Fig.1: a block diagram of the radio modem, showing how two microcontrollers can be linked together.

Low cost, simple construction and easy interfacing makes this project ideal for a whole range of low-speed wireless data applications. Remote control and sensing are two obvious uses and there are undoubtedly many more. Even if you're just learning about microcontrollers, you'll be able to get two PICAXEs talking in no time!

The seeds of this project were sown when I got a call from Mr Vineyard, whose grapes kept freezing during the depths of winter. He said he needed a system that would turn on a misting water spray over the vines when the temperature dropped below a certain level. Apparently, Jack Frost would then freeze the water rather than the grapes. This seemed a bit dubious but I was assured that this is a well-known method of frost damage control.

The only complicating factor was that the temperature sensors needed to be amongst the vines which were up to half a kilometre away from the shed that housed the water control valves. Given half a dozen sensors, the amount of wiring needed was clearly impractical. And Mr Vineyard was very keen to have a temperature readout in his home so he could keep an eye on things.