Hack and / - Temper Temper

A $15 USB thermometer and some spare parts are all I needed to control my refrigerator with Linux.

If loving Linux ever became a crime and I were hauled into court, I think the prosecution's argument would go something like this:

Your honor, I need to submit only two pieces of evidence to make my case. First, I present Exhibit A: a stack of Linux Journal magazines of which the defendant is a columnist.

And your second piece of evidence?

Your honor, the defendant's refrigerator is powered by Linux.

(The audience gasps.) Order! I've heard enough! Guilty!

I can't help it. I mean, why wouldn't you power your fridge with Linux if you had the chance? In my case, we recently purchased a new fridge for our house, which meant our spare fridge was headed for the garage where I would use it for beer fermentation. Fridges are well-insulated, and it seemed ideal for the task at hand, but the problem I ran into was that the built-in thermostat for the fridge would go up to only around 45–50°F at its warmest. To ferment ales, I needed to maintain temperatures between 60–72°F.

When most people convert fridges to ferment beer, they purchase a purpose-built device from their local brew shop. Essentially, you plug your fridge in to the device, plug the device in to the wall, and then set the analog thermostat on the device to your desired temperature. A temperature probe goes into your fridge, and when it gets too warm, the fridge is powered on. These devices range from around $70 to more than $100, depending on whether they are analog or digital, and I almost bought one until I realized I could do the same thing with an old Linux laptop, a couple pieces of hardware and a few scripts.

If you are into home automation at all, you are familiar with the X10 suite of home automation gadgets. Essentially, you can connect lamps and appliances to different X10 gadgets and then power them on with a remote control. There's even a remote control that connects to a serial port, so you can control everything from a computer. Linux has a program called bottlerocket that works great with X10 serial port controllers, and I had used one to control my DSL modem for many years, but that's something for another column.

The TEMPer USB Thermometer

So, I had a laptop and could control the fridge power, but I still needed a thermometer that worked under Linux and was relatively cheap. I discovered a great little USB-powered thermometer made by a company named TEMPer. It's small, cheap (less than $15 shipped), supports temperatures between –40°C and +120°C, and with a little effort, it works under Linux. It turns out many Linux administrators are using these devices to monitor temperatures in their data centers.

Figure 1. TEMPer USB Thermometer (photo from the Amazon product page)

Apparently, the older versions of this thermometer showed up as a USB-to-serial interface; however, the newer models, including the one I bought, show up as a USB Human Interface Device when you plug it in:

Apr 16 14:44:33 muriel kernel: [11601.992205] usb 1-1: 
 ↪new low speed USB device using uhci_hcd and address 2
Apr 16 14:44:33 muriel kernel: [11602.182910] usb 1-1: 
 ↪configuration #1 chosen from 1 choice
Apr 16 14:44:33 muriel kernel: [11602.188481] usb 1-1: 
 ↪New USB device found, idVendor=1130, idProduct=660c
Apr 16 14:44:33 muriel kernel: [11602.188529] usb 1-1: 
 ↪New USB device strings: Mfr=0, Product=2, SerialNumber=0
Apr 16 14:44:33 muriel kernel: [11602.188563] usb 1-1: 
 ↪Product:  PCsensor Temper
Apr 16 14:44:35 muriel kernel: [11604.090148] usbcore: 
 ↪registered new interface driver hiddev
Apr 16 14:44:35 muriel kernel: [11604.119323] input:  
 ↪PCsensor Temper as /class/input/input7
Apr 16 14:44:35 muriel kernel: [11604.140885] input,hidraw0: 
 ↪USB HID v1.10 Keyboard [ PCsensor Temper] 
 ↪on usb-0000:00:07.2-1
Apr 16 14:44:35 muriel kernel: [11604.170151] input:  
 ↪PCsensor Temper as /class/input/input8
Apr 16 14:44:35 muriel kernel: [11604.188677] input,hidraw1: 
 ↪USB HID v1.10 Device [ PCsensor Temper] 
 ↪on usb-0000:00:07.2-1
Apr 16 14:44:35 muriel kernel: [11604.188931] usbcore: 
 ↪registered new interface driver usbhid
Apr 16 14:44:35 muriel kernel: [11604.188980] usbhid: 
 ↪v2.6:USB HID core driver

At first I thought I could get the temperature from this thermometer through some /proc or /sys interface, but unfortunately, the thermometer is more proprietary than that. The Linux community is resourceful though, and a quick search turned up a number of guides on how to pull the temperature from Linux (see Resources for the most helpful guide I found). Essentially, you need to install a few custom Perl modules, including a special one created just for this device that depends on Perl 5.10, so you need a relatively new distribution for this to work (I used the latest stable Debian release).


Kyle Rankin is Chief Security Officer at Purism, a company focused on computers that respect your privacy, security, and freedom. He is the author of many books including Linux Hardening in Hostile Networks, DevOps Troubleshooting and The Official Ubuntu