ESP8266 and Si7021 temperature sensor example

The  Si7021  I2C Humidity  and  Temperature  Sensor  is  a  monolithic  CMOS  IC integrating   humidity   and   temperature   sensor   elements,   an   analog-to-digital converter, signal processing, calibration data, and an I2C Interface. The Si7021 offers an accurate, low-power, factory-calibrated digital solution ideal for measuring humidity, dew-point, and temperature, in applications ranging from HVAC/R and asset tracking to industrial and consumer platforms.

Features

Precision Relative Humidity Sensor ± 3% RH (max), 0–80% RH
High Accuracy Temperature Sensor ±0.4 °C (max), –10 to 85 °C
0 to 100% RH operating range
Up to –40 to +125 °C operating range
Wide operating voltage – (1.9 to 3.6 V)
Low Power Consumption – 150 μA active current

Connection

Code

No 3rd party libraries required

[codesyntax lang=”python”]

#include <Wire.h>

// SI7021 I2C address is 0x40(64)
#define si7021Addr 0x40

void setup()
{
  Wire.begin();
  Serial.begin(9600);
  Wire.beginTransmission(si7021Addr);
  Wire.endTransmission();
  delay(300);
}

void loop()
{
  unsigned int data[2];
  
  Wire.beginTransmission(si7021Addr);
  //Send humidity measurement command
  Wire.write(0xF5);
  Wire.endTransmission();
  delay(500);
    
  // Request 2 bytes of data
  Wire.requestFrom(si7021Addr, 2);
  // Read 2 bytes of data to get humidity
  if(Wire.available() == 2)
  {
    data[0] = Wire.read();
    data[1] = Wire.read();
  }
    
  // Convert the data
  float humidity  = ((data[0] * 256.0) + data[1]);
  humidity = ((125 * humidity) / 65536.0) - 6;

  Wire.beginTransmission(si7021Addr);
  // Send temperature measurement command
  Wire.write(0xF3);
  Wire.endTransmission();
  delay(500);
    
  // Request 2 bytes of data
  Wire.requestFrom(si7021Addr, 2);
  
  // Read 2 bytes of data for temperature
  if(Wire.available() == 2)
  {
    data[0] = Wire.read();
    data[1] = Wire.read();
  }

  // Convert the data
  float temp  = ((data[0] * 256.0) + data[1]);
  float celsTemp = ((175.72 * temp) / 65536.0) - 46.85;
  float fahrTemp = celsTemp * 1.8 + 32;
   
  // Output data to serial monitor
  Serial.print("Humidity : ");
  Serial.print(humidity);
  Serial.println(" % RH");
  Serial.print("Celsius : ");
  Serial.print(celsTemp);
  Serial.println(" C");
  Serial.print("Fahrenheit : ");
  Serial.print(fahrTemp);
  Serial.println(" F");
  delay(1000);
}

[/codesyntax]

Testing

Open the serial monitor and you should see soemthing like this

Humidity : 50.55 % RH
Celsius : 22.20 C
Fahrenheit : 71.96 F
Humidity : 50.54 % RH
Celsius : 22.19 C
Fahrenheit : 71.94 F
Humidity : 50.96 % RH
Celsius : 26.13 C
Fahrenheit : 79.04 F
Humidity : 51.84 % RH
Celsius : 27.84 C
Fahrenheit : 82.11 F
Humidity : 53.12 % RH
Celsius : 27.97 C
Fahrenheit : 82.34 F

Link
GY-21 Humidity Sensor with I2C Interface Si7021 for Arduino