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.

The JSN-SR04T Waterproof Ultrasonic Sensor As well as the familiar ultrasonic ranging module HC-SR04, another one seen on online markets is the JSN-SR04 (and variants). The JSN-SR04 ultrasonic ranging ...

The treat in question is detected by an SR-04 ultrasonic distance sensor connected to an Arduino Nano. All in all this was a highly successful build as far as neighborhood entertainment value goes.