contact closure

A contact closure is a device that detects the open or closed status of a circuit. Depending on the open/closed status, the contact closure does something. With open (aka "0") as a binary false, and closed (aka "1") as binary true, this device can used to receive commands from another system.

For example, a fire alarm system may need to interface with an architectural lighting system such that it could trigger the lights to turn on in the event of an emergency. Two different systems made by two different manufacturers could otherwise require custom software to interface between the two systems, but contact closures allow the fire alarm system to output to the lighting system in a simple but effective way. In this case, if the circuit is Open, that state might be defined as "normal operations," but when the contact closure sees that the circuit is Closed, it triggers the "Emergency Panic Mode," which tells the lighting system to turn on the lights so that everyone can safely leave the building.

Because of how universal contact closures are, they are most often used to interface with non-native devices -- devices made by other manufacturers. So while it could be a fire alarm system telling a lighting system to turn on the lights, the lighting system could also flip a relay (to alter the open/closed status of a circuit) to tell a motor to open the window shades or to tell an igniter to turn the fireplace on.

When more than two states exist that need to be controlled, another circuit can be added, allowing four, and another, to control 8. This is exactly the same premise of how binary is used to create the modern numbering system. For example, if three circuits were being monitored by a contact closure, each could result in a different action being enacted. The following example is of a button station for an architectural lighting system talking to a building automation system in someone's house.

000 - #0 - Audio Off / Doors Locked / Window Shades Closed
001 - #1 - Audio Off / Doors Locked / Window Shades Open
010 - #2 - Audio Off / Doors Unlocked / Window Shades Closed
011 - #3 - Audio Off / Doors Unlocked / Window Shades Open
100 - #4 - Audio On / Doors Locked / Window Shades Closed
101 - #5 - Audio On / Doors Locked / Window Shades Open
110 - #6 - Audio On / Doors Unlocked / Window Shades Closed
111 - #7 - Audio On / Doors Unlocked / Window Shades Open

The contact closure is one of the most primitive but powerful means of communication between systems, given it's universal nature and interoperability between devices, regardless of who they are manufactured by. A sensor for an alarm system could detect if a door is open or closed by opening a circuit or closing it, which then would be detected by a contact closure for the security system. While initially the sensor and the alarm system may both be made by the same company, later on if that sensor becomes faulty it could be replaced by any other sensor on the market that operates on the same premise. Contact closures make integrating various systems and components a simple task once broken down to the most simplistic binary true/false statements.

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