I bought these simple alarm system for my apartment to be surprised at night not from potential burglars. The installation is quite simple. The two parts with the supplied adhesive tape on the door (or wherever) and fixed after you only have the hidden switch from "off" to "alarm" or "bell" pose. Already, anyone who opens the door, greeted with a deafening noise. The sound of the alarm system is really so outrageously loud that you can feel how the eardrum vibrates. I have an additional adhesive tape over the speaker mounted to dampen the volume a bit. But the alarm is still loud enough to wake the whole probably house at night. The sensor also works very well and does not need, as in the picture, are mounted in the exact position. Otherwise the device would in fact do not work with a normal door. All in all a simple, cheap and extremely loud deterrent.
