Extending the Range of a Wireless Temperature Sensor with Walkie-Talkies

Here I describe a hack where I extend the signal from a 433.92MHz wireless temperature (and humidity) sensor to a few kilometers by repeating it using PMR446 walkie-talkies. For visitors from the U.S./Canada, this roughly translates to saying "using FRS walkie-talkies to a few miles". I used AFSK (audio frequency-shift keying) modulated AX.25 frames (a.k.a. packet radio) (at 1200 bps) to be compatible with amateur radio standards. Please keep in mind that this is a hack and I tried to use stuff that I had already lying around. Things could probably have been accomplished easier, but I had fun and learned new things.

Introduction

After reverse engineering a wireless weather station, I wanted to deploy more temperature sensors in our garden. One interesting place would be our greenhouse. However, it is behind our garage and not very close to our house (and my receiving server). The sensor was occluded by several stone walls, and signals did not come through---not even after building a Yagi-Uda antenna at the receiving side. I went through my equipment and decided to make a system where a cheap China-quality 433MHz receiver is placed in the range of the sensor. The receiver is connected to an ATmega88 microcontroller which decodes the sensor readings. The Atmega drives a walkie-talkie, and translates the reading into a series of AFSK beeps. My server is connected to another walkie-talkie, and using its sound card it translates the beeps back into temperature/humidity pairs. Schematically, that looks like this:

My description of this project (hack) is split into two parts:

Here is a graph produced with data that traveled from the sensor at 433.92MHz to my microcontroller, which AFSK modulated it and transmitted it via a walkie-talkie at 446.08125MHz to a receiver connected to my server which captured it with its sound card. Yeah, a bit crazy indeed.

Between 09:00 and 10:00, there is a flat line segment of 10 minutes. At that position a frame (with 10 measurements of 1 minute each) was lost. Unless I missed it, this is the only place where things went wrong, which yields a success rate of >99% for this test run. Not bad I think, considering the amount of hackery.