A proximity detector with a Raspberry Pi, HC-SR04 proximity detector and some Python
Using a Raspberry Pi, a HC-SR04, three resitors, one led and some Python, create a simple proximity and presence detector.
A good introduction to the HC-SR04
Using the Raspberry, we connect to the HC-SR04 and periodically poll it to determine the distance to any object in front of it.
If there's an object nearer than 150cm (configurable), blink a led!
My kids love watching TV from 50cm away and I've given up asking them to move up.
Next step (if the led is not deterrent enough) is Home Assistant integration, to actually turn of the TV!