Weather machine

28.06.2009 21:16

Here's another piece of electronics I saved from being dumped in a landfill: it's a wireless temperature and humidity sensor from one of those home weather stations. My sister gave it to me after I expressed my interest in it, but unfortunately I was too late to save the broken base station with the receiver. Now I'm playing with the idea of building a new receiver from scratch.

On the outside the sensor has a couple of buttons for setting up its connection with the receiver, self-test and choosing temperature display in Celsius or Fahrenheit (Kelvin fans are left out in the cold). There's a small LCD display on the unit that shows the current sensor readings.

Weather station transmitter, top side

Top side

Weather station transmitter, bottom side

Bottom side

There are no markings of any kind that would identify the manufacturer. That could be intentional, guessing from the sloppy way it's soldered together. Looks like the cost of assembling one of these things together was the primary concern of the designers: there's a single IC in the middle that uses direct chip attachment. Everything is SMD except for a couple of through-hole components that appear to be soldered by hand - in fact, whoever did it forgot to solder one of the pins of the trimmer capacitor (so I'm wondering if the base station was actually working just fine). Battery, buttons and LCD display all connect to the PCB by just being pressed against it - connectors really must cost a fortune these days.

Here's a schematic of the RF part. It appears to be a classical case of a cheap transmitter in the 433 MHz band. Basically it's a tuned oscillator: you have an amplifier and two tuned resonant circuits on both sides. One is a R433A SAW resonator and the other is a LC circuit that can be tuned with the trimmer. Because of the resonance the feedback can be minimal. In this case just the parasitic capacitance in the transistor's base-collector junction (Cbc) appears to do the trick. The antenna is just a fancy trace on the circuit board. I'm guessing there's only a simple amplitude modulation going on: either the transmitter is on, or it's off. But I have yet to hook up an oscilloscope to the circuit the verify that.

Schematic of a 433 MHz weather station transmitter

I'm planning to build a simple super-regenerative receiver, reverse engineer the data protocol and hook it up to an Arduino or some similar microcontroller. I know I could probably get a receiver module already built, but I want to get my hands dirty and finally try working with some RF circuit designs.

What I found so far on the internet seems encouraging. There's a really nice document about super-regenerative receivers by Eddie Insam and the protocol for weather stations has been broken before.

Posted by Tomaž | Categories: Analog

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