A look at the SI7021 Temperature Sensor with Arduino and Raspberry Pi examples

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In this article we look at the SI7021 Temperature  Sensor with examples for the Arduino and Raspberry Pi

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

  • Relative Humidity Sensor:
    • Si7013/21: ± 3% RH (maximum) @ 0-80% RH
    • Si7020: ± 4% RH (maximum) @ 0-80% RH
    • Si7006: ± 5% RH (maximum) @ 0-80% RH
  • Temperature Sensor:
    • Si7013/20/21: ±0.4°C accuracy (maximum) @ -10 to +85°C
    • Si7006: ±1.0°C accuracy (maximum) @ -10 to +85°C
  • 0 to 100% RH operating range
  • Up to -40 to +125°C operating range
  • Wide operating voltage range (1.9 to 3.6V)
  • Low Power Consumption: 2.2µW average power at 3.3V and 1 sample per second
  • I2C host interface
  • Integrated on-chip heater
  • 3mm x 3mm QFN package
  • Excellent long term stability
  • Factory calibrated
  • Optional factory-installed filter/cover
    • Lifetime protection during reflow and in operation
    • Protects against contamination from dust, dirt, household chemicals and other liquids
    • AEC-Q100 automotive qualified (Si7013/20/21)

 

Raspberry Pi and Si7021

Parts List

Part Type Link
Si7021 sensor Atmospheric Humidity Temperature Sensor Breakout Barometric Pressure BMP280 SI7021 for Arduino GY-21P
Raspberry Pi Raspberry Pi Zero W (Wireless) (new 2017 model)
dupont wire Free shipping Dupont line 120pcs 20cm male to male + male to female and female to female jumper wire

Schematics and Connections

 

pi and si7021
pi and si7021

Code

Python example

Save the following as SI7021.py

import smbus
import time
 
# Get I2C bus
bus = smbus.SMBus(1)
bus.write_byte(0x40, 0xF5)
 
time.sleep(0.3)
 
# SI7021 address, 0x40  Read 2 bytes, Humidity
data0 = bus.read_byte(0x40)
data1 = bus.read_byte(0x40)
 
# Convert the data
humidity = ((data0 * 256 + data1) * 125 / 65536.0) - 6
 
time.sleep(0.3)
bus.write_byte(0x40, 0xF3)
time.sleep(0.3)
 
# SI7021 address, 0x40 Read data 2 bytes, Temperature
data0 = bus.read_byte(0x40)
data1 = bus.read_byte(0x40)
 
# Convert the data and output it
celsTemp = ((data0 * 256 + data1) * 175.72 / 65536.0) - 46.85
fahrTemp = celsTemp * 1.8 + 32
 
print "Relative Humidity is : %.2f %%" %humidity
print "Temperature in Celsius is : %.2f C" %celsTemp
print "Temperature in Fahrenheit is : %.2f F" %fahrTemp

Run the code by typing – sudo python SI7021.py on the command line, you will see the following

si7021 output
si7021 output

Java example

This time we explore the world of Java on the Raspberry Pi

First you need to install PI4j – http://pi4j.com/install.html . I’ll sum it up as its easy to install from a terminal

The simplest method to install Pi4J on your RaspberryPi is to execute the following command directly on your RaspberryPi.
curl -s get.pi4j.com | sudo bash

Now for the java code – this is courtesy of a controleverything example

// Distributed with a free-will license.
// Use it any way you want, profit or free, provided it fits in the licenses of its associated works.
// SI7021
// This code is designed to work with the SI7021_I2CS I2C Mini Module available from ControlEverything.com.
// https://www.controleverything.com/content/Humidity?sku=SI7021_I2CS#tabs-0-product_tabset-2
 
import com.pi4j.io.i2c.I2CBus;
import com.pi4j.io.i2c.I2CDevice;
import com.pi4j.io.i2c.I2CFactory;
import java.io.IOException;
 
public class SI7021
{
	public static void main(String args[]) throws Exception
	{
		// Create I2C bus
		I2CBus bus = I2CFactory.getInstance(I2CBus.BUS_1);
		// Get I2C device, SI7021 I2C address is 0x40(64)
		I2CDevice device = bus.getDevice(0x40);
 
