Security System Wiring
Hardwired Zones:
There are two basic types of alarm circuit (also called a 'zone') -- these are open and closed, and require that the sensors hooked up to them to be either normally open or normally closed. Most hardwired systems will allow you to designate each zone to be either open or closed, so you can mix and match sensors as you desire -- as long as the open and closed sensors are on different zones. You also can connect multiple sensors to most wireless transmitters in a similar fashion.
In order to connect more than one sensor to a particular zone, you need to know what type of sensors (open or closed) you are using -- the procedure for wiring each is different. In a closed circuit zone, you must wire all of the sensors in series with each other; in an open circuit zone, you must wire all of the sensors in parallel with each other. An explanation of series and parallel circuits follows, along with a diagram (please forgive my lack of artistic ability):
Series Circuits: In a series circuit, electrical current only has one path through which to travel. With normally closed security sensors, you need to create a 'loop' which connects (allows current to pass between) two terminals in your security panel when the protected entrances are closed . The current must pass through each sensor in the line before going to the next; therefore if any one of the sensors in tripped due to an unauthorized entry, the current running through the loop is interrupted, and the alarm panel can take appropriate action. This is like many types of holiday lights where each bulb helps to complete the circuit; if you remove one, the circuit is broken and all of the lights go out.
Parallel Circuits: In a parallel circuit, the current in the circuit can pass through multiple paths. With normally open contacts, the circuit is incomplete until one of the sensors trips, which then completes the circuit. This is the way your 110 volt power is run in your house; all of your lights and appliances are in parallel with one another.