This uses a 36114-MP radar module to control a 5-digit red 7-segment LED display which normally shows our house street address. It is mounted on the side of the house facing the driveway. However when the radar detects a moving person nearer than 18 feet it changes selected segments so that the display changes to "HELLO", which contains only characters which can be displayed using 7 segments.

These are photos of the device.

Here are the block and circuit diagrams.