monitor in the sendSensorData method.

Notice that the sensor is normally in idle mode. But when you send it a request for the temperature,
it is switched to waitingForSensorData mode. This prevents problems where we get another request while
waiting for a previous request to complete. You don't want to process two overlapping requests, since that would
cause a problem on the I2C bus itself.

Pro tip: You can improve this busy behavior by adding the Stash behavior extension to your actor.
Stash all unexpected messages while in busy mode. Be sure to call unstashAll() when going back to idle
mode so that all stashed messages become available again for processing.

Once the temperature data is retrieved the sensor is returned to its idle state.

To complete the application you need to write one more piece of code. The code that is going to connect the sensor
to the monitor.

class Program extends App {
  val actorSystem = ActorSystem("temperature-monitor")

  val sensor = actorSystem.actorOf(TemperatureSensor.props, "temperature-sensor")
  val monitor = actorSystem.actorOf(TemperatureMonitor.props(sensor), "temperature-monitor")  

When you compile this application on your Raspberry PI you will see temperature data
scrolling slowly over your screen.

Adding Akka Remote Actors to make it awesome

Getting the current temperature is kind of the thing that everyone does using Python. With ReactivePI and Akka though
you can do something much nicer. How about monitoring the temperature data from your computer which talks to the temperature sensor actor on the Raspberry PI?

The scenario I will be showing you shortly works like this. The monitor actor instance will be running on your laptop/computer.
The temperature sensor will be running on the Raspberry PI. To make this possible you first need to make sure you can run the application
without actually starting any actor. Just make sure that the actor system is up and available across the network.

To do this I made use of a trick in the application startup logic:

import akka.actor.ActorSystem

object Program extends App {
  val parser = new scopt.OptionParser[StartupOptions]("temperature-monitor") {
    head("monitor", "0.1")

    // Specify a run mode (client or server)
    opt[String]('m',"mode") action { (x,c) =>
      // Copy the settings and set the client/server mode based on the provided option
      c.copy(sensor = x.equalsIgnoreCase("sensor"), monitor = x.equalsIgnoreCase("monitor"))
    } text("Application mode")

    checkConfig(opts => {
      if(!opts.sensor && !opts.monitor) {
        failure("Invalid application mode. Allowed values: sensor or monitor")
      } else {

  parser.parse(args, StartupOptions()) match {
    case Some(options) =>
      val actorSystem = ActorSystem("temperature-monitor")

      // When the application is run in monitor mode, it will try to access the
      // temperature sensor on the raspberry PI.
      if(options.monitor) {
        val sensor = actorSystem.actorOf(TemperatureSensor.props, "temperature-sensor")
        val monitor = actorSystem.actorOf(TemperatureMonitor.props(sensor), "temperature-monitor")
    case None =>

I added a commandline option parser. The parser itself is a whole different story, but for this application it adds the possibility to pass in the -m option.
This sets the application either in sensor or monitor mode. In sensor mode only the actor system is started. When the application is started in monitor mode,
the monitor as well as the sensor are started.

Sounds weird, starting the sensor on the computer that is going to work as the monitor. But it really isn't. The final piece of the puzzle is in the configuration
of the sensor and monitor version of the application. When you start the application in sensor mode you have to make sure that the following content is available
in the application.conf file of the application:

akka {
  actor {
    provider = "akka.remote.RemoteActorRefProvider"
  remote {
    enabled-transports = ["akka.remote.netty.tcp"]
    netty.tcp {
      hostname = ""
      port = 2552

This enables Akka remoting, setting the listen port to 2552 and the published IP-address to
When you start the application in sensor mode on your Raspberry PI you will notice that it will start to listen on port 2552
and go idle after that.

The monitor application config is a bit more complicated:

akka {
  actor {
    provider = "akka.remote.RemoteActorRefProvider"
    deployment {
      /temperature-sensor {
        remote = "akka.tcp://temperature-monitor@<remote-ip>:2552"
  remote {
    enabled-transports = ["akka.remote.netty.tcp"]
    netty.tcp {
      hostname = ""
      port = 2552

When you start the application in monitor mode on your computer, it creates a new actor with the name temperature-sensor.
Instead of starting the actor locally Akka will automatically
deploy the actor to the actor system running in sensor mode on your Raspberry PI.

Try it out yourself!

With the config in place and the startup logic modified you can now run your temperature monitor anywhere you like.
In fact you can even add multiple sensors by starting multiple actors configured to be deployed to different Raspberry PI systems
running in sensor mode.

Want to try this yourself? Check out the demo code at https://github.com/reactivepi/samples/

Willem Meints

Willem Meints

I write and talk about new technology. I like to help others get the most out of the technology they use. Within Info Support I work together with others to help projects start AI and other tech.

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