Question 12.9: You want to measure temperature using an accurate digital se......
You want to measure temperature using an accurate digital sensor.
Use the DS18B20 digital temperature sensor. This device is more accurate than the TMP36 used in Recipe 12.7 and uses a one-wire digital interface, so it doesn’t require an ADC chip.
Although called one-wire, this just refers to the data pin. You do need at least one other wire to connect to a one-wire device.
To make this recipe, you will need:
• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)
• DS18B20 temperature sensor (see “Integrated Circuits” on page 381)
• 4.7kΩ resistor (see “Resistors and Capacitors” on page 380)
Fit the components onto the breadboard as shown in Figure 12-16. Make sure that you get the DS18B20 the right way around.
Both Occidentalis and newer versions of Raspbian have support for the one-wire interface used by the DS18B20 enabled. If the upcoming program doesn’t work, then try:
| $ sudo apt-get upgrade |
Open an editor (nano or IDLE) and paste in the following code. As with all the program examples in this book, you can also download the program from the Code section of the Raspberry Pi Cookbook website, where it is called temp_DS18B20.py.
| import os, glob, 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 while True: print(“temp C=%f\ttemp F=%f” % read_temp()) time.sleep(1) |
When the program is run, it will report the temperature once a second in both degrees Celsius and Fahrenheit:
| $ python temp_DS18B20.py temp C=25.187000 temp F=77.336600 temp C=25.125000 temp F=77.225000 temp C=25.062000 temp F=77.111600 temp C=26.312000 temp F=79.361600 temp C=27.875000 temp F=82.175000 temp C=28.875000 temp F=83.975000 |
Discussion
At first sight, the program looks a little odd. The interface to the DS18B20 uses a file-like interface. The file interface for the device will always be in the folder /sys/bus/w1/devices/ and the name of the file path will start with 28, but the rest of the file path will be different for each sensor. The code assumes that there will only be one sensor and finds the first folder starting with 28. Within that folder will be a file called w1_slave, which is opened and read to find the temperature.
The sensor actually returns strings of text like this:
| 81 01 4b 46 7f ff 0f 10 71 : crc=71 YES 81 01 4b 46 7f ff 0f 10 71 t=24062 |
The remainder of the code extracts the temperature part of this message. This appears after t= and is the temperature in one-thousandths of a degree Celsius.
The read_temp function calculates the temperature in degrees Fahrenheit and returns both.
Besides the basic chip version of the DS18B20, you can also buy a version encapsulated in a rugged and waterproof probe.
See Also
To find out about logging readings, see Recipe 12.12.
This recipe is heavily based on an Adafruit tutorial.
To measure temperature using a less accurate TMP36 analog sensor, see Recipe 12.7.
Take a look at the datasheet for the DS18B20.
| Table A-3. Prototyping equipment | |
| Description | Suppliers |
| M-M jumper wires | SparkFun: PRT-08431, Adafruit: 759 |
| M-F jumper wires | SparkFun: PRT-09140, Adafruit: 825 |
| F-F jumper wires | SparkFun: PRT-08430, Adafruit: 794 |
| Half-sized breadboard | SparkFun: PRT-09567 Adafruit: 64 |
| Pi Cobbler | Adafruit: 1105 |
| Table A-6. Integrated circuits | |
| 7805 voltage regulator | SparkFun: COM-00107 |
| L293D motor driver | SparkFun: COM-00315, Adafruit: 807 |
| ULN2803 Darlington driver IC | SparkFun: COM-00312, Adafruit: 970 |
| DS18B20 temperature sensor | SparkFun: SEN-00245, Adafruit: 374 |
| MCP3008 eight-channel ADC IC | Adafruit: 856 |
| TMP36 temperature sensor | SparkFun: SEN-10988, Adafruit: 165 |
| Table A-4. Resistors and capacitors | |
| 270Ω 0.25W resistor | Mouser: 293-270-RC |
| 470Ω 0.25W resistor | Mouser: 293-470-RC |
| 1kΩ 0.25W resistor | Mouser: 293-1k-RC |
| 3.3kΩ 0.25W resistor | Mouser: 293-3.3k-RC |
| 4.7kΩ 0.25W resistor | Mouser: 293-4.7k-RC |
| 10 kΩ trimpot | Adafruit: 356, SparkFun: COM-09806, Mouser: 652-3362F-1-103LF |
| Photoresistor | Adafruit: 161, SparkFun: SEN-09088 |
| 220nF capacitor | MCM: 31-0610, Mouser: 80-C322C224M5U5HA |
