How to measure temperature?

Question

What can I connect to the RPi to measure temperature? I think devices connected to the I²C or SPI would make most sense.

Here is a question about DHT-22 and other 1-wire devices. But at this stage it seems that 1-wire is difficult on the RPi due to the critical timings

Answer 1

Here's how to connect a MCP9804.

You can use it like this:

root@raspberrypi:~# modprobe i2c-dev
root@raspberrypi:~# modprobe i2c-bcm2708 
root@raspberrypi:~# i2cdetect -y 0
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 1f 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --                         
root@raspberrypi:~# i2cget -y 0 0x1f 0x5 w
0x67c1

Converting 0x67c1 to a temperature is a little convoluted. The MSB is 0xc1 and the LSB is 0x67

The first 4 bits of the MSB are dropped and which leaves the temperature in 16ths of a degree

(0xc1&0xf)*16+0x67/16.0 = 22.4375 degrees 

Python Example
In addition to loading the i2c modules above, you'll need to install the python-smbus package. You can reduce self-heating by shutting down the MCP9804 between readings.

#!/usr/bin/env python
import time
from smbus import SMBus

class MCP9804(object):
    def __init__(self, bus, addr):
        self.bus = bus
        self.addr = addr

    def wakeup(self):
        self.bus.write_word_data(self.addr, 1, 0x0000)

    def shutdown(self):
        self.bus.write_word_data(self.addr, 1, 0x0001)

    def get_temperature(self, shutdown=False):
        if shutdown:
            self.wakeup()
            time.sleep(0.26) # Wait for conversion

        msb, lsb =  self.bus.read_i2c_block_data(self.addr, 5, 2)

        if shutdown:
            self.shutdown()

        tcrit = msb>>7&1
        tupper = msb>>6&1
        tlower = msb>>5&1

        temperature = (msb&0xf)*16+lsb/16.0
        if msb>>4&1:
            temperature = 256 - temperature
        return temperature



def main():
    sensor = MCP9804(SMBus(0), 0x1f)
    while True:
        print sensor.get_temperature()
        time.sleep(1)


if __name__ == "__main__":
    main()

Answer 2

You may use the Raspberry Pi built in serial port, and connect it to a digital thermometer IC (e.g. DS1620)

You can find out serial port interfacing of Raspberry Pi here

• P1 (LEFT BOTTOM) - 3.3V
• P6 - GND
• P8 GPIO14 - TX
• P10 GPIO15 - RX

Important: Remember the RPi UART runs at TTL 3.3V - Be careful not to use High Voltage 5v/12volt Uart direct to the RPi. It will cause damage!

Answer 3

I've tried two approaches to temperature sensing. For I2C, I used a TMP102 module which is similar to what gnibbler describes. Here's my post on that:

For 1-wire, Adafruit recently released there own image, and it contains 1-wire support. I was able to read a DS18B20 1-wire temp sensor with it. More details in this post:

Finally, another approach is to use analog temp sensor and an external ADC. Adafruit has a nice tutorial on this.

Answer 4

A simple, cheap USB "HID TEMPer" thermometer also works, and is much easier to connect for those who are yet to fiddle with UARTs or GPIO, like me.

My RPi provides enough power to drive it directly from the USB port without a hub.

To set this up with Raspbian Wheezy, I followed these instructions which were written for Ubuntu (disclaimer: link is for a post on my own blog). For the Raspberry Pi, I only had to make one small tweak to set LIBUSB_LIBDIR when installing the Device::USB perl module so it could find libusb in the non-standard arm location. The complete instructions follow.

To get a simple reading without any of the munin stuff, install the dependencies as follows (as root):

apt-get install libusb-dev
export LIBUSB_LIBDIR=/usr/lib/arm-linux-gnueabihf
cpan Inline::MakeMaker
cpan Device::USB::PCSensor::HidTEMPer

Create readtemp.pl:

#!/usr/bin/perl
use strict;
use Device::USB::PCSensor::HidTEMPer;

my $pcsensor = Device::USB::PCSensor::HidTEMPer->new();
my @devices = $pcsensor->list_devices();
foreach my $device (@devices) {
    print $device->internal()->celsius()."\n" if defined $device->internal();
}

And run that as root to see the output. In my case, it's a little chilly in the garage this evening:

day@pi:~$ sudo ./readtemp.pl 
16.5

Answer 5

I'm currently reading this book and like it. Going that route my vision is that you'd have a temperature sensor, an arduino, and an xbee radio glued together. That's your remote sensor which could be anywhere as long as it's in range of the home station. Then for the home station have a rasberry and another xbee. I'm guessing that it might be easier to also have the home station xbee on an arduino, and then have the arduino and rasberry talk to each other. With that you could have multiple remote sensors, and different types of sensors.