I would like to design a lock which will automatically open once someone is on a specific side of a door ("outside").

There are easy (software) and complicated (hardware) elements to take into account but one of the points which i do not know to approach is the side detection.

How is this typically done?

An example is my car which will let the door close only if the key is outside the car. If it is next to the car outside, the car will lock. A few centimeters in (the key touches the door from the inside) and it is not possible to lock the door anymore.

  • i think that the car locking system works on the key's distance from a sensor, not on the position of the key relative to the car ..... that is why you should never leave your key near the front door .... a signal booster can be used to open your car and to start it
    – jsotola
    Mar 18, 2019 at 6:28

1 Answer 1


Jsotola is right. The cars use the distance to know when they have to close doors.

In your case, you can use an extra sensor in the inside part. Maybe, an RFID or NFC tag, an ultrasonic sensor, or a laser or infrared sensors. With this element, you can detect if people are in the inside part.

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