In the microcontroller era today, Arduino ... the sensor. This sensor only responds to 38 KHz modulated infrared light which makes it immune to disturbance caused by the ambient light. Specifications ...

[ladyada] has a freshly-published and amazingly thorough tutorial on passive infrared ... most PIR sensors (like the one above) come in a simple-to-interface module containing all the support ...

In the real world, missile systems use advanced radars, infrared sensors, and other hardware ... data for the launcher, an Arduino Uno uses an ultrasonic sensor pair mounted atop a servo.