Microwave movement sensor kit
This one was featured in the home handyman magazine. It simply consists of a microwave sensor and relay. Hooking this up is easy as shown in the picture below.
You can power it up with an old 5V cellphone charger power supply that you probably have laying around at home.
Its operation is easy to understand when the microwave sensor pics up movement it will trigger the relay (you will hear the relay click). The relay, in turn, is used to switch anything including a siren. The picture below shows how to connect a light to a relay but please be careful, 220V will kill you (definitely not for children). A 12V siren is probably the best to connect for newcomers in electronics.
This Microwave movement sensor kit includes: