schouten_tjeerd
Published © MIT

Arduino Chilled Mirror Hygrometer

Measure the dew point and humidity with an Arduino-based Chilled Mirror Hygrometer.

IntermediateFull instructions provided14,700
Arduino Chilled Mirror Hygrometer

Things used in this project

Hardware components

Arduino UNO
Arduino UNO
×1
12V 10A power supply
×1
12V 40mm high power fan
×1
12V 40mm TEC
×1
40mm heat sink
×1
small mirror
×1
Thermally conductive adhesive
×1
Thermal paste
×1
BTS7960 motor driver
×1
3 mm LED: Yellow
3 mm LED: Yellow
×1
OPT101 light sensor
×1
IRF520 MOSFET module
×1
DS18B20 Programmable Resolution 1-Wire Digital Thermometer
Maxim Integrated DS18B20 Programmable Resolution 1-Wire Digital Thermometer
×1
SparkFun Humidity and Temperature Sensor Breakout - Si7021
SparkFun Humidity and Temperature Sensor Breakout - Si7021
×1

Software apps and online services

Arduino IDE
Arduino IDE

Hand tools and fabrication machines

Hot glue gun (generic)
Hot glue gun (generic)

Story

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Schematics

Chilled Mirror Hygrometer schematic

Code

Chilled Mirror Hygrometer

C/C++
Arduino code
#include <math.h>
#include <avr/wdt.h>  //Watchdog crash detection

//These are custom libraries.
#include "Si7021.h" //humidity sensor with heater
#include <OneWire.h> //DS18B20 temp sensor
#include <DallasTemperature.h> //DS18B20 temp sensor

//Timer library: https://github.com/brunocalou/Timer
#include "timer.h"
#include "timerManager.h" 

//Define the hardware pins on the Arduino board.
#define coolingPWM 6
#define heatingPWM 5
#define coolingEnable 13
#define heatingEnable 12
#define tecFan 7
#define opticalSensor 0 //Analog in
#define oneWireBus A3 //DS18B20 temp sensor

//The state of the TEC.
#define COOLING 0
#define HEATING 1
#define OFF 2

//Timers
Timer timerMainLoop;
Timer timerTecCooling;
Timer timerSampleNoise;

//Temperature sensor (humidity not used). 
Si7021 si7021;

//DS18B20 temp sensor
OneWire oneWire(oneWireBus); 
DallasTemperature sensors(&oneWire);

float humidity = 0;
float ambientTemp = 0;
float opticalDewpoint = 0;

//Set these to an initial higher value to get the Serial Plotter range correct.
float mirrorTemp = 30; 
float optical = 30; 
float dewPoint = 15; //initial value must be lower than the mirror temp.
float relativeHumidity = 30;

int tecState = OFF;
bool cooling = false;

int intervalTecCooling = 200; //How often the TEC timer is updated in ms.
float opticalThreshold = 0.5f; //0.5 //The amount of degrees C the optical reading has to drop below the reference in order to flag condensation detection. This must be a bigger number than the signal noise.
int pwmIncrement = 1; 
int startPwm = 100;
int maxPwm = 255;
int intervalMainLoop = 200;  
int tecPwm = 0;
int noiseSampleIndex = 0;
int noiseSampleAmount = 10;
float noiseSampleHighest = 0;
float noiseSampleLowest = 10000;
bool noiseSampling = false;



float calculateHumidity(float TD, float T){

  //The dew point cannot be higher than the temperature.
  if(TD > T){

    TD = T;
  }

  //August-Roche-Magnus approximation. 
  float rh = 100*(exp((17.625*TD)/(243.04+TD))/exp((17.625*T)/(243.04+T)));

  return rh;
}


//Set the TEC to heating, cooling, or off.
void SetTEC(int state, int amount){

  tecState = state;

  //Note that for both heating and cooling, the heating AND cooling pin need to be set to high. Ask the PCB designer why.
  //Driver used to control the TEC: BTS7960 motor driver board. Note that PWM to drive a TEC is not efficient and it is better to use a variable current source. 
  switch(state)
  {
    case COOLING: 
      digitalWrite(heatingEnable, HIGH); 
      analogWrite(heatingPWM, 0);   
      digitalWrite(coolingEnable, HIGH);
      analogWrite(coolingPWM, amount); 
      break;
      
    case HEATING: 
      digitalWrite(coolingEnable, HIGH);
      analogWrite(coolingPWM, 0); 
      digitalWrite(heatingEnable, HIGH); 
      analogWrite(heatingPWM, amount);   
      break;

    case OFF: 
      digitalWrite(coolingEnable, LOW);
      analogWrite(coolingPWM, 0); 
      digitalWrite(heatingEnable, LOW); 
      analogWrite(heatingPWM, 0); 
      break;
      
    default: 
      digitalWrite(coolingEnable, LOW);
      analogWrite(coolingPWM, 0); 
      digitalWrite(heatingEnable, LOW); 
      analogWrite(heatingPWM, 0);
  }
}

void setup() {

  //Watchdog crash detection. This is for safety because you don't want the TEC to be stuck in heating mode.
  wdt_enable(WDTO_2S); //WDTO_500MS //WDTO_1S
  
