Those hide-a-key rocks have always looked so fake to me. It seems silly to trust the security of your home to something so flimsy. Other than the doormat it seems like a fake rock will be the first place a thief looks.
I decided to turn that weakness into a strength.
I built a decoy hide-a-key fake rock that, instead of having a house key inside, will have an alarm that triggers when a potential thief flips it over. The current prototype is built with Arduino which means it was nice and quick to design and assemble although the components are large, since they're designed for breadboard prototyping. Even still, the enclosure inside of the fake rock is still large enough to almost entirely encase the components.
I coded this device in C with Arduino using a tilt sensor to activate the alarm routine and a reset button to set it back to standby mode. The code for the program can be found here.