I am trying to use my Pi to monitor a flat (temperature, humidity, door bell, windows and doors, camera, ...). What matters most to me at the beginning is to check whether doors or windows are opened or closed, or rather when they last got opened or closed. For the same reasons, I want the devices to be small and discreet. Wired switches are not an option for me.
I need help to find a small, battery-powered reed switch which is able to send an encoded signal, via radio, once a window or door is opened (something like http://tinyurl.com/q2cgdee). On the other side, a receiver would be connected to the Pi, which receives the encoded signal, which can be traced back to specific reed switch/transmitter.