This project describes a motion sensor alarm based on a Passive Infra-Red (PIR) sensor module. There are many vendors that manufacture the PIR sensor modules and almost all of them are pretty much the same in function. They have a single output that goes high (or low, based on specification) when the motion is detected. In this project, a PIC12F635 microcontroller continuously monitors the output from the sensor module and turns a buzzer on when it goes active.
Certain crystalline materials have the property to generate a surface electric charge when exposed to thermal infrared radiation. This phenomenon is known as pyroelectricity. The Passive Infra-Red (PIR) sensor module works on the same principle. The human body radiates heat in the form of infrared radiation which is maximum at about 9.4 um. The presence of human body creates a sudden change in the IR profile of the surrounding that is sensed by the pyroelectric sensor. The PIR sensor module has an instrumentation circuit on board that amplifies this signal to appropriate voltage level to indicate the detection of motion.