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1-Wire Digital Temperature Sensor For The Raspberry Pi By Greg Swain & Nicholas Vinen If you just want to measure temperature, then using a Sense HAT with the Raspberry Pi (RPi) is overkill. A much cheaper, easier and more accurate option is to use a Dallas DS18B20 1-Wire Digital Thermometer Sensor. T HE DALLAS DS18B20 temperature sensor looks just like a TO-92 transistor but is actually much more complicated. Its internal chip not only includes a temperature sensor but also has a 12-bit on-board digital-to-analog converter, a 1-wire serial interface and all the necessary control logic. It’s accurate to within ±0.5°C over the range of -10°C to +85°C and has a full operating temperature range from -55°C to +125°C. In addition, each DS18B20 has a unique 64-bit serial code, which allows multiple DS18B20s to function on the same 1-Wire bus. If you want to use multiple sensors, it’s just a matter of connecting them in parallel. A waterproofed and pre-wired version of the sensor is available and this can be purchased from the SILICON CHIP Online Shop for just $5 plus postage. The sensor itself is housed in a small metal tube and is hooked up to a 1metre long cable. Apart from cost, the big advantage of the DS18B20 is that it’s far more accurate than the Sense HAT. It’s not only inherently accurate but can be well-separated from the RPi so that it is unaffected by heat generated by the RPi’s ARM CPU. As a result, it doesn’t require time for the readings to stabilise after switch on and we don’t need to compensate for heatsoak from the RPi. Connecting it to the RPi Interfacing the DS18B20 sensor to the RPi’s GPIO port couldn’t be easier. As shown in Fig.1, there are just three wires to connect: the red wire goes to the +3.3V pin, the black wire goes to GND and the yellow data wire goes to GPIO4. In addition, a 4.7kΩ pull-up resistor must be connected between the data line and the +3.3V supply. The easiest way to connect the wires is to terminate them in 1-way header sockets which are then plugged into RPi’s GPIO port. Alternatively, you can cut two sections from a pin header socket (eg, Altronics P5380), plug them into the GPIO port and solder the sensor’s leads to the pins. A small clamp attached to the case lid can be used to secure the cable in place. The 4.7kΩ resistor can be connected by soldering its leads directly to the GPIO pads underneath the PCB. Getting it going You will need to install Raspbian on the RPi and set it up as described in the January and February 2016 is- RASPBERRY PI GPIO TEMPERATURE SENSOR 34  Silicon Chip GPIO4 Fig.1: here’s how to connect the DS18B20 sensor leads to the Raspberry Pi’s GPIO port. The red wire goes to +3.3V, the black wire to GND and the yellow data wire goes to the GPIO4 pin. In addition, a 4.7kΩ pull-up resistor is connected between GPIO4 and the +3.3V pin. +3.3V GND 4.7k PULLUP RESISTOR SOLDERED TO +3.3V & GPIO4 PINS UNDERNEATH RASPBERRY PI PCB siliconchip.com.au The Raspberry Pi connects to your router via WiFi and streams the temperature readings to a web server so they can be accessed over the internet. sues of SILICON CHIP. If you don’t have a Sense HAT module, you can leave out Steps 8-10 of the January article as they are not relevant. Once you’ve done that and connected the DS18B20 temperature sensor, power up the RPi. You now need to tell it how to detect the sensor. To do that, enter the following command in a terminal window: sudo nano /boot/config.txt then move the cursor to the bottom of the file and add the following line: dtoverlay=w1-gpio Hit Ctrl-o and Ctrl-x to save the file and exit Nano, then reboot (sudo reboot) the RPi so that the changes take effect. (Note: on older versions of Raspbian, it may be necessary to add the lines w1-gpio and W1_therm to the end of /etc/modules). Once the RPi is up and running again, enter the following commands into a terminal window: cd /sys/bus/w1/devices ls Your DS18B20 temperature sensor’s unique ID address will now be listed siliconchip.com.au Fig.2: once the DS18B20 has been connected to the RPi, a number of commands are run in a terminal window to determine its ID address (see text). in the terminal window followed by w1_bus_master1 – see Fig.2. If you multiple sensor’s connected in parallel, then multiple IDs will be listed (one for each sensor). In our case, the sensor’s ID is 28011581aefaff and we can now open the sensor’s file to view the temperature reading as follows: cd 28-011581aefaff cat w1_slave This will return two lines of data, as shown in Fig.2. The first line is a cyclic redundancy check (CRC) and if it ends in “YES”, then the reading was successful. The second line displays the temperature reading, in this case t=23812. This is the temperature in °C x 1000, so we simply divide by The 4.7kΩ pull-up resistor can be directly soldered to the GPIO pins. 1000 to get the temperature: ie, 23812 ÷ 1000 = 23.812°C. Python program We don’t want to have to go through this rigmarole every time we want to March 2016  35 import time import os import re Fig.3: readtemp.py sensor_names = {"011581aefaff": "indoor"} #Substitute your sensor's ID def list_onewire_sensors(): path = '/sys/bus/w1/devices' return [f for f in os.listdir(path) if not os.path.isfile(os.path.join(path, f)) and f.sta def read_onewire_sensor(name): path = '/sys/bus/w1/devices' file = open(os.path.join(path,name,'w1_slave'), 'r') line1 = file.readline() line2 = file.readline() file.close() if line1.endswith(' YES\n'): info = re.search('(\\d+)\n?