This is not an ordinary door stopper. Besides as a door stopper it is doubled as an alarm too.  When someone pushes the door, the 100 dB alarm will warn you that someone is trying to enter. This is a good way to prevent people to get in secretly or to frighten away intruders.

Security Door Stopper Alarm-pictures

It is totally easy to use, just power it with 2 AA batteries and put at the base of the door, that’s it. The device also portable so you can bring it to anywhere you need to feel protected.