Beaglebone and SI7021 sensor example in Python

In this article we look at another Humidity  and  Temperature  Sensor – this time its the Si7021  and we will connect it to our Beaglebone and we will have a python example

First lets take a look at the sensor in question

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

This was my sensor of choice

Parts Required

Schematic/Connection

Code Example

[codesyntax lang=”python”]

import smbus
import time


bus = smbus.SMBus(2)

# SI7021 address, 0x40(64)
# Read data, 2 bytes, Humidity MSB first
rh = bus.read_i2c_block_data(0x40, 0xE5, 2) 
#what really happens here is that master sends a 0xE5 command (measure RH, hold master mode) and read 2 bytes back
#if you read 3 bytes the last one is the CRC!
time.sleep(0.1)
# Convert the data
humidity = ((rh[0] * 256 + rh[1]) * 125 / 65536.0) - 6


# SI7021 address, 0x40(64)
# Read data , 2 bytes, Temperature MSB first
temp = bus.read_i2c_block_data(0x40, 0xE3,2)
#what really happens here is that master sends a 0xE3 command (measure temperature, hold master mode) and read 2 bytes back 
#if you read 3 bytes the last one is the CRC!
time.sleep(0.1)

# Convert the data
cTemp = ((temp[0] * 256 + temp[1]) * 175.72 / 65536.0) - 46.85
fTemp = cTemp * 1.8 + 32

# Output data to screen

print ("Humidity %%RH: %.2f%%" %humidity)
print ("Temperature Celsius: %.2f C" %cTemp)
print ("Temperature Fahrenheit: %.2f F" %fTemp)

[/codesyntax]

Output

Run this example and you should see the following –

debian@beaglebone:/var/lib/cloud9/$ python si7021.py
Humidity %RH: 39.00%
Temperature Celsius: 14.38 C
Temperature Fahrenheit: 57.88 F
debian@beaglebone:/var/lib/cloud9/$ python si7021.py
Humidity %RH: 39.00%
Temperature Celsius: 14.24 C
Temperature Fahrenheit: 57.63 F
debian@beaglebone:/var/lib/cloud9/$

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