Humidity Monitoring on the Raspberry Pi

First published: 17^th July 2013

I've been using DS18S20 temperature sensors to monitor a warehouse for years using a PIC microcontroller. However, after receiving my raspberry pi, I was asked about humidity monitoring. A few searches later, I had found quite a few different sensors at a range of prices:

(prices are for comparison only)

As I wanted to store the readings in a database, a digital input would be most convenient. As I already have a 1-wire bus for the RPi, the DS1923 is the obvious choice, until you notice the price and that it is a logging device, with an internal lithium battery, so it has a limited lifespan. Great for initialising, sealing into the environment to be recorded (a shipping container or cellar, perhaps) and retrieving weeks or months later, but not so good for continuous, live monitoring.

The capacitive and impedance devices are generally cheap, but the design of a cheap, reliable circuit to convert that to digital is a bit beyond my rudimentary electronics knowledge. That seemed to leave the HIH-6120-021-001 with its I²C interface. But then I found a useful article by Dan Awtrey that connects a HIH-3605 humidity sensor to a DS2438 1-wire device. Now (as you can see from the table above) I couldn't find a source for the HIH-3605, but surely the same method would work with any of Honeywell's voltage-output humidity sensors, if the voltages were within range?

I decided to try humidity sensing on the RPi using both the I²C bus and the 1-Wire bus.