Removed Si7021 and added HYT221 humidity sensor

CHANGELOG.md

@@ -85,3 +85,13 @@ All notable changes to this project will be documented in this file.
 There is a bug in the Si7021 chip: when the sensor is exposed to extreme environmental conditions for a longer period (very high or very low humidity), the humidity register will wrap around and give wrong values. (https://community.silabs.com/s/question/0D51M00007xeOo9SAE/si7021-relative-humidity-reading-underflowing-?language=en_US).
 Workaround: let's assume the humidity will never go below 15%. In the Netherlands a humidity below 20% is rare, but can occur and the minimum value ever measured in De Bilt was 6% (on 1 april 1965). At several other sites in the Netherlands the humidity went as low as 10%. But as this was a one time event and the measurements are doubtful, this event was not registered in the original database. (https://nl.wikipedia.org/wiki/Relatieve_luchtvochtigheid)
 In rare cases, the humidity value could creep up above 15% (when the actual humidity is 100%) and still give wrong values.
+
+## [0.3.0] - 2024-05-02
+
+### Added
+
+Support for HYT-221 humidity sensor
+
+### Removed
+
+Support for si7021 humidity sensor (this sensor was not suited for outdoor measurments)

TODO.md

@@ -0,0 +1,12 @@
+# Todo list for weather station with ModBus over RS-485
+
+## Reset firmware via ModBus coil bit
+
+Soemtimes the weater station needs to be reset. For example if the I2C communication with the sensors is lost. The pressure sensor tends to have this probem occacionally.
+
+## Implement support for new humidity sensor
+
+Heater function of former sensor needs to be disabled as the new sensor does not have (or need) a heater.
+
+
+

build-doc/weather_station.html

@@ -5,7 +5,7 @@
   <meta name="generator" content="pandoc" />
   <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
   <meta name="author" content="M.T. Konstapel" />
-  <meta name="dcterms.date" content="2024-01-22" />
+  <meta name="dcterms.date" content="2024-05-02" />
   <title>Weather station</title>
   <link rel="stylesheet" href="./css/mvp.css" />
 <style type="text/css">
@@ -53,6 +53,9 @@
             id="toc-specifications">Specifications</a></li>
             <li><a href="#schematic"
             id="toc-schematic">Schematic</a></li>
+            <li><a href="#problems-i-encounter-after-four-months-of-use"
+            id="toc-problems-i-encounter-after-four-months-of-use">Problems
+            I encounter after four months of use</a></li>
             <li><a href="#software-dependencies"
             id="toc-software-dependencies">Software
             dependencies</a></li>
@@ -75,7 +78,7 @@
 <h1 class="title">Weather station</h1>
 <p class="subtitle">with ModBus RTU interface</p>
 <p class="author">M.T. Konstapel</p>
-<p class="date">2024-01-22</p>
+<p class="date">2024-05-02</p>
 <p><a href="./weather_station.pdf"><i>PDF version</i></a></p>
 </header>
 <main>
@@ -868,6 +871,29 @@ Address  : 14</code></pre>
 <h1 id="schematic">Schematic</h1>
 <p><a href="./images/weather_station_schematic.pdf"><img
 src="./images/weather_station_schematic.svg" alt="Schematic" /></a></p>
+<h1 id="problems-i-encounter-after-four-months-of-use">Problems I
+encounter after four months of use</h1>
+<p>The Si7021 humidity sensor is not made for outdoor use. The datasheet
+is clear about that. But a lot of people use this cheap sensor for
+weather stations anyway. So I choose this sensor for my design. But that
+was not smart. After an initial time without any problems, the sensor
+started to saturate. This happened during a very wet and mild winter we
+had. My first solution was to utilize the build in heater to drive of
+the moisture. That worked, but only for a short period. The heater blew
+up and the sensor started to report humidity levels above 100% and
+because of a bug in the firmware of the sensor, the humidity register
+wrapped around and the sensor reported humidity levels of 0-30%. I tried
+to find a software solution, which worked for a while, but the sensor
+deteriorated even more. To the point of being totally useless. So I
+searched for another, better sensor. And I found the HYT-221 from
+Innovative Sensor Technology. This sensor is designed to work outdoors
+and can even be used in saunas, where the humidity levels are always
+high and the air is condensating. The datasheet specifically mentions
+outdoor weather stations as an application. The only downside is its
+price: it is ten times more expensive than the Si7021.</p>
+<p>The sensor can be controlled via the I2C bus, so implementing the new
+sensor in the firmware was very easy. From version 0.3.0 onward, this
+sensor is used. The Si7021 is removed from the code.</p>
 <!---
 # Bill of materials
 
