Friday, September 21, 2007

Wireless Temperature Sensor

I've got some wireless temperature sensors that I was curious about.

The remote unit stopped working a while back, so I figured I'd open it up and see if I could find the problem. Yes, I checked the batteries first :)

There isn't anything obviously wrong with it, just a little crud left over from assembly. I figured maybe it absorbed water while it was hanging outside, so I cleaned it up. I put the batteries back in and it was working again. So I have no idea what was wrong, but at least it's going.

I opened up the receiver as well, just to take a look at the data stream that it receives from the transmitter.

As you can see, I've had to open it before to replace the battery wire. The little daughter board is the radio receiver and decoder, it connects to the main board with a 4 pin header. The protocol is a basic 433MHz OOK. The bit rate is very low, 2Hz. Here is the decoded signal.

Most of the bits (chips?) are 0.05s wide with 0.5 seconds between them. There are also some 0.14s bits (usually in pairs, but occasionally single). Sometimes the delay between bits is 1s, rarely 1.5s. Periodically there will be 10-15s of silence. I'll have to run the signal through one of the microcontrollers I have set up to capture data streams like this to get a good idea of what is going on. The scope doesn't have a long enough memory to let me see a wide enough window to see what's really happening.
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