[Editor’s note: As many mentioned in the comments, 433 MHz is a licensed ham band in the USA (ITU Region 2), so you can’t use it without a license. (Get one, it’s easy.) In the USA ...

There’s a huge market for 433 MHz alarm system hardware out there, from PIR motion detectors to door and window sensors. If you want to put them to work, all you need is a receiver, a network ...