Raspberry Pi and SI7021 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

pi-and-si7021_bb

Code

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

Testing

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

si7021-output

 

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

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