====== Temperature sensors ======

===== 18B20 (digital) =====

Wiring:
  * Waterproof:
    * Red: 3.3V
    * Yellow: 4.7 k pull-up (5.1k fine) and gpio pin nominated in ''/boot/config.txt''
    * Black: ground
  * Kit module:
    * 1 (labelled ''-''): ground
    * 2: 3.3V
    * 3 (labelled ''S''): 4.7k pull-up and signal

Wire as many as you like in parallel.  There are limits to numbers and to wiring length, but they're not simple.

At the end of ''/boot/config.txt'' (replace 21 with pin number):

  dtoverlay=w1-gpio,gpiopin=21

Save and reboot.

  ls -l /sys/bus/w1/devices

Grab its address (format ''28-xxxxxxxxxxx'').  Read the temperature with

  cat /sys/bus/w1/devices/28-[address]/w1_slave

The numerical value after ''t='' is the temperature in thousandths of a degree C.