How do automatic doors know when to open & close?

Written by andrew mikael
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How do automatic doors know when to open & close?
Home garage doors are one application of automatic door openers. (red garage door image by green 308 from

Automatic doors use several automated methods to open and close, depending on their use and placement. These methods usually require a sensor mechanism on both sides of the door and sometimes include a secondary sensor for safety.

Pressure Sensors

Doors with pressure sensor activators have a large pad placed in front of the opening. The pad works like a scale, detecting the amount of weight placed on the pad at any given moment. When the pad senses a weight heavy enough to represent a person, the door opens. Since the pad also recognises when a person moves off the pad, the same mechanism can hold the door open and close it. Pressure sensors are also used as a safety mechanism, such as the ones lining the bottom of certain garage doors. If the door begins to close on any resistant object, the sensors tell the door to open.

Infrared Sensors

Motion detectors often use infrared technology to decide when a door needs to open. Infrared sensors detect differences in temperature and open when they detect the bulk of warmer temperature in a human form passing in front of the detector.

Other Motion Sensors

Motion detectors that don't use infrared rely on microwave or ultrasonic waves. These motion sensors both send out pulses of either microwave energy or ultrasonic sound and measure how the pulse bounces off objects and returns to the sensor. When something disrupts the regular angles of the pulses, the door opens. Some door controls may use more than one type of sensor to reduce the likelihood of failing to open or opening at an incorrect time.


Some automatic doors, such as garage doors, use a human interface remote to control opening and closing. These doors only activate when they receive the appropriate signal from the transmitter, usually a specific radio wave frequency.


Since motion detecting door sensors may not know when a person has moved out from between the doors, secondary sensor technology tells the door if people or objects are blocking the door from closing, ensuring no one is injured by a door closing inadvertently. Often this secondary sensor is an infrared curtain projected in the doorway. Elevator doors also use this technology to avoid closing when blocked.

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