Alarm Contacts
From CT.wiki
An alarm contact is any method used to open/close the alarm circuit, triggering the alarm. An alarm contact will have an electrical characteristic, and a physical design.
Electrically, alarm contacts can be either Normally Open (NO) or Normally Closed (NC). The "normal" describes what the condition of the contact will be without any forces acting on it. In other words, "normal" would be the condition of the contact when it is new in the box. A NO contact will not carry electricity until it is activated, while a NC contact will carry electricity until it is activated.
One area that confuses many people is when you add in the concept of NO and NC loops. A "loop" is the alarm industry's term for an individual alarm circuit, and could contain multiple alarm contacts. Just like contacts, a NO loop will not pass electricity in its normal (non-alarm) state, while a NC loop will pass electricity in its normal state. A NC loop may contain both NO contacts and NC contacts! The contacts in a NC loop are wired in series, i.e. the output from contact 1 connects to the input to contact 2, output from #2 to the input of #3, etc. The contacts in a NO loop are wired in parallel, i.e. all contact inputs tied together, and all contact outputs tied together.
The physical design of an alarm contact can take many different shapes. Probably the most common is the magnetic contact switch, AKA magnetic reed switch. These switches can come in either NO or NC forms. In a magnetic reed switch, two small metallic "reeds" are enclosed in a small glass tube. One reed is connected to the input, while another is connected to the output (in reality, there is no "in" or "out" on a reed switch. It will work in either direction). When in the presence of a magnetic field, the reeds will bend, either making the electrical connection (NO switches) or breaking the electrical connection (NC switches). The most common use for a magnetic reed switch is in the normally closed loops for doors and windows. Using a NO contact, the switch is placed in the door/window casing, while a magnet is hooked to the door/window frame. When the door/window is closed, the magnet causes the switch to close, thus making the circuit. When someone opens the door, the switch opens, breaking the circuit, and tripping the alarm.
Some other types of alarm contacts are listed below. Note that some of these contacts are actually relays attached to some type of electronic sensor, and thus require some type of power supply.
