Interfacing HC SR501 Motion with Arduino Uno

The HC-SR501 is a passive infrared (PIR) motion sensor that detects the change in infrared radiation as a warm body moves across its Fresnel-lensed sensing cone. It outputs a simple digital HIGH when motion is detected and LOW otherwise — no library, no bus protocol, just a digital read.

It's the standard part for motion-activated lighting, security alarms, and occupancy sensing. The catch that trips people up is the two onboard trim pots and one jumper: the sensitivity pot sets detection range (up to ~7m), the delay pot sets how long the output stays HIGH after triggering (5s to 300s, defaulting to a very long ~5min-ish setting out of the box that makes the sensor look "stuck" on first test), and the jumper selects single-trigger (L, output resets during the delay window) vs repeatable-trigger (H, output stays high as long as motion continues). It also needs a 30–60 second warm-up period after power-on to stabilize — false or missed triggers in that window are normal.