@@ -906,7 +932,7 @@ option) any later version.</p>
 </main>
 <footer>
 <p>&copy;
- 2024-01-22
+ 2024-05-02
  M.T. Konstapel
  <a href="https://meezenest.nl/mees/">https://meezenest.nl/mees/</a>
 </p><p>This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">Creative Commons Attribution-ShareAlike 4.0 International License</a>.

build-doc/weather_station.md

@@ -2,7 +2,7 @@
 title: Weather station
 subtitle: with ModBus RTU interface
 author: M.T. Konstapel
-date: 2024-01-22
+date: 2024-05-02
 website: https://meezenest.nl/mees/
 page_back: https://meezenest.nl/mees/weather_station.html
 logo: ./images/mees_logo.svg
@@ -392,6 +392,12 @@ As a housing for the prototype, I used an old beehive. These are weatherproof an
 
 [![Schematic](./images/weather_station_schematic.svg)](./images/weather_station_schematic.pdf)
 
+# Problems I encounter after four months of use
+
+The Si7021 humidity sensor is not made for outdoor use. The datasheet is clear about that. But a lot of people use this cheap sensor for weather stations anyway. So I choose this sensor for my design. But that was not smart. After an initial time without any problems, the sensor started to saturate. This happened during a very wet and mild winter we had. My first solution was to utilize the build in heater to drive of the moisture. That worked, but only for a short period. The heater blew up and the sensor started to report humidity levels above 100% and because of a bug in the firmware of the sensor, the humidity register wrapped around and the sensor reported humidity levels of 0-30%. I tried to find a software solution, which worked for a while, but the sensor deteriorated even more. To the point of being totally useless. So I searched for another, better sensor. And I found the HYT-221 from Innovative Sensor Technology. This sensor is designed to work outdoors and can even be used in saunas, where the humidity levels are always high and the air is condensating. The datasheet specifically mentions outdoor weather stations as an application. The only downside is its price: it is ten times more expensive than the Si7021.
+
+The sensor can be controlled via the I2C bus, so implementing the new sensor in the firmware was very easy. From version 0.3.0 onward, this sensor is used. The Si7021 is removed from the code.
+
 <!---
 # Bill of materials
 

firmware/weather_station.ino

@@ -32,6 +32,9 @@
  *             - Changed some variables to the propper standard (uint8_t, uint16_t, etc.)
  *             - SparkFun wind interrupt now calculates over 3 seconds in stead of 1 second (KNMI standard)
  *
+ * 2-24-05-02: - Removed cope for si7021
+ *             - Added code for HYT221 humidity sensor
+ *
  * See CHANGELOG.md
  *
  **********************************************************************************/
@@ -43,8 +46,7 @@
 #include "i2c.h"
 
 //Temperature and humidity sensor
-#include "i2c_SI7021.h"
+#define HYT_ADDR 0x28     // I2C address of the HYT 221, 271, 371 and most likely the rest of the family
-SI7021 si7021;
 
 // Pressure sensor
 #include "i2c_BMP280.h"
@@ -152,136 +154,49 @@ struct MeasuredData {
   bool HeaterStatus = 0;
 } MeasuredData;
 
