Avian’s Blog

Last month at a project meeting in Ghent, Hans Cappelle told me about PCsensor TEMPerHUM hygrometers and thermometers. At around 23 EUR per piece, these are relatively cheap, Chinese made USB-connected sensors. I was looking for something like that to put into a few machines I have running at different places. Knowing the room temperature can be handy. For instance, once I noticed that the CPU and motherboard temperature on one server was rising even though fan RPMs did not change. Was it because the change in the room temperature or was it a sign that heat sinks were getting clogged with dust? Having a reading from a sensor outside of the server's box would help in knowing the reason without having to go and have a look.

When I took them out of the box, I was a bit surprised at their weight. I assumed that the shiny surface was metallized plastic, but in fact it looks like the case is actually made of metal. Each came with a short USB extension cable so you can put it a bit away from the warm computer they are plugged in.

My devices identify as RDing TEMPERHUM1V1.2 on the USB bus (vendor ID 0x0c45, product ID 0x7402). They register two interfaces with the host: a HID keyboard and a HID mouse. The blue circle with TXT in it is actually a button. When you press it, the sensor starts simulating someone manually typing in periodic sensor readouts. As far as I understand, you can set some settings in this mode by playing with Caps lock and Num lock LEDs. I haven't looked into it further though since I don't currently care about this feature, but it is good to know that sensors can be used with only a basic USB HID keyboard driver and a text editor.

On Linux, udev creates two hidraw devices for the sensor. To get a sensor readout from those without having to press the button, Frode Austvik wrote a nice little utility. For v1.2 sensors, you need this patched version.

The utility is written in C and is easy to compile for more exotic devices. The only real dependency is HID API. HID API is only packaged for Debian Jessie, but the 0.8.0 source package compiles on Wheezy as well without any modifications. OpenWRT has a package for HID API. The sensor however does not work on my TP-Link router because it uses USB v1.10 and the router's USB ports apparently only support USB v2.x (see OpenWRT ticket #15194).

By default, udev creates hidraw device files that only root can read. To enable any member of the temperhum group to read the sensor, I use the following in /etc/udev/rules.d/20-temperhum.rules:

Update: Previously, I made the hidraw device world-writable. That might be a security problem, although probably a very unlikely one.

SUBSYSTEM=="hidraw", ATTRS{idVendor}=="0c45", ATTRS{idProduct}=="7402", MODE="0660", GROUP="temperhum"

Finally, what about the accuracy? I left one of the sensors (connected via an extension cable) to sit overnight in a room next to an Arduino with a DS18B20 and the old wireless weather monitor I use at home. The two spikes you see on the graphs are from when I opened the windows in the morning.

The DS18B20 has a declared accuracy of ±0.5°C, while the Si7021 sensor that TEMPerHUM is supposedly using should be accurate to ±0.4°C. However, TEMPerHUM is showing between 1.5°C and 2.0°C higher reading than DS18B20. This might be because the USB fob is heating itself a little bit, or it might be some numerical error (or they might be using off-spec chips). Other people have reported that similar sensors tend to overestimate the temperature.

For relative humidity measurements I don't have a good reference. The difference between my weather monitor and TEMPerHUM is around 10% according to this graph. Si7021 should be accurate to within 3% according to its datasheet. My weather monitor is certainly worse than that. So I can't really say anything about the humidity measurement except that it isn't obviously wrong.

In conclusion, except for the incompatibility with some Atheros SoCs I mentioned above, these sensors seem to be exactly what they say they are. Not super accurate but probably good enough for some basic monitoring of the environment.