Wireless door/window security sensor containing a Reed switch, AtMega328P controller and RFM12B radio, powered by 3 x LR44 coin cell batteries.

This sensor integrates with EmonCMS software, part of OpenEnergyMonitor project.

In order to make the sensor as small as possible, I had to use improvised battery holder connectors. I couldn't find any ready made connectors, surface mount or through-hole, which could fit three batteries into my small plastic case. These header pins are not so easy to solder, but they work pretty good!

Connector J4 (2mm pitch) is used as ISP port, to program Arduino bootloader into ATmega328. This tutorial has detailed procedure on how to do that. Once the bootloader is in, all the programming and reprogramming is done through J5 connector (0.1" pitch) in a normal way through Arduino software and a compatible Serial-to-USB converter.

More information on this project can be found on my blog page: