Beaglebone and BME280 humidity sensor python example

In this article we look at another sensor – this time its the BME280 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 BME280 is a humidity sensor especially developed for mobile applications and wearables where size and low power consumption are key design parameters. The unit combines high linearity and high accuracy sensors and is perfectly feasible for low current consumption, long-term stability and high EMC robustness.

The humidity sensor offers an extremely fast response time and therefore supports performance requirements for emerging applications such as context awareness, and high accuracy over a wide temperature range.

Parts Required

Schematic/Connection

Code Example

This is a controleverything example .Save this as bme280.py

[codesyntax lang=”python”]

import smbus
import time

# Get I2C bus
bus = smbus.SMBus(2)

# BME280 address, 0x76(118)
# Read data back from 0x88(136), 24 bytes
b1 = bus.read_i2c_block_data(0x76, 0x88, 24)

# Convert the data
# Temp coefficients
dig_T1 = b1[1] * 256 + b1[0]
dig_T2 = b1[3] * 256 + b1[2]
if dig_T2 > 32767 :
    dig_T2 -= 65536
dig_T3 = b1[5] * 256 + b1[4]
if dig_T3 > 32767 :
    dig_T3 -= 65536

# Pressure coefficients
dig_P1 = b1[7] * 256 + b1[6]
dig_P2 = b1[9] * 256 + b1[8]
if dig_P2 > 32767 :
    dig_P2 -= 65536
dig_P3 = b1[11] * 256 + b1[10]
if dig_P3 > 32767 :
    dig_P3 -= 65536
dig_P4 = b1[13] * 256 + b1[12]
if dig_P4 > 32767 :
    dig_P4 -= 65536
dig_P5 = b1[15] * 256 + b1[14]
if dig_P5 > 32767 :
    dig_P5 -= 65536
dig_P6 = b1[17] * 256 + b1[16]
if dig_P6 > 32767 :
    dig_P6 -= 65536
dig_P7 = b1[19] * 256 + b1[18]
if dig_P7 > 32767 :
    dig_P7 -= 65536
dig_P8 = b1[21] * 256 + b1[20]
if dig_P8 > 32767 :
    dig_P8 -= 65536
dig_P9 = b1[23] * 256 + b1[22]
if dig_P9 > 32767 :
    dig_P9 -= 65536

# BME280 address, 0x76(118)
# Read data back from 0xA1(161), 1 byte
dig_H1 = bus.read_byte_data(0x76, 0xA1)

# BME280 address, 0x76(118)
# Read data back from 0xE1(225), 7 bytes
b1 = bus.read_i2c_block_data(0x76, 0xE1, 7)

# Convert the data
# Humidity coefficients
dig_H2 = b1[1] * 256 + b1[0]
if dig_H2 > 32767 :
    dig_H2 -= 65536
dig_H3 = (b1[2] &  0xFF)
dig_H4 = (b1[3] * 16) + (b1[4] & 0xF)
if dig_H4 > 32767 :
    dig_H4 -= 65536
dig_H5 = (b1[4] / 16) + (b1[5] * 16)
if dig_H5 > 32767 :
    dig_H5 -= 65536
dig_H6 = b1[6]
if dig_H6 > 127 :
    dig_H6 -= 256

# BME280 address, 0x76(118)
# Select control humidity register, 0xF2(242)
#		0x01(01)	Humidity Oversampling = 1
bus.write_byte_data(0x76, 0xF2, 0x01)
# BME280 address, 0x76(118)
# Select Control measurement register, 0xF4(244)
#		0x27(39)	Pressure and Temperature Oversampling rate = 1
#					Normal mode
bus.write_byte_data(0x76, 0xF4, 0x27)
# BME280 address, 0x76(118)
# Select Configuration register, 0xF5(245)
#		0xA0(00)	Stand_by time = 1000 ms
bus.write_byte_data(0x76, 0xF5, 0xA0)

time.sleep(0.5)

# BME280 address, 0x76(118)
# Read data back from 0xF7(247), 8 bytes
# Pressure MSB, Pressure LSB, Pressure xLSB, Temperature MSB, Temperature LSB
# Temperature xLSB, Humidity MSB, Humidity LSB
data = bus.read_i2c_block_data(0x76, 0xF7, 8)

# Convert pressure and temperature data to 19-bits
adc_p = ((data[0] * 65536) + (data[1] * 256) + (data[2] & 0xF0)) / 16
adc_t = ((data[3] * 65536) + (data[4] * 256) + (data[5] & 0xF0)) / 16

# Convert the humidity data
adc_h = data[6] * 256 + data[7]

# Temperature offset calculations
var1 = ((adc_t) / 16384.0 - (dig_T1) / 1024.0) * (dig_T2)
var2 = (((adc_t) / 131072.0 - (dig_T1) / 8192.0) * ((adc_t)/131072.0 - (dig_T1)/8192.0)) * (dig_T3)
t_fine = (var1 + var2)
cTemp = (var1 + var2) / 5120.0
fTemp = cTemp * 1.8 + 32

# Pressure offset calculations
var1 = (t_fine / 2.0) - 64000.0
var2 = var1 * var1 * (dig_P6) / 32768.0
var2 = var2 + var1 * (dig_P5) * 2.0
var2 = (var2 / 4.0) + ((dig_P4) * 65536.0)
var1 = ((dig_P3) * var1 * var1 / 524288.0 + ( dig_P2) * var1) / 524288.0
var1 = (1.0 + var1 / 32768.0) * (dig_P1)
p = 1048576.0 - adc_p
p = (p - (var2 / 4096.0)) * 6250.0 / var1
var1 = (dig_P9) * p * p / 2147483648.0
var2 = p * (dig_P8) / 32768.0
pressure = (p + (var1 + var2 + (dig_P7)) / 16.0) / 100

# Humidity offset calculations
var_H = ((t_fine) - 76800.0)
var_H = (adc_h - (dig_H4 * 64.0 + dig_H5 / 16384.0 * var_H)) * (dig_H2 / 65536.0 * (1.0 + dig_H6 / 67108864.0 * var_H * (1.0 + dig_H3 / 67108864.0 * var_H)))
humidity = var_H * (1.0 -  dig_H1 * var_H / 524288.0)
if humidity > 100.0 :
    humidity = 100.0
elif humidity < 0.0 :
    humidity = 0.0

# Output data to screen
print "Temperature in Celsius : %.2f C" %cTemp
print "Temperature in Fahrenheit : %.2f F" %fTemp
print "Pressure : %.2f hPa " %pressure
print "Relative Humidity : %.2f %%" %humidity

[/codesyntax]

Output

Run this example and you should see the following

debian@beaglebone:/var/lib/cloud9/$ python bme280.py
Temperature in Celsius : 22.02 C
Temperature in Fahrenheit : 71.63 F
Pressure : 998.04 hPa
Relative Humidity : 0.00 %
debian@beaglebone:/var/lib/cloud9/$ python bme280.py
Temperature in Celsius : 25.27 C
Temperature in Fahrenheit : 77.48 F
Pressure : 998.06 hPa
Relative Humidity : 0.00 %
debian@beaglebone:/var/lib/cloud9/$ python bme280.py
Temperature in Celsius : 26.65 C
Temperature in Fahrenheit : 79.97 F
Pressure : 998.04 hPa
Relative Humidity : 0.00 %

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