		// Send humidity measurement command
		device.write((byte)0xF5);
		Thread.sleep(300);
 
		// Read 2 bytes of humidity data, msb first
		byte[] data = new byte[2];
		device.read(data, 0, 2);
 
		// Convert humidity data
		double humidity = (((((data[0] & 0xFF) * 256) + (data[1] & 0xFF)) * 125.0) / 65536.0) - 6;
 
		// Send temperature measurement command
		device.write((byte)0xF3);
		Thread.sleep(300);
 
		// Read 2 bytes of temperature data, msb first
		device.read(data, 0, 2);
 
		// Convert temperature data
		double cTemp = (((((data[0] & 0xFF) * 256) + (data[1] & 0xFF)) * 175.72) / 65536.0) - 46.85;
		double fTemp = (cTemp * 1.8 ) + 32;
 
		// Output data to screen
		System.out.printf("Relative Humidity : %.2f %% %n", humidity);
		System.out.printf("Temperature in Celsius : %.2f C%n", cTemp);
		System.out.printf("Temperature in Fahrenheit : %.2f F%n", fTemp);
	}
}

Now you have to compile and run the program like this

$> sudo pi4j SI7021.java
si7021 java output
si7021 java output

Arduino and Si7021

The good thing about the Arduino example is that this will work with many boards if you have enabled support in the IDE. I have tried this on an ESP32, ESP8266 and Micro:bit

Parts List

 

Part Type Link
Si7021 sensor Atmospheric Humidity Temperature Sensor Breakout Barometric Pressure BMP280 SI7021 for Arduino GY-21P
Arduino UNO R3 CH340G/ATmega328P, compatible for Arduino UNO
dupont wire Free shipping Dupont line 120pcs 20cm male to male + male to female and female to female jumper wire

Schematics and Connections

 

arduino and si7021
arduino and si7021

Code

#include <Wire.h>
 
const int ADDR =0x40;
int X0,X1,Y0,Y1,Y2,Y3;
double X,Y,X_out,Y_out1,Y_out2;
 
void setup()
{ 
  Serial.begin(9600);   
  Wire.begin();                                   
  delay(100);  
  Wire.beginTransmission(ADDR);
  Wire.endTransmission();                        
}
 
void loop()
{
  /**Send command of initiating temperature measurement**/
  Wire.beginTransmission(ADDR);
  Wire.write(0xE3);
  Wire.endTransmission();
 
  Serial.print("Temp");
  Serial.print("\t");
  Serial.println("Humidity");
 
  /**Read data of temperature**/
  Wire.requestFrom(ADDR,2);
 
  if(Wire.available()<=2);
  {
    X0 = Wire.read();
    X1 = Wire.read();
    X0 = X0<<8;
    X_out = X0+X1;
  }
 
  /**Calculate and display temperature**/
  X=(175.72*X_out)/65536;                        
  X=X-46.85;
  Serial.print(X);
  Serial.print("C");
  Serial.print("\t");
 
  /**Send command of initiating relative humidity measurement**/
  Wire.beginTransmission(ADDR);                     
  Wire.write(0xE5);
  Wire.endTransmission(); 
 
  /**Read data of relative humidity**/
  Wire.requestFrom(ADDR,2);
  if(Wire.available()<=2);
  {
    Y0 = Wire.read();
    Y2=Y0/100; 
    Y0=Y0%100;
    Y1 = Wire.read();
    Y_out1 = Y2*25600;
    Y_out2 = Y0*256+Y1;
  }
 
  /**Calculate and display relative humidity**/
  Y_out1 = (125*Y_out1)/65536;                     
  Y_out2 = (125*Y_out2)/65536;
  Y = Y_out1+Y_out2;
  Y=Y-6;
  Serial.print(Y);
  Serial.println("%");                     
 delay(300);
 
 
Serial.println(); 
delay(1000);
 
}

Output

Open the serial monitor, you should see something like this

Temp    Humidity
23.12C    52.83%

Temp    Humidity
24.04C    53.13%

Temp    Humidity
26.28C    53.83%

Temp    Humidity
27.42C    54.57%

Temp    Humidity
28.27C    55.32%

Temp    Humidity
27.94C    56.11%

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