-// State machine implementing smart heater to prevent saturation of the sensor
+void ReadHYT221 (void)
-char HeaterSi7021 (float humidity)
 {
-  static int state=0;
+  double humidity;
-  static unsigned long StatemachineTimer=0;
+  double temperature;
-  bool TempValid=1;
+  
-  bool Heater=0;
+  Wire.beginTransmission(HYT_ADDR);   // Begin transmission with given device on I2C bus
-
+  Wire.requestFrom(HYT_ADDR, 4);      // Request 4 bytes 
-  // If Smart heater algorithm is disabled, reset the statemachine forever.
+  
-  // TempValid bit is also forced to 1, but it could be that we just came out of a heater period.
+  // Read the bytes if they are available
-  // We assume that the client on the other side of the ModBus is smart enough to understand.
+  // The first two bytes are humidity the last two are temperature
-  if ( (MeasuredData.StatusBits & 0x04) == 0)
+  if(Wire.available() == 4) {                   
-    state = 0;
+    int b1 = Wire.read();
-
+    int b2 = Wire.read();
-  switch (state)
+    int b3 = Wire.read();
-  {
+    int b4 = Wire.read();
-    // Default state: humidity is below HUMIDITY_THRESHOLD
+    
-    case 0:
+    Wire.endTransmission();           // End transmission and release I2C bus
-      Heater = 0;
+    
-      if (humidity >= HUMIDITY_THRESHOLD) {
+    // combine humidity bytes and calculate humidity
-        StatemachineTimer = millis();
+    int rawHumidity = b1 << 8 | b2;
-        state = 1;
+    // compound bitwise to get 14 bit measurement first two bits
-      }
+    // are status/stall bit (see intro text)
-      break;
+    rawHumidity =  (rawHumidity &= 0x3FFF);
-    // Humidity went above HUMIDITY_THRESHOLD. See if humidity stays above HUMIDITY_THRESHOLD for more than an hour, if so turn on heater
+    humidity = 100.0 / pow(2,14) * rawHumidity;
-    case 1:
+    
-      if (humidity >= HUMIDITY_THRESHOLD) {
+    // Scale for more decimal positions when converted to integer value for ModBus
-        if ( (millis() - StatemachineTimer) >= 3.6e+6 ) {
+      MeasuredData.Humidity = 100 * humidity;
-        //if ( (millis() - StatemachineTimer) >= 300000 ) { // short delay for testing
-          Heater = 1;
-          TempValid = 0;
-          StatemachineTimer = millis();
-          state = 2;
-        } else {
-          Heater = 0;
-        }
-      } else {
-        Heater = 0;
-        state = 0;
-      }      
-      break;
-
-    // Heater is now on, let the sensor cook for 5 minutes
-    case 2:
-      if ( (millis() - StatemachineTimer) >= 300000 ) {
-        StatemachineTimer = millis();
-        Heater = 0;
-        state = 3;
-      } else {
-        Heater = 1;
-      }
-      TempValid = 0;
-      break;
-    // Heater is now off, let the sensor cool for 15 minutes
-    case 3:
-      if ( (millis() - StatemachineTimer) >= 900000 ) {
-        TempValid = 1;            // Sensor cooled, so we can take a valid temperature reading
-        if (humidity >= HUMIDITY_THRESHOLD) {
-          // Humidity still above HUMIDITY_THRESHOLD, repeat heating/cooling
-          Heater = 1;
-          StatemachineTimer = millis();
-          state = 2;
-        } else {
-          // Humidity below HUMIDITY_THRESHOLD, reset statemachine
-          Heater = 0;
-          state = 0;
-        }
-      } else {
-        Heater = 0;
-        TempValid = 0;
-      }
-      break;
-
-    default:
-      Heater = 0;
-      state = 0;
-      break;
-  }
-
-  return TempValid<<1 | Heater;
 
