Reading the temperature from a DS18B20 Digital Thermometer

## Pin connections

-[http://www.jamesrobertson.me.uk/dynarex/2016/feb/20/raspberry-pi-to-ds18b20-connections.xml]

## Configuring the sensor

Append the following entry in the file /boot/config.txt:

<pre>
dtoverlay=w1-gpio,gpiopin=21
</pre>

Reboot the Raspberry Pi.

## Identifying the device unique ID

Use the following commands:

<pre>
cd /sys/bus/w1/devices
ls
</pre>

In my directory I observed the file device unique ID *28-0000049a0085*. I then typed `cd 28-0000049a0085`, followed by `ls`, followed by `cat w1_slave`.
I observed the following file contents:

<pre>
22 01 4b 46 7f ff 0e 10 4a : crc=4a YES
22 01 4b 46 7f ff 0e 10 4a t=18125
</pre>

## Example

    #!/usr/bin/env ruby

    # file: ds18b20.rb

    def gettemp(id: nil)

      return 'gettemp error: please supply an id' unless id
      filename = 'w1_slave'
      filepath = File.join('/sys/bus/w1/devices/', id, filename)

      unless File.exists? filepath then
        return 'gettemp error: file doesn\'t exist -> ' + filepath
      end

      buffer = File.read filepath
      a = buffer.lines
      crc = a.shift =~ /YES$/
      crc ? (a.shift[/t=(\d+)/,1].to_f / 1000).round(3)  : 99999

    end

    if __FILE__ == $0 then
      puts "Temp : %s" % gettemp(id: '28-0000049a0085')
    end 

Output:

<pre>
Temp : 18.125
</pre>

Note: The above code can also be executed from within a Docker container.

## Resources

* DS18B20 Digital Temperature Sensor Chip Dallas http://www.jamesrobertson.me.uk/cpages/2016/feb/20/ds18b20-digital-temperature-sensor-chip-dallas-ebay-co-uk.html
* Raspberry Pi 1- Wire Digital Thermometer Sensor (DS18B20) http://www.raspberrypi-spy.co.uk/2013/03/raspberry-pi-1-wire-digital-thermometer-sensor/

ds18b20 temperature sensor raspberrypi