Raspberry Pi security alarm – the basics
A significant number of homes already have wireless sensors installed.  Almost all of these sensors (glass break, motion, contact, key fob, etc) can be monitored via sdr using rtl_433.I always thought it would be a pretty viable start up business to sell a security system in a box for people with existing security equipment.  Connect the security system in a box and then open and close your door/window/etc to begin monitoring those wireless events.Shameless plug: I wrote the rtl_433 GE/Interlogix sensor protocol decoder.