Walk up proximity detector

Ladies and Gents, I've been asked to provide some sort of proximity sensor for a client. Ideally it would draw a meager amount of current so it could run off batteries and have a detection range of 1 to 3 meters.

What they want is that when you walk up to a display it will automatically start the program, run for about 30 seconds and repeat.

Your thoughts or comments would be greatly appreciated.

HC-SR04 from Amazon
ultrasonic range detector has 2m range cheap and easy

Use a PIR detector.

Cheers

Al.

+1

the advertisement gadgets activating animations for people passing by storefronts are built arounds PIR detectors ...

Alain