HTU31D Humidity Sensor
The HTU31D  is an I2C digital humidity sensor with a typical accuracy of ±2% at 20% to 100% relative humidity. It also provides temperature with an accuracy of ±0.2°C from 0 to 100°C.
A breakout is available from Adafruit :
The I2C address is #x40 or 64.
Reading the temperature
The following floating-point function reads the temperature in degrees Celsius:
(defun htu31d-temp () (with-i2c (s #x40) (write-byte #x40 s) ; conversion (restart-i2c s) (delay 20) (write-byte #x00 s) ; read temperature (restart-i2c s 2) (- (* 165 (/ (logior (ash (read-byte s) 8) (read-byte s)) 65535)) 40)))
> (htu31d-temp) 22.3594
Reading the humidity
The following floating-point function reads the humidity in %RH:
(defun htu31d-humidity () (with-i2c (s #x40) (write-byte #x40 s) ; conversion (restart-i2c s) (delay 20) (write-byte #x10 s) ; read humidity (restart-i2c s 2) (* 100 (/ (logior (ash (read-byte s) 8) (read-byte s)) 65535))))
> (htu31d-humidity) 60.0412
Enabling the heater
The sensor also incorporates a heater, to combat condensation. Here's a routine to enable or disable the heater:
(defun htu31d-heater (on) (with-i2c (s #x40) (write-byte (if on #x04 #x02) s)))
To turn the heater on call:
or to turn if off call:
- ^ HTU31D Datasheet on TTI.com.
- ^ Adafruit HTU31D Temperature & Humidity Sensor Breakout Board on Adafruit.