  Serial.begin(9600); //9600 //57600

  pinMode(coolingPWM, OUTPUT);
  pinMode(heatingPWM, OUTPUT);
  pinMode(coolingEnable, OUTPUT);
  pinMode(heatingEnable, OUTPUT);
  pinMode(tecFan, OUTPUT);
  pinMode(opticalSensor, INPUT);

  //Setup the timers
  timerMainLoop.setInterval(intervalMainLoop);
  timerMainLoop.setCallback(mainLoop);
  timerMainLoop.start();

  timerTecCooling.setInterval(intervalTecCooling);
  timerTecCooling.setCallback(tecCoolingCallback);

  timerSampleNoise.setInterval(intervalTecCooling);
  timerSampleNoise.setCallback(sampleNoiseCallback);

  //si7021 temp sensor setup.
  uint64_t serialNumber = 0ULL;
  si7021.begin();
  serialNumber = si7021.getSerialNumber();

  //DS18B20 onewire temperature sensor
  sensors.begin(); 

  //Disable the temp sensor debug logging in order to get the graph to work correctly.
  /*
  Serial.print("Si7021 serial number: ");
  Serial.print((uint32_t)(serialNumber >> 32), HEX);
  Serial.println((uint32_t)(serialNumber), HEX);
  //Firware version
  Serial.print("Si7021 firmware version: ");
  Serial.println(si7021.getFirmwareVersion(), HEX);
  */

  startNoiseSampling();
  
}

//Get the optical sensor reading.
float getOptical(){

  int opt = analogRead(opticalSensor);
  float optFactored = (float)opt / 30.0f;

  return optFactored;
}

//Timer callback.
void tecCoolingCallback(){
  
  digitalWrite(tecFan, HIGH);

  //Slowly increase the power of the TEC.
  tecPwm += pwmIncrement;

  //Clamp
  if(tecPwm > maxPwm){

    tecPwm = maxPwm;
  }

  //Set the TEC cooling amount
  SetTEC(COOLING, tecPwm); 

  //Is condensation detected?
  if(optical <= (noiseSampleLowest - opticalThreshold)){

    //Log the dew point;
    dewPoint = mirrorTemp;
    opticalDewpoint = optical;

    stopTec();
  }
}

void startNoiseSampling(){

  noiseSampling = true;
  noiseSampleHighest = 0;
  noiseSampleLowest = 10000;
  timerSampleNoise.start();
}

void sampleNoiseReset(){

  timerSampleNoise.stop();
  noiseSampleIndex = 0;
  noiseSampling = false;
}

void sampleNoiseCallback(){

  
  if(noiseSampleIndex > noiseSampleAmount){

    sampleNoiseReset();
    startTecCooling();
  }

  else{

    if(optical > noiseSampleHighest){

      noiseSampleHighest = optical;
    }

    if(optical < noiseSampleLowest){

      noiseSampleLowest = optical;
    }
  }

  noiseSampleIndex++;
}


void startTecCooling(){

  cooling = true;

  digitalWrite(tecFan, HIGH); 

  tecPwm = startPwm;

  //Start the TEC counter callback.
  timerTecCooling.start();
}

void stopTec(){

  cooling = false;

  //Turn the TEC fan off.
  digitalWrite(tecFan, LOW); 
  
  timerTecCooling.stop();

  //No cooling, no heating
  SetTEC(OFF, 0);  
}


//Non blocking timer.
void mainLoop(){

  //DS18B20 temp sensor for ambient temperature.
  sensors.setResolution(10); //has to be done before each temperature measurement. Note that a higher resolution is slower.
  sensors.requestTemperatures();
  ambientTemp = sensors.getTempCByIndex(0);  

  //si7021 temps sensor for mirror.
  mirrorTemp = si7021.measureTemperature();

  //Get the optical sensor reading.
  optical = getOptical();

  relativeHumidity = calculateHumidity(dewPoint, ambientTemp);

/*
 //Readable format
  Serial.print("dewPoint: ");
  Serial.println(dewPoint, 2);
  Serial.print("mirrorTemp: ");
  Serial.println(mirrorTemp, 2);
  Serial.print("ambientTemp: ");
  Serial.println(ambientTemp, 2);
  Serial.print("relativeHumidity: ");
  Serial.println(relativeHumidity, 2);

*/

  //For the Serial Plotter
  Serial.print(mirrorTemp, 2);
  Serial.print(" ");
  Serial.print(optical, 2);
  Serial.print(" ");
  Serial.println(dewPoint, 2);
 // Serial.print(" ");
 // Serial.println(relativeHumidity, 2);
  
  
  

  //Wait for the condensation to disappear
  if(!cooling && !noiseSampling && (optical >= noiseSampleLowest)){
  
    startNoiseSampling();
  }  
}

void loop() {

  //Watchdog crash detection
  wdt_reset();

  //Update all timers.
  TimerManager::instance().update();
}

Credits

schouten_tjeerd
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