$', line2) if info and int(info.group(1)) > 0: return int(info.group(1)) / 1000.0 return '?'; def getmsg(entities): tm = time.strftime("%d/%m/%Y %H:%M:%S", time.localtime()) msg = "[%s]" % tm deg = "°" if entities else u"\u00B0" onewire_sensors = list_onewire_sensors() for sensor in onewire_sensors: temp = read_onewire_sensor(sensor); msg += ' '+( sensor_names[sensor[3:]] if sensor[3:] in sensor_names else sensor[3:])+' return msg while True: print(getmsg(False)) time.sleep(5) read the temperature, so the answer is to automate the procedure using a simple Python program. Better still, we can then stream the readings to Apache Web Server on the RPi, so that we can access the readings over the internet (or on the local network) using a web browser. Let’s get the Python program running first. Fig.3 shows the code – just download the readtemp.py file from the SILICON CHIP website to the RPi’s /home/pi folder, then launch Python 3 from a terminal window using the following commands: xhost + (if you're running it headless) sudo idle3 & Wait until Python 3 launches, then open /home/pi/readtemp.py and click Run Module. You should see the temperature readings appear as shown in Fig.4, with the reading updated every 5s. OK, let’s take a closer look at the pro36  Silicon Chip gram. The new code has two functions to assist with reading the DS18B20 1-Wire sensor temperature, plus some extra code to display the results. The list_onewire_sensors function scans the /sys/bus/w1/devices directory for subdirectories starting with “28” and adds the sensor’s name, which includes its unique serial number, to an array. The returned array thus contains one entry for each DS18B20 sensor that has been detected. The read_onewire_sensor function receives the name of one of the sensors (from the list) and interrogates that sensor. It reads two lines, both of which contain the full sensor response. As stated, the first one has a flag indicating whether the CRC is valid, while the second line contains the decoded temperature value. This function reads both lines and, if the CRC is valid, returns the temperature reading in °C as a floating point value. If an error occurs, it returns a string containing a question mark. We also have a list called sensor_ names which can be used to map the cryptic unique ID for a given sensor to a more useful name such as “indoor”, “outdoor”, etc. That way, you can name the sensors and the temperature for each sensor is then displayed after its name. The test script also initialises a string with the current date and time. It then calls the first helper function to get a list of sensors and then, for each sensor, gets the name and temperature and adds it on to the end of that string. Finally, that string is printed, as shown in Fig.4. If you want to change the update time, just edit the last line of the script. Web access We’ve also adapted the index.py script originally used last month so siliconchip.com.au artswith('28-')] Fig.5 (above): the temperature readings as displayed in a web browser. The rapid increase in temperature was the result of briefly holding the temperature sensor between two fingers. ': '+('?' if temp == '?' else ('%.3f' % temp)+deg+'C') played. You can edit the sensor_names = line to distinguish between them – just add a comma after each preceding entry and add each additional sensor’s ID address and name (with everything enclosed by one set of parentheses). Apart from adding code to support the DS18B20 sensors, we’ve also improved the program to give more consistent time intervals between readings. Basically, the page load time has now been taken out of the equation. Setting up the web server Fig.4: this is the output that appears when running readtemp.py in Python on the RPi. The reading updates about every five seconds but this is easy to change. that the Apache 2 Web Server could serve readings from the Sense HAT. We’ve left the code to display the readings from the Sense HAT in place but wrapped it with an “exception handler” so that if the Sense HAT isn’t installed, its absence is ignored. We then use the same code from the readtemp.py script to append the DS18B20 reading(s) to the end of each siliconchip.com.au line that’s displayed. If you can physically connect both the Sense HAT and some DS18B20 temperature sensors, all the readings at any particular instant will be shown on the same line. If you only have a DS18B20 sensor connected, its readings will be displayed as shown in Fig.5. And if you have several DS18B20s connected in parallel, all their readings will be dis- Installing and getting the Apache Web Server going is straightforward – just follow the step-by-step procedure described in the February 2016 article. As before, the index.py script (embedded in index.py.zip) can be downloaded from the SILICON CHIP website, unzipped and copied to /var/www/html. Don’t forget to set up password access and a dynamic DNS service (eg, Duck DNS) if you want to access the temperature readings over the internet. It’s also a good idea to install Fail2Ban, to temporarily ban anyone who makes too many failed log-in attempts. Once the set-up is complete, you can browse to the RPi’s web server by entering your local IP, WAN IP or dynamic DNS address into your browser to display the temperature readings. By default, the reading updates every five seconds but you can easily change this by altering the two interval=5 values in index.py; eg, interval=30 updates the reading every 30 seconds. Alternatively, you can add a switch to the website address, as described on SC page 58 of the February issue. March 2016  37