A PIR (Passive Infrared) sensor works on the principle of detecting changes in infrared radiation emitted by warm objects like humans or animals. Every object that has heat emits infrared energy. When a person moves within the sensor’s field of view, it causes a sudden change in the detected infrared pattern. The PIR sensor senses this variation and sends an output signal to activate lights, alarms, or other control devices. It doesn’t emit anything itself — it only receives infrared energy.
An LDR (Light Dependent Resistor) sensor works on the principle of photoconductivity. Its resistance decreases as light intensity increases. In darkness, the LDR has high resistance, but when light falls on it, electrons gain energy and flow more easily, reducing resistance. This change helps the LDR sensor control light-based devices like automatic street lights, brightness controls, and solar tracking systems.