Controlling the Humidity with an Embedded Linux System

Charles Darwin, in his Beagle Diary that led to the book Voyage of the Beagle, wrote while in Peru, “On the hills near Lima, at a height but little greater, the ground is carpeted with moss, and beds of beautiful yellow lilies, called Amancaes. This indicates a very much greater degree of humidity, than at a corresponding height at Iquique.” Like Darwin, I always have been conscious of humidity. For years, I've struggled with the humidity in my music room, as my Carlos Pina concert-grade classical guitar went out of tune frequently with wild swings in humidity. Pennsylvania winters are cold and dry, summers hot and humid, and this plays havoc on my classical guitar.

Commercially available humidifiers and dehumidifiers have humidity sensors that are far too coarse for certain applications. One such application is the humidity control for my music room. Being an embedded developer for my entire career, with a particular interest in embedded applications for Linux, I decided to build my own humidity controller for my music room. After a bit of research, I settled on a hardware architecture that includes a Cirrus EP9301 ARM9-based controller, several solid-state relays and a capacitive humidity/temperature sensor. Linux was my selection as the embedded OS, and with several Linux device drivers to control the relays and monitor the humidity and temperature, the basis of a humidity controller was born.

I decided to use the humidifying and dehumidifying capability of my retail humidifier and dehumidifier units. The humidity controller that I built switches power on and off to the humidifier and dehumidifier, essentially assuming the role of the humidity sensor. To finish off the humidity controller, I added a Web interface that allows me to monitor and control the system through any network-attached browser, such as Firefox.

Before I began developing the embedded humidity controller, I had to decide on the system-level requirements. Even though this was for personal use only, it's always good practice to do a bit of systems engineering on the front end of the design process. I decided on the following requirements:

Figure 1 shows the overall embedded hardware architecture of the humidity controller. The ARM9-based controller I selected is the TS-7200 from Technologic Systems. In addition to the controller board, I used a TS-RELAY8 peripheral board connected to the TS-7200's PC/104 bus. The daughter board contains eight SPDT relays. To house the system, I used a TS-ENC720 enclosure. Figure 2 shows the main board and peripheral board mounted on the back plate of the enclosure.

The capacitive humidity/temperature sensor is a Sensirion SHT11, which is controlled through a two-wire data/clock interface. The SHT11 control interface connects to two of the TS-7200's discrete I/O pins. Switching power on and off is accomplished with the single pole double throw (SPDT) relays on the peripheral board. I used a pair of relays for the humidifier and another pair for the dehumidifier. I used a pair as it seemed much safer to switch both the 120V and neutral lines, rather than just the 120V.

Figure 1. Hardware Architecture

Figure 2. Hardware

The TS-7200 single-board computer (SBC) runs Linux on an ARM9-based processor. The system's software architecture is shown in Figure 3. Two Linux drivers are required: one to sense the humidity (and temperature, which came almost free) and the second to control the position of the relays. A user-mode application on top of the drivers periodically polls the humidity and temperature data, and controls the relay position depending on SNMP MIB configuration settings. The SNMP MIB is managed by the Linux snmpd dæmon. The SNMP MIB also serves as the basic bridge to an Apache custom module that exposes the MIB data to a Web browser for control and monitoring of the entire humidity control system. Each component of the humidity control system is described in more detail later in this article.

Figure 3. Software Architecture