Final answer:
A door sensor or door contact sensor is what triggers an open door alert when the door is left open. It uses a simple mechanism where a completed electrical circuit between the door and frame signals a closed door, and a broken circuit signals an open one.
Step-by-step explanation:
The device on a door that triggers an open door alert if the door is left open is commonly known as a door sensor or door contact sensor.
These sensors typically consist of two parts: one is attached to the door, and the other to the door frame. When the door is closed, the two parts of the sensor are next to each other, completing an electrical circuit.
If the door is opened, the circuit breaks, and the system is alerted, leading to the open door alert. This is especially useful in security systems or as part of a smart home setup, as it allows monitoring of whether doors are securely closed.