-}
+    // combine temperature bytes and calculate temperature
-// Read Si7021 sensor and process data
+    b4 = (b4 >> 2); // Mask away 2 least significant bits see HYT 221 doc
-void ReadSi7021 (void)
+    int rawTemperature = b3 << 6 | b4;
-{
+    temperature = 165.0 / pow(2,14) * rawTemperature - 40;
-    char result=0x2;
-    float humidity;
-
-    si7021.triggerMeasurement();
-    si7021.getHumidity(humidity);
-
-    // There is a bug in the Si7021 chip: when the sensor is exposed to extreme environmental conditions for a longer period (very high or very low humidity),
-    // the humidity register will wrap around and give wrong values. (https://community.silabs.com/s/question/0D51M00007xeOo9SAE/si7021-relative-humidity-reading-underflowing-?language=en_US).
-    // Workaround: let's assume the humidity will never go below 15%. In the Netherlands a humidity of below 20% is rare, but can occur and the minimum value ever measured
-    // in De Bilt was 6% (on 1 april 1965). At several other sites in the Netherlands the humidity went as low as 10%. But as this was a one time event and the
-    // measurements are doubtful, this event was nor registered in the original database. (https://nl.wikipedia.org/wiki/Relatieve_luchtvochtigheid)
-    // In rare cases, the humidity value could creep up above 15% (when the actual humidity is 100%) and still give wrong values.
-    if (humidity>100 || humidity<15)
-      humidity = 100;
- 
-    //If humidity is larger than HUMIDITY_THRESHOLD switch on heater to get more acurate measurement and prevent memory offset
-    result = HeaterSi7021(humidity);
-    MeasuredData.StatusBits &= 0xFFFC;  // Reset heater status bits to zero
-    MeasuredData.StatusBits |= result;  // And set the proper bits to one if there are any. The result is we copied the status bits to the register
-
-    // Temperture and humidity readings are valid (as the sensor is not heated)
-    if (result & 0x2) {
-      si7021.getTemperature(MeasuredData.Temperature);
-      // Scale for more decimal positions when converted to integer value for ModBus
-      MeasuredData.Temperature *= 100;
-      //Serial.print(F("Valid temp"));
-
-      si7021.getHumidity(MeasuredData.Humidity);
-      // See humidity bug workaround above.
-      if (MeasuredData.Humidity>100 || MeasuredData.Humidity<15)
-        MeasuredData.Humidity = 100;
-      // Scale for more decimal positions when converted to integer value for ModBus
-      MeasuredData.Humidity *= 100;      
-    }
-    // Statemachine thinks it is time to switch on the heater
-    if (result & 0x1) {
-      //Serial.print(F("Heater on."));
-      MeasuredData.HeaterStatus = 1;
-      
-    } else {
-      //Serial.print(F("Heater off."));
-      MeasuredData.HeaterStatus = 0;
-    }
-    si7021.setHeater(MeasuredData.HeaterStatus);
 
+    // Scale for more decimal positions when converted to integer value for ModBus
+    MeasuredData.Temperature = 100 * temperature;
+    
+    //Serial.print(MeasuredData.Humidity);
+    //Serial.print("% - Temperature: ");
+    //Serial.println(MeasuredData.Temperature);
+  }
+  else {
+    Serial.println("Not enough bytes available on wire.");
+  }
 }
 
 // Read BMP280
@@ -467,26 +382,11 @@ void setup() {
   mb.Ireg (SensorRainSinceMidnightIreg, 0);
   mb.Ireg (SensorSnowFallIreg, 0);
  
-  Serial.println(F("Weather station v0.2.6"));
+  Serial.println(F("Weather station v0.3.0"));
   Serial.println(F("(C)2024 M.T. Konstapel"));
   Serial.println(F("This project is free and open source"));
   Serial.println(F("More details: https://meezenest.nl/mees/"));
 
-  //Initialize Si7021 sensor
-  Serial.print(F("Humidity sensor SI7021 "));
-  if (si7021.initialize())
-    Serial.println(F("found"));
-  else
-  {
-    Serial.println(F("missing"));
-    while(1) {
-      digitalWrite(LED_BUILTIN, HIGH);  // turn the LED on (HIGH is the voltage level)
-      delay(500);                      // wait for half a second
-      digitalWrite(LED_BUILTIN, LOW);   // turn the LED off by making the voltage LOW
-      delay(500);  
-    }
-  }
-
   // The standard library of the Si7021 sets the heater element to the default 3.1mA, but we want the full power
   // We could alter the library, but than we break compatibility. So for this one time we do a raw-write to the 
   // heater register.
@@ -601,7 +501,7 @@ void loop() {
     digitalWrite(LED_BUILTIN, HIGH);  // LED as heartbeat
 
     // Read temperature and humidity
-    ReadSi7021();
+    ReadHYT221();
 
     // Read pressure and temperature
     ReadBMP280();