The MagPi - July 2018

raspberrypi.org/magpi July 2018 73

BUILD A WEATHER STATION Feature

OPEN IDLE

In Raspbian, select Menu > Programming >

Python 3 (IDLE). Create a new Python file,

save it as bme280_sensor.py in the /home/pi/

weather-station directory and add the code

from bme_280_sensor.py to it.

DEEP BREATH

While the code is running, exhale onto the

BME280 sensor. You should see the humidity

value (and possibly the temperature) increase.

Press CTRL+C to exit the program.

UPDATE THE CODE

Replace the while True loop with a

function called read_all() that returns

the humidity, pressure, and temperature

readings, in that order. Update your code using

bme_280_sensor_2.py as a guide.

GROUND TEMPERATURE

The BME280 will report the air temperature,

but this can be warmer than the ground,

particularly if it’s frosty. A DS18B20 thermal

probe (magpi.cc/JILuTd) stuck into the soil is

a useful supplemental measurement.

WIRING THE SENSOR

Normally the DS18B20 comes with three bare

wires, so the easiest way to test it is to use

PCB mount screw terminal blocks that can be

plugged into breadboards. Add your DS18B20 to

your circuit as shown in Figure 1.

EDIT THE BOOT

Open the file /boot/config.txt:

sudo nano /boot/config.txt

Edit it by adding the line below at

the bottom:

Right The Adafruit BME280 I^2 C

or SPI Temperature Humidity

STEP 1 Pressure Sensor

STEP 2

STEP 3

STEP 4

STEP 5

import bme280

import smbus2

from time import sleep

port = 1

address = 0x77 # Adafruit BME280 address. Other BME280s

may be different

bus = smbus2.SMBus(port)

bme280.load_calibration_params(bus,address)

while True:

bme280_data = bme280.sample(bus,address)

humidity = bme280_data.humidity

pressure = bme280_data.pressure

ambient_temperature = bme280_data.temperature

print(humidity, pressure, ambient_temperature)

sleep( 1 )

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bme280_sensor.py

dtoverlay=w1-gpio

Then open /etc/modules:

sudo nano /etc/modules

Add the lines below at the bottom

of the file:

w1-gpio

w1-therm

Reboot the Raspberry Pi.

Open the file /home/pi/weather-station/

ds18b20_therm.py in IDLE and run it (see

‘Software download’ box on page 71 to use

Git to clone this file). You should see the

temperature printed out in the Python

Shell window.

import bme280

import smbus2

from time import sleep

port = 1

address = 0x76

bus = smbus2.SMBus(port)

bme280_load_calibration_params(bus, address)

def read_all():

bme280_data = bme.sample(bus,address)

return bme280_data.humidity, bme280_data.pressure,

bme280_data.temperature

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bme280_sensor_2.py

CODE

Language: Python

Name:

bme280_sensor.py,

bme280_sensor_2.py

Link:

magpi.cc/GhJlde

Name:

ds18b20_therm.py

Link:

magpi.cc/NQGLtb

STEP 6