As part of my project to bring our central heating into the 21st century I have been working on a custom wireless sensor platform. My key aim here is to develop a low-cost, open solution with security designed in from the start; something that a lot of sub-GHz protocols seem to totally omit. I’m avoiding ZigBee because, as much as I want to like it, I keep coming back to the issue that its complexity just pushes up the system cost. In a domestic environment a simple sub-GHz solution can easily cover a typical property without resorting to meshing.
The design I have gone for combines temperature and light sensing with a pulse input for connection to the gas and electricity meters. The microcontroller is an ATMEGA328 and the radio is a Hope RF RFM23B, which is based on a SiLabs Si4431. This is a synthesised transceiver so the potential for congestion can be avoided through the use of listen-before-talk and frequency hopping, the latter also offering a way around the duty cycle restrictions in the 868 MHz band.
The printed antenna needs to be tuned by trimming it to length, which will no doubt be the subject of a future post. Once the optimal dimensions have been determined this can be applied to any future spin of the boards in advance. No prizes for spotting the deliberate mistake with the microstrip track passing under the RF module – I am hopeful it won’t have too great an effect!
The boards in the photograph just came back from iTeadStudio in about a fortnight. The quality is absolutely excellent considering the <£10 price tag for the 10 circuits. I have loaded a couple and firmware development is in progress.