Saturday, June 6, 2015

Inside stuff - Auraton 2020 TX/RX thermostat

I got this pair of items from a colleague, requesting a repair. The units would not bind to each other, or rather, the RX unit (controlling the heating) would not receive anything from the TX unit.

The transmitter, pictured below, is quite a nice low-power unit. It contains the temperature sensor and provides a basic programming interface for controlling the heating - based on day of week, hour, vacation mode.

The receiver unit plugs into a free outlet, without hogging it:

I would like to see more products that take a plug space return it to the user. In this case, this plug would probably take 2 or even 3 spaces on a 45 degree German-style plug.

The manual for the unit is hosted here: which also provided the "fix" for the radio binding problem.

The TX unit doesn't have too much to show us, most of the programming and display stuff is done by the black blob on the right side.

More interesting is the radio transmission unit, it's all made out of discrete components! In this day and age I would expect the one-chip solutions to be better and cheaper.

I've measured the output of the unit with an RTL8192 dongle and it seemed to work fine.

The RX unit shows initially a transformer (dark red) with heavy wiring and a smaller relay (orange) for closing the 'remote' switch on a central heating unit.

The other side of the board shows the contacts for the remote activation (TH1), a PIC12C508A microcontroller, the bridge rectifier (D2-D5) going to the Zener (Z1) that makes a crude power supply. Since the uC only consumes milliamps they can get away with only using a zener - which usually can't provide more than 20 mA without burning out.
The relay would draw more than 25mA, but it is controlled via a transistor (Q3?) from the rectified (but unregulated) voltage.
The D1 diode is for clamping the high-voltage spike produced by the relay coil.

Sorry if I might have gotten things reversed a bit, it's been a few months since I took the pictures.

The receiver unit is again made with discrete components.

Plugging an oscilloscope showed some signals going through the radio and signal coming out of data. This meant that the radio link functioned correctly. It also meant that the chip on the RX might be fried, but there was some blinking on startup and otherwise the unit seemed to work fine. No traces of black/blue smoke either.

Time to take out the manual:

Well, the RX unit has a sticker with "088" on the back. After programming that code in - instead of "085" - it worked!

Moral of the story: if you take the batteries out make sure you have the manual handy.

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