Raspberry Pi Sensors

This is a Python 3 package that enables Raspberry Pi to read various sensors (and interact with some non-sensors). It has been tested on Python 3.5/Raspbian.

Supported devices include:

• DS18B20 temperature sensor
• BMP180 pressure and temperature sensor
• HTU21D humidity and temperature sensor
• MCP3004 A/D Converter (MCP3008 also compatible)
• LCD1602 display

The chief motivation for this package is educational. I am teaching a Raspberry Pi course, and find it very troublesome for students having to download a separate library every time they use another sensor. With this package, download once and they are set (for my course, anyway). I hope you find it useful, too.

Installation

It is best to update Linux first.

sudo apt-get update
sudo apt-get upgrade

Install this package:

sudo pip3 install sensor

But the sensor package would not work by itself. Communicating with sensors often requires some sort of serial protocol, such as 1-wire, I2C, or SPI. You have to know which sensor speaks which, and set up Linux and Python accordingly.

Enable 1-Wire

sudo nano /boot/config.txt, add this line:

dtoverlay=w1-gpio

Reboot.

Enable I2C

sudo apt-get install i2c-tools python3-smbus

sudo nano /etc/modules, make sure this line is there:

i2c-dev

sudo nano /boot/config.txt, add this line (or uncomment it):

dtparam=i2c_arm=on

Reboot.

Enable SPI

sudo apt-get install python3-dev
sudo pip3 install spidev

sudo nano /boot/config.txt, add this line (or uncomment it):

dtparam=spi=on

Reboot.

Know your sensor's address

Unlike many libraries out there, this library knows no default bus number and no default device address. I want learners to be explicitly aware of those numbers, even if they are fixed.

For example:

• I2C bus is numbered 1
• SPI bus is numbered 0

To find out individual sensor's address:

• For 1-wire sensors, go to /sys/bus/w1/devices/
• For I2C sensors, use i2cdetect -y 1
• For SPI sensors, you should know which CS pin you use

My sensors don't give simple numbers

Unlike many libraries out there, this library does not return a simple Celcius degree when reading temperatures, does not return a simple hPa value when reading pressure, does not return a simple RH% when reading humidity, etc. Instead, I return a namedtuple representing the quantity, which offers two benefits:

• No more conversion needed. Suppose you get a Temperature called t, you may access the Celcius degree by t.C as easily as you do Fahrenheit by t.F.
• Namedtuples may have methods. For example, a Pressure has a method called altitude(), which tells you how high you are above mean sea level.

DS18B20

• Temperature, 1-wire

• To find out the sensor's address:

  $ cd /sys/bus/w1/devices/
  $ ls
  28-XXXXXXXXXXXX  w1_bus_master1
  

Read the sensor as follows:

from sensor.DS18B20 import DS18B20

ds = DS18B20('28-XXXXXXXXXXXX')
t = ds.temperature()  # read temperature

print(t)    # this is a namedtuple
print(t.C)  # Celcius
print(t.F)  # Fahrenheit
print(t.K)  # Kelvin

BMP180

• Pressure + Temperature, I2C
• Use i2cdetect -y 1 to check address. It is probably 0x77.

from sensor.BMP180 import BMP180

# I2C bus=1, Address=0x77
bmp = BMP180(1, 0x77)

p = bmp.pressure()  # read pressure
print(p)            # namedtuple
print(p.hPa)        # hPa value

t = bmp.temperature()  # read temperature
print(t)               # namedtuple
print(t.C)             # Celcius degree

p, t = bmp.all()  # read both at once
print(p)          # Pressure namedtuple
print(t)          # Temperature namedtuple

# Look up mean sea level pressure from local observatory.
# 1009.1 hPa is only for example.
a = p.altitude(msl=1009.1)

print(a)     # Altitude
print(a.m)   # in metre
print(a.ft)  # in feet

HTU21D

• Humidity + Temperature, I2C
• Use i2cdetect -y 1 to check address. It is probably 0x40.

from sensor.HTU21D import HTU21D

# I2C bus=1, Address=0x40
htu = HTU21D(1, 0x40)

h = htu.humidity()  # read humidity
print(h)            # namedtuple
print(h.RH)         # relative humidity

t = htu.temperature()  # read temperature
print(t)               # namedtuple
print(t.F)             # Fahrenheit

h, t = htu.all()  # read both at once

MCP3004

• Analog sensors (e.g. photoresistor) cannot interface with Raspberry Pi directly. They have to go through an A/D converter.

from sensor.MCP3004 import MCP3004

# SPI bus=0, CS=0, V_ref=3.3V
mcp = MCP3004(bus=0, addr=0, vref=3.3)

mcp.voltage(0)  # read voltage on channel 0

LCD1602

• Not a sensor, obviously. Useful for displaying sensor data.
• Use i2cdetect -y 1 to check address. It is probably 0x27.

from sensor.LCD1602 import LCD1602

# I2C bus=1, Address=0x27
lcd = LCD1602(1, 0x27)

lcd.display('Nick Lee', 1)   # my name on line 1
lcd.display('Hong Kong', 2)  # my city on line 2

lcd.clear()