What's the difference between PIR and mmWave motion sensors?

PIR (passive infrared) sensors detect heat-based movement and are fast, cheap, and battery-friendly, but they fail to detect a still person. mmWave radar sensors (like the Aqara FP2 class) can detect breathing and small movements, so they keep lights on while you're reading or sleeping, but they're typically wired, pricier, and more prone to ghost triggers from fans or pets. Most homes are best served by PIR for hallways and bathrooms, with mmWave reserved for living rooms and offices where presence detection matters.