HTU21D-F Humidity Sensor
The HTU21D-F  is an I2C digital humidity sensor with a typical accuracy of ±2 % over the range 5 % to 95 % RH. It also provides temperature with an accuracy of ±1 °C from -30 °C to 90 °C. It has an operating voltage range of 1.5 V to 3.6 V, so you'll need a regulator and logic-level conversion if you want to use it on a 5 V board.
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 tmp () (with-i2c (s #x40) (write-byte #xE3 s) (restart-i2c s 2) (- (* 175.72 (/ (logior (ash (read-byte s) 8) (read-byte s)) 65536)) 46.85)))
Reading the humidity
The following floating-point function reads the humidity in %RH:
(defun hum () (with-i2c (s #x40) (write-byte #xE5 s) (restart-i2c s 2) (round (- (* 125 (/ (logior (ash (read-byte s) 8) (read-byte s)) 65536)) 6))))
Enabling the heater
The sensor also incorporates a heater, to combat condensation. Here's a routine to enable or disable the heater:
(defun hot (on) (with-i2c (s #x40) (write-byte #xE6 s) (write-byte (if on #x06 #x02) s)))
To turn the heater on call:
or to turn if off call:
- ^ HTU21D-F Datasheet on TE Connectivity.
- ^ Adafruit HTU21D-F Temperature & Humidity Sensor Breakout Board on Adafruit.