This is an Arduino-based Proximity Sensor circuit wired using the popular HC-SR04 ... If everything goes right,you should see the burning eye of the red LED when there is no object in the safe-zone ...

Instead, it relies on an ultrasonic distance sensor to detect a hand (or foot) waved in front of the door. An Arduino Leonardo runs the show, sounding a buzzer when the ultrasonic sensor is triggered.

Enclosed are the ultrasonic sensor, a temperature sensor and an LED indicating the power status. On the other side of the CAT5 cable can be found an Arduino compatible board with an XBee shield ...