What Is the Difference Between a Door and Window "Contact" and a "Sensor"?

There is no real difference between a door and window "contact" and a "sensor". These terms are often used interchangeably. Technically, a door and window "sensor" could refer to more devices than a door and window "contact". But regardless, all of these devices monitor doors and windows.

Whether you call a door and window monitoring device a "contact" or a "sensor" really doesn't matter. You can use these terms interchangeably. Anyone with knowledge of security systems will know what you are talking about. These are some of the most common devices used with alarm panels, and they are a staple of any security system. Knowing when a door or window is opened is a basic security function, so you will definitely want some of these devices installed and enrolled with your system.

No matter how you refer to these devices, they usually work in the same way. They consist of two (2) main parts. There is a larger piece called a "sensor" and a smaller piece called a "magnet". The larger sensor is typically placed on the stationary door or window frame. The smaller magnet is then placed on the moving part of the door or window. For proper operation, the magnet must be in very close proximity to the sensor when the door or window is closed. In most cases, a spacing gap of less than half an inch is advised. It's actually preferable to have the magnet and sensor in direct contact when the door or window is closed if possible. That is why these devices are often called "contacts".

When the door or window is opened, the magnet will become separated from the sensor. This will release a metal reed switch inside the sensor. The sensor will then know to send out a signal to the control panel. This will let the system know that the door or window has been opened. The system will then respond based on the programming settings for that zone. The associated zone for the sensor will remain faulted until the door or window is closed.

Please note that there are also some recessed devices, in which the sensor and magnet are installed inside the door or window and its frame. This is as opposed to being mounted on the outer surface. Door and window monitoring devices used in this fashion are appropriately referred to as "recessed sensors" or "recessed contacts". However, their manner of operation is still the same. The only difference is that these devices will not be visible from the outside when the door or window is closed. Meanwhile, the standard devices mounted on the outer surface of a door or window are appropriately referred to as "surface-mounted contacts" or "surface-mounted sensors".

When you talk about door and window "sensors", you are generally referring to door and window "contacts". So whether you call these devices "contacts" or "sensors" won't make much of a difference. But there is one notable exception to this rule. Some door and window monitoring devices don't use a magnet and a sensor with an internal metal reed switch. There are certain recessed devices that function in a different manner. These alternative devices consist of a recessed plunger switch that is installed in the door or window frame. When the door or window is closed, the plunger switch will be pressed down. Opening the door or window will cause the plunger switch to pop back up. The sensor will recognize this and send an alert to the panel. The system will then know that the door or window was opened.

Since these devices don't involve the use of a sensor and magnet in direct contact, it is inappropriate to refer to these devices as "door and window contacts". The more appropriate term would be "recessed door and window plunger switches", or just "recessed switches". You might also still refer to these devices as "door and window sensors", as they meet the criteria of a sensor that is used to monitor doors and windows.

However, recessed plunger switches are much less commonly used than standard door and window contact sensors. So when you say "door and window sensors", it is generally understood that you are talking about contact devices that use a larger sensor with an internal metal reed switch and a smaller accompanying magnet. If you are actually talking about a recessed door and window plunger switch, you should make sure to be clear that you are referring to one of those devices instead. Otherwise it could lead to confusion.

But in general, both "door and window contacts" and "door and window sensors" will identify the same devices that use a sensor and a magnet. Basically, you can call these devices "sensors" or "contacts", and a security expert will know what you are talking about either way. Call them whichever term you prefer more, or use them interchangeably.