Computing stuff tied to the physical world

Cresta sensor

In Software on Apr 16, 2010 at 00:01

Another day, another sensor:

Dsc 1339

This is a Cresta unit (no serial number on it), with a simple outside temperature sensor (which has been outside for a some time). Here’s the OOK scope fingerprint:

Screen Shot 2010 04 12 at 13.50.25

Here’s the baseline, i.e. the background signal with the sensor switched off:

Screen Shot 2010 04 12 at 14.06.37

Some clear peaks, at values 100, 135, 175, and 195, roughly. These correspond to pulse intervals centered around 400 µs, 570 µs, 890 µs, 1080 µs, respectively.

My hunch is that the 400/570 and 890/1080 pairs are really just the same pulse, skewed by the size of the preceding pulse.

In this case, it turns out that the sensor sends 3 copies of the packet, with a 1..3 counter value inside. Seeing that counter in the raw bits indicates that this sensor is not using Manchester encoding, simply 1 = 1 and 2x 0 = 0:

Screen Shot 2010 04 13 at 20.11.30

I get better results with the sensor at least 1 meter away from the receiver – a very strong signal probably adds extra spikes or shifts pulses too far apart.

Time to write a decoder for this thing:

Screen Shot 2010 04 13 at 20.15.06

With the base unit turned on to see what the values being reported are, I was again able to determine the basic details of the packet layout. Each 8-bit byte is followed by a 9-th parity bit. So this hex data isn’t as easy to read:

Screen Shot 2010 04 13 at 20.18.56

But the expected temperature values are in there, and the 1..3 channel selection is as well. Again, I’ll save the checksum verification and the post-processing in JeeMon for another day.