A proximity sensor is a device that is able to sense the presence of objects that enter its field without any physical contact with it. These sensors emit a field, often electromagnetic, and the sensor detects any changes in the field’s return signal to determine whether anything has disturbed it; when this occurs it sends a proximity alert through a sound or other notification.

Proximity alerting can also be implemented using a smartphone or tablet computer. Proximity sensors built into these devices can be used to recognize hand gestures made near them or recognize when the phone is held near a user’s ear, for example. A smartphone’s GPS capabilities can also be used to determine when the device is near a predefined location. Users can set a waypoint with a specified radius and when the phone is within that radius it issues an alert.