PIR Sensor HC-SR501

The HC-SR501 is a passive infrared (PIR) motion sensor module that detects movement by measuring changes in infrared radiation emitted by warm objects such as humans, using a pyroelectric sensor and Fresnel lens to widen the sensing area, and its wide operating voltage range (typically 4.5 V to 20 V DC) allows easy use with microcontrollers and development boards while the digital output switches to a high (≈3.3 V) signal when motion is detected and low (0 V) when no motion is present; the module also features adjustable sensitivity and delay time controls that let you fine-tune the detection range (about 3–7 m) and the time the output stays HIGH after detection, making the HC-SR501 ideal for security, automation, lighting control and DIY electronics projects.