Record temperature from Lexibook ASM30 sensor with Arduino

Needed hardware

Arduino Uno R3 board (available on Amazon) with USB cable
433Mhz RF transmitter and receiver (available on eBay)
Lexibook ASM30 temperature sensor: I had one since many years and I noticed it communicates with 433RF signals with the base

Assembly

It is very simple. Just connect the RF 433 receiver like this:

GND to Arduino pin GND
VCC to Arduino pin 5V
DATA to Arduino DIGITAL pin 2

Then I suppose you have the Arduino connected with a USB cable to a PC running the Arduino development environment.

Reverse engineering Lexibook ASM30 temperature sensor

I tried RFControl sniffer to see if some signals can be discovered. RFControl outputs pulse timing if the same code is received twice.

The ASM30 sensor sends data every 30 seconds, corresponding to the time when the LED is flashing. And RFControl detects some data exactly at the same time!

Analysing the timings show several things :

There is a short timing (<500µs) every other pulse
Some pulses have medium duration (around 1900µs)
Some pulses have long duration (around 4500µs)
The full frame is sent 3 times with a timing >9500µs between frames

My asumption was a short pulse was a separator, a medium pulse was a “0” bit and a long pulse a “1” bit. So I tried to decode bytes knowing the expected temperature which is displayed on the sensor itself: so easy, the 3rd byte is the temperature x10 simply encoded in decimal.

So now, this is what I found:

First byte can vary but I don't know yet what is it. Maybe contains the sensor ID
Second byte is always the same: “11110000” (240). Probably not an ID (or I am really lucky!), maybe a separator
Third byte is the temperature in °C x10
2 next bits are the channel you can choose on the sensor itself: 1, 2 or 3. This is to be able to know the source of the temperature when you have several sensors.
Last 2 bits are always “00”, probably stop bits

The arduino sketch

The following sketch reads the temperature and outputs to the serial line in JSON format. In this sketch, I use RFControl library as is, so it is not very optimized for my needs. However, for now it works and it is enough!

SmartHomeBox_sgripon.ino

#include <RFControl.h>
 
// Save current temperature and channel
int temperature = 0;
int channel = 0;
 
void setup() {
 
  // Initialize serial line
  Serial.begin(115200);
 
  // Start receiving RF signals
  RFControl::startReceiving(0);
}
 
// The function prints current data to the serial line
void printData() {
    Serial.print("{ ");
    Serial.print("\"temp\": ");
    Serial.print(temperature);
    Serial.print(", \"channel\": ");
    Serial.print(channel);
    Serial.print(" }\n");
}
 
void loop() {
 
  // Decode data when received
  if(RFControl::hasData()) {
    unsigned int *timings;
    unsigned int timings_size;
    unsigned int pulse_length_divider = RFControl::getPulseLengthDivider();
    RFControl::getRaw(&timings, &timings_size);
 
    int iTemperatureTmp = 0;
    int iChannelTmp = 0;
 
    for(int i=0; i < timings_size; i++) {
      unsigned long timing = timings[i] * pulse_length_divider;
 
      // Temperature is at bit 16*2 coded in decimal x 10 on 8 bits
      if( (i >= 16*2) && (i < (16*2 + 8*2)) ) {
        if(i == 16*2) iTemperatureTmp = 0;
 
        if(timing < 1000) {
          // bit separator, do nothing
        }
        else if(timing < 2000) {
          iTemperatureTmp = iTemperatureTmp<<1;
        }
        else if(timing < 5000) {
          iTemperatureTmp = iTemperatureTmp<<1;
          iTemperatureTmp++;
        }
      }
      // Sensor Channel is after byte 3, on 2 bits, decimal encoded
      else if((i >= 24*2) && i <= 26*2) {
        if(i == 24*2) channel = 0;
        if(timing < 1000) {
          // bit separator, do nothing
        }
        else if(timing < 2000) {
          iChannelTmp = iChannelTmp<<1;
        }
        else if(timing < 5000) {
          iChannelTmp = iChannelTmp<<1;
          iChannelTmp++;
        }
      }
      // Other data
      else {
        // Do nothing for now
      }
    }
 
    temperature = iTemperatureTmp;
    channel = iChannelTmp;
 
    RFControl::continueReceiving();
  }
 
  printData(); 
}

Next Step

Add temperature sensor in Domoticz using a respberry pi.

References

RFControl RF 433MHz sniffer library for Arduino: RFControl

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