Mittwoch, 18. Januar 2012

A portable wireless temperature sensor node

I recently wanted to compare the energy consumption in our house with the temperature inside the fridge. It is an old fridge we bought used together with our kitchen, so i expected it to consume quite a lot of energy.

This is what the actual power consumption of the AC phase, the fridge is connected to, looks like:

(I scaled the Y-Axis to 200W so the big peeks get cut off)

You can clearly see, that there is a regular pattern in that graph. There seem to be two different devices consuming power: One is active for about an hour consuming roughly 100W and then pauses for ca. 2 hours. The other device takes about 40 W and turns on for 40 minutes, 14 times a day. Both powers add up when both devices are running simultaneously, that explains the humps on the curve.

My guess was, that the 100 W device is the old fridge and the 40 W device the pretty up-to-date freezer. So - how to find out? One could simply plug out both devices  - but that would defrosts the groceries inside the freezer and would be much too simple :)

So i build a wireless sensor for embedding in the fridge :) This is what i came up with:

It is a small plastic box containing a Jeenode equipped with a 433 MHz RFM12 transceive, with the antenna folded inside the box. It is attached to a 3xAA battery box and to a DS1820 one-wire temperature sensor via a 3.5 mm audio jack.

Most of the time it is in deep power-down mode, but once every minutes it wakes up and transmits the temperature reading out of the fridge to the 433 MHz accesspoint. I found it quite impressive that it manages to establish a bidirectional communication out of what is basically a faraday cage! This is what the reading from inside the fridge looks like:

As you can see, it matches perfectly the power waveform above: whenever the 100 W device is running, temperature falls - until it reaches the lower point of the controllers hysteresis. Power then turns off and the fridge start to warm up, unitl it reaches the upper hysteresis temperature .. and so on.

So, that was fun - next!

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