Some home security systems are complex and should be installed by a professional. However, there are many good home security systems sold in kit forms that can be installed by any do-it-yourselfer. Most simple systems use a bell, loud buzzer, or other sound source to note intrusion.
Installation involves mounting the sound source or sounder in a location where it can be easily heard. Then attach a circuit of switches to the sounder and connect a battery to the system. Since the sounder operates from the battery, it remains an effective alarm system even in the event of a power failure. Alternately, battery-operated remote wireless sensors and controls can be installed. However, make sure that batteries are replaced regularly.
In electrical terminology, this alarm system is called a closed-circuit system. When the doors and windows are shut, the attached switches are closed. Because all the switches are in a wiring loop, opening any one of them breaks the loop and triggers the sounder circuit. Simply closing the door or window, moreover, does not restore the switch loop circuit continuity and does not stop the sound until either the battery becomes exhausted or someone switches off the circuit from battery to sounder.
There is an electronic switch built into the sounder. This switch is turned on by breaking the magnetic-switch loop. A key-operated switch in the sounder circuit allows you to turn the alarm system completely off when it is not needed. Also, once the alarm goes on, operating the key switch is the only way to silence the sounder. Only someone who has a key to the switch can reset the system.