There are several types of door alarms so let's start out with the one I think is the best, which is the door stop alarm. When discovering how door alarms work remember that sometimes simpler is better. It works very simply. It blocks the door just like any door stop, and if you've ever tried to open a door with one under it you know it's pretty much impossible. There is a pressure sensitive switch in the base and when anyone pushes on it, this activates an ear splitting alarm. Another reason I like this device is because it's portable. You can use it at home, take it to the office with you and even use it in a hotel while on business or vacation.

Next let's talk about a unit that's not only a portable door alarm but also a window alarm as well. When looking into how door alarms work this warning device functions the way many of them do. It has an attachment with a sprung metal clip that you slide between the door and the jam (or the window and the sill). With the door (or window) closed the connection between the two pieces of metal are held in place completing an electrical circuit. If however the door (or window) is opened they spring apart, the connection (and the electrical circuit) is broken and the siren goes off. Really, understanding how door alarms work is not that difficult.

What you may think of when you think "door alarm" is 2 contacts, 1 mounted to the door, the other mounted to the frame, connected by wiring to a box or keypad, like you would get form ADT or any of the big alarm companies. This alarm functions the exact same way as the previous unit described above. When the door is opened, and the contacts separate, the connection is broken which causes the alarm to go off. The only difference is it can't be moved.

Another type of alarm, although not technically a door alarm, is a PIR (Passive InfraRed) motion sensor. These sensors can be connected to a base unit either by wire or wireless transmitter. They operate by detecting variances between a persons body heat and the normal background temperature. To finish understanding how door alarms work I'd like to talk about a dual purpose instant door alarm and personal body alarm. This one works by creating an incomplete electrical circuit. It comes with a chain that is placed on any metal door knob. It sends an electrical signal through the chain, to the knob, which has no where to go until someone touches it. When someone touches the knob they ground the circuiut and that instantaneously triggers the alarm. A dissadvantage of this alarm is that it can not be used on a metal door because the door would gruond the circuit and the alarm would go off constantly.