Temperature and Humidity sensing with DHT22 or DS18B20

What we are doing today is somewhat different than a led or a switch. Instead of just turning a GPIO on and off, or reading the state of a GPIO we are going to connect to and communicate with a "smart" device.

We will send it digital requests
it will respond with digital answers.
All actually quite complex.
Fortunately we don't have to concern ourselves with the complexity.
We install libraries to do this for us.
Thanks to the folks that build these libraries.

                  DHT22

Or


                                    DS18B20

DHT22

Download the DHT Library for DHT22

Download the code from github

cd  #to ensure that you are in the pi home directory
git clone https://github.com/adafruit/Adafruit_Python_DHT.git
cd Adafruit_Python_DHT

Install the Library and Dependancies

sudo apt-get update
sudo apt-get install build-essential python-dev python-openssl

If you see an error that a package is already installed or at the latest version, don't worry you can ignore it and move on.

Build and Install the Library

sudo python setup.py install

Circuit

GPIO pin is pin 16 which is board pin 36

Code for DHT22

#! /usr/bin/python

import RPi.GPIO as GPIO
import time

import Adafruit_DHT

# Set up parameters
cycleTime = 5 # seconds
sensor = 22 # sensor type
pin = 16 # sensor data pin

# Loop forever
while 1:

    #get the humidity and temperature.
    humidity, temperature = Adafruit_DHT.read_retry(sensor, pin)
    print temperature, ", ", humidity
    print('Temp={0:0.1f}*C Humidity={1:0.1f}%\n'.format(temperature, humidity))
    time.sleep(cycleTime)

And when the above is run we get:

$ ./temperature.py
21.2999992371 , 53.0999984741
Temp=21.3*C Humidity=53.1%

21.1000003815 , 52.7999992371
Temp=21.1*C Humidity=52.8%

21.1000003815 , 52.9000015259
Temp=21.1*C Humidity=52.9%

DS18B20

Wire it up and Connect it to the Pi

DS18B20 Connections to the pi

Data to pin 7 on the pi
- to pin 9 on the pi
+ to pin 17 on the pi
4.7K Ohm pull-up resistor between the signal and + pins on the prototype board

See the reference section below for pictures of the connections

Enable the 1 wire interface

sudo raspi-config
choose Interfacing Options and select it and hit enter

Then Choose 1-Wire and select it and hit enter

Then select yes and hit enter.

Reboot: sudo reboot

Test that it works

sudo modprobe w1-gpio # modprobe adds modules to the Linux Kernel

sudo modprobe w1-therm

cd /sys/bus/w1/devices

28-051701206aff w1_bus_master1

cd 28* (This will take you to the 28 (whatever) directory)

cat w1_slave

        75 01 4b 46 7f ff 0c 10 16 : crc=16 YES

              75 01 4b 46 7f ff 0c 10 16 t=23312

Code for DS18B20

#! /usr/bin/python import osimport globimport time os.system('modprobe w1-gpio')os.system('modprobe w1-therm') base_dir = '/sys/bus/w1/devices/'device_folder = glob.glob(base_dir + '28*')[0]device_file = device_folder + '/w1_slave' def read_temp_raw(): f = open(device_file, 'r') lines = f.readlines() f.close() return lines def read_temp(): lines = read_temp_raw() while lines[0].strip()[-3:] != 'YES': time.sleep(0.2) lines = read_temp_raw() equals_pos = lines[1].find('t=') if equals_pos != -1: temp_string = lines[1][equals_pos+2:] temp_c = float(temp_string) / 1000.0 #temp_f = temp_c * 9.0 / 5.0 + 32.0 #return temp_c, temp_f return temp_c while True: print(read_temp()) time.sleep(1)

Reference

DHT22

DS18B20

BCM GPIO#	2nd func	pin#	pin#	2nd func	BCM GPIO#
-	+3V3	1	2	+5V	-
GPIO2	SDA1 (I2C)	3	4	+5V	-
GPIO3	SCL1 (I2C)	5	6	GND	-
GPIO4	GCLK	7	8	TXD0 (UART)	GPIO14
-	GND	9	10	RXD0 (UART)	GPIO15
GPIO17	GEN0	11	12	GEN1	GPIO18
GPIO27	GEN2	13	14	GND	-
GPIO22	GEN3	15	16	GEN4	GPIO23
-	+3V3	17	18	GEN5	GPIO24
GPIO10	MOSI (SPI)	19	20	GND	-
GPIO9	MISO (SPI)	21	22	GEN6	GPIO25
GPIO11	SCLK (SPI)	23	24	CE0_N (SPI)	GPIO8
-	GND	25	26	CE1_N (SPI)	GPIO7
(Models A and B stop here)
EEPROM	ID_SD	27	28	ID_SC	EEPROM
GPIO5	-	29	30	GND	-
GPIO6	-	31	32	-	GPIO12
GPIO13	-	33	34	GND	-
GPIO19	-	35	36	-	GPIO16
GPIO26	-	37	38	-	GPIO20
-	GND	39	40	-	GPIO21

Homework

1) Turn the LED on above a set temperature, and off below that temperature.

You will need to use an "if" statement to check the temperature. E.G.:

if temperature > 20:
	GPIO.output(pin, 1)
else:
	GPIO.output(pin, 0)

You may want to consider deadband ...

2) Add the push button into your temperature code. Each push of the button changes the set temperature by +.05 degrees C. Up to a predetermined temperature. Then the next push drops the temperature to a low limit. E.G. Start at 18 °C and go up in steps to 22°C.