B4R Tutorial Reading remote sensor data with B4J, Python and Node.js

I am working on a little thermometer project for my greenhouse using an ESP8266 and a temperature sensor. You have multiple sensors to choose from such as DHT11/22, BMP180 etc, but no matter which sensor, you need to transmit the readings.

My first attempt was with AsyncStreams + B4RSerializator and while this works, it is unnecessary complex for such simple tasks as transmitting a sensor reading. @monki kindly pointed me to another direction using UDP packets. This turned out to be a much better approach.

Advantages of UDP broadcasts
UDP broadcast messages have several advantages:
  • Very lightweight
  • No server required. The sensor broadcasts to any listening receiver
  • You can have as many receivers as you like. For example, you can build a weather station with displays in both your living room and bedroom
  • Very easy to deal with in other platforms
The last point is important. Erel has already shown a simple broadcast receiver using B4J in the weather station tutorial:

Simple UDP broadcast receiver in B4J:
Sub Process_Globals
    Private usocket As UDPSocket
End Sub

Sub AppStart (Args() As String)
    usocket.Initialize2("usocket", 51042, 8192, True, False) 'reuse is set to true
    Log($"Broadcast address: ${usocket.GetBroadcastAddress}"$)
End Sub

Private Sub usocket_PacketArrived (Packet As UDPPacket)
    If Packet.Length <> 8 Then
        Log("Invalid data")
        Dim raf As RandomAccessFile
        raf.Initialize3(Packet.Data, True)
        raf.CurrentPosition = Packet.Offset
        Dim temperature, pressure As Float
        temperature = raf.ReadFloat(raf.CurrentPosition)
        pressure = raf.ReadFloat(raf.CurrentPosition)
        Log($"Temperature: $1.1{temperature}°"$)
        Log($"Pressure: $1.1{pressure} mBar"$)
    End If
End Sub

However, I wanted to run the receiver on my Raspberry Pi 3 with limited storage so I didn't want to install Java. Luckily, we have easy alternatives.

First, let's check if there is anything going on on the specific port. On Linux and Mac you can use netcat for this. There is probably also a Windows version.
Checking UDP packets with netcat:
netcat -lu -p 51042

The output is just garbage since the floating point numbers are represented as their equivalent ascii characters without interpretation. But it will tell us if there are packets broadcast to the given port.

Python receiver
Now let's implement the receiver in Python. It will show the sensor readings from the B4R program in the weather station tutorial:

Broadcast receiver in Python:
#!/usr/bin/env python
# coding=UTF-8
port = 51042
from socket import *
import struct
s.bind(('', port))
while True:
   f1 = m[0][0:4]
   f2 = m[0][4:8]
   temperature = struct.unpack('f',f1)[0]
   pressure = struct.unpack('f',f2)[0]
   print "%s °C, %s mBar" % (temperature, pressure)

Node.js receiver
I have also node.js installed on my Raspi, and since I do a lot of node work, I prefer that over Python. Here is the node version:
Broadcast receiver in Node.js:
#!/usr/bin/env node
const PORT = 51042;
const dgram = require("dgram");
const socket = dgram.createSocket({ type: "udp4", reuseAddr: true });
socket.on("message", function(message, info) {
   const temperature = message.readFloatLE()
   const pressure = message.subarray(4).readFloatLE()
   console.log(`${temperature} °C, ${pressure} mBar`)

Broadcast receiver with web server
It is easy to implement a simple web server in node. I wanted the Rasbi to provide a JSON file for my Android app. Thus, the nodejs script on my Rasbi is responsible for collecting the packets from the ESP8266 and my B4XPages app can display the data. This is better than having the phone app receive the broadcasts because this will only work if you're at home.

This simple script will both receive the sensor broadcasts and publish the JSON data on localhost:8001. You can choose any port you like.
Broadcast receiver and web server in node:
#!/usr/bin/env node

const UDP_PORT = 51043
const HTTP_PORT = 8001

const http = require('http')
const data = { temperature: "no reading", pressure: "no reading", time: new Date().toLocaleTimeString()}

const dgram = require("dgram")

const socket = dgram.createSocket({ type: "udp4", reuseAddr: true })

socket.on("message", function(message, info) {
  data.temperature = message.readFloatLE()
  data.pressure = message.subarray(4).readFloatLE()
  data.time = new Date().toLocaleTimeString()

http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'application/json; charset=utf-8'})

JSON output from web server:
"temperature": 23.60211181640625,
"pressure": 999.6287841796875,
"time": "14:50:30"
Multiple sensors
What if you have multiple sensors? For example a sensor in each room as well as an outdoor sensor?

You have two choices. Either you can use slightly different reading intervals for you sensors, for example every 57 and 61 seconds. This means that they will rarely collide even though they broadcast on the same port. And when they do collide, the next reading will be fine again.

A more robust approach is to use a separate port for each sensor. This example for node shows two receivers listening on separate ports. The info parameter in the message function tells us which sensor (IP and port) sent the message, so we never mix up the data even though we use the same message function for multiple sensors.

Multiple receivers in node:
#!/usr/bin/env node
const PORT1 = 51042;
const PORT2 = 51043;

const dgram = require("dgram");

const socket1 = dgram.createSocket({ type: "udp4", reuseAddr: true });
const socket2 = dgram.createSocket({ type: "udp4", reuseAddr: true });

socket1.on("message", message);
socket2.on("message", message);


function message(msg, info) {
   const temperature = msg.readFloatLE()
   const pressure = msg.subarray(4).readFloatLE()
   console.log(`${info.address}:${info.port} says: ${temperature} °C, ${pressure} mBar`)

Output from multiple sensors: says: 23.665287017822266 °C, 999.4187622070312 mBar says: 5.898721812782782 °C, 999.6021217711182 mBar

Sending UDP messages
This tutorial mainly deals with the receiving end, but to make it complete, here is the B4R code taken from the weather station tutorial that actually sends the sensor data:
Reading and broadcasting data from BMP180:
Private Sub Timer1_Tick
    If Not(bmp180.GetTemperature) Then
        Log("Error retrieving the temperature.")
        Dim buffer(8) As Byte
        'double in B4R is 4 bytes
        Dim raf As RandomAccessFile
        raf.Initialize(buffer, True)
        raf.WriteDouble32(bmp180.LastResult, raf.CurrentPosition)
        bmp180.GetPressure(0, bmp180.LastResult)
        raf.WriteDouble32(bmp180.LastResult, raf.CurrentPosition)
        usocket.BeginPacket(ip, port)
    End If
End Sub
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