Raspberry Pi Temp Sensor Part 1
Over the last few years I have been teaching a class called Mechatronics that combines together engineering and robotics with a goal for students to build real world technology to solve a problem. In building student learning for open ended tasks, I needed a project that could combine many skills together. I had previously used Arduino's to enable a programmable device with I/O but I was happy bring in other technologies. I had also been coding more and more with Swift and making Apps with Xcode. As my students had also begun their journey with App Development in other classes, I thought it would be a great opportunity to leverage this knowledge and see how I could show live data from a temperature sensor in an App. So this lead me to the path of using a Raspberry Pi to become the hub of the project as I could basically build a database and web server that stored and shared data collected from a temperature sensor. This data could then be shown via an iOS app.
Essentially this series of guides will cover:
- Setting up a DS18B20 temperature sensor
- Writing a Python script to read the sensor
- Setup of LAMP. Linux, Apache, MySQL & PHP for the webserver
- Writing the values from the temp sensor to the MySQL database
- Writing PHP scripts to access the MySQL database from a web browser
- Outputting the results as JSON for a mobile app to read.
- An simple iOS app to read our temp sensor values and display them
Acknowledgement
Now in my Googling, I came across an excellent website that covered all these requirements. Unfortunately as we followed through the guides we found that due to age we had to make changes along the way and was only solved by spending time on Google looking for answers. With permission, my guides will follow closely to what these guides had instructed. There are so many little changes that it will be easier to reproduce the work rather than identifying the changes needed to get everything working.
Download
Once you have your Raspberry Pi you will need to install Raspbian and I also recommend using an app called Etcher to install onto the SD Card.
www.raspberrypi.org/downloads/raspbian/
Once installed the first time you boot your Pi you will be able to run a software update, which I highly recommend. Otherwise you will need to run the following commands from the Terminal:
sudo apt-get update sudo apt-get upgrade
Also you will need to go into the Raspberry Pi Configuration under Preferences to enable SSH and 1-Wire. Once you have enabled these settings you can remote setup your Pi via SSH (which makes for a much easier experience) rather than using a keyboard, mouse and monitor connected.
To SSH open Terminal and type:
ifconfig
and depending if you are using wifi or Ethernet you will see something like
inet 192.168.0.20
Which really does depend on the network you are using.
On your computer you can then SSH using:
ssh pi@192.168.0.20
Wiring Up
To wire up the circuit you will need a bread board, a 10k Ohm resistor some prototyping wires and the temperature sensor. As I wanted to set this up for an aquarium I used a water proof one:
Waterproof DS18B20 Digital temperature sensor
core-electronics.com.au/waterproof-ds18b20-digital-temperature-sensor.html
The circuit uses 1-wire comms which is great feature because each temperature sensor can be wired to all connect to the same pin on the Raspberry Pi. Apparently you can have up to 75 temperature sensors reading at 1 sec intervals connected straight into the same pin. By default this pin is number 4.
Bread Board 
Pi Wiring 
Connecting Pi to Board
Reading the Temperature Sensor
We need to load the drivers for the 1-wire comms and the temp sensor into the Pi kernel. Modprobe is a Linux program to add a loadable kernel into the Linux kernel. In your terminal enter:
sudo modprobe w1-gpio sudo modprobe w1-therm
Now change your working directory using the following command:
cd /sys/bus/w1/devices/
This is where the devices running on the 1-wire will be. So to find our newly created device just list the contents of the directory with ls.
ls
Now you should see something listed like
28-00000622fd44 w1_bus_master1
This is the serial number for the device. To interrogate that device we need to go into its directory. Make sure you use the serial number of your own sensor!!
cd 28-00000622fd44
The sensor writes to a file called w1_slave so if we just read that file we can now finally know what temperature it is. Enter:
cat w1_slave
cat simply displays the contents of a file.
You will get the following output:
0b 01 4b 46 7f ff 05 10 a8 : crc=a8 YES 0b 01 4b 46 7f ff 05 10 a8 t=16687
The second line shows the temp “t = xxxxx” in degrees Celsius. This may seem high but that’s because you need to format the data, as the value for t is the temperature to 3dp. So read it as t = xx.xxx . For example 16.687.
Now this is great and all but it’s a bit on the laborious side so lets write a Python script to do all of the hard work for us.
Writing a Python script
Go back to your home directory:
cd ~
and make a new directory called tempLog:
mkdir tempLog
Go into the new directory:
cd tempLog
and create a new python file in nano (which is just a text editing app):
sudo nano getTemp.py
Copy the code below taking care to use your own value for the sensor. Explanation beneath the code.
import os
import time
import datetime
import glob
from time import strftime
os.system('modprobe w1-gpio')
os.system('modprobe w1-therm')
temp_sensor = '/sys/bus/w1/devices/28-00000622fd44/w1_slave'
def tempRead():
t = open(temp_sensor, 'r')
lines = t.readlines()
t.close()
temp_output = lines[1].find('t=')
if temp_output != -1:
temp_string = lines[1].strip()[temp_output+2:]
temp_c = float(temp_string)/1000.0
return round(temp_c,1)
while True:
temp = tempRead()
print temp
datetimeWrite = (time.strftime("%Y-%m-%d ") + time.strftime("%H:%M:%S"))
print datetimeWrite
break
We first import all of the Python libraries we need. We then load the gpio and therm kernels again using modprobe. Rather than write it out again and again we also use a variable to store our path to the sensor’s w1_slave file, I called it temp_sensor.
A method called tempRead is defined. In this method we read in the w1_slave file using open ‘r’ The file is then converted in to an array of strings called ‘lines’ using readlines.
If you remember it is the second line of the w1_slave file that contains the temperature so we search lines[1] using find for “t=”. Which will give us an index value of where that string occurs. If the returned value from that find is not -1(which means it didn’t find it) we then strip out the temperature reading as a string from the index where “t=” was 2 to the end of the line. If you didn’t 2 you would get “t=xxxxx” as the temperature, we want “xxxxx”.
Then the string is converted to a float, divided by 1000 to put the temperature into proper format of xx.xxx and then returned to the while loop.
When the script is run the first thing it does is go to the while loop. Here it will call our previously defined method tempRead to get the value of the sensor. It will then print the value to the output. Then just to make things look a bit nicer it gets the current date and time and prints it out on the output one line below. Break then stops the while loop and ends the script. I used a while loop so that if you wish you can remove break and insert time.sleep(10) for example so that the temperature and current date and time is output every 10 seconds.
Hopefully that all makes sense. Close the nano file by pressing ctrl and x. It will prompt you to save, press y to confirm writing and the filename.
Now lets give it a go. While still in the tempLog directory enter:
sudo python getTemp.py
You will get something like this:
pi@raspberrypi ~/tempLog $ sudo python getTemp.py 16.4 2014-12-24 11:51:08
We now have the basic workings of our temperature sensor and logger setup. In part 2 we will set up the web server and MySQL table for our data.