A PIR sensor is used for motion detection of human beings. This sensor consists of two sensing elements. When a
human being passes through both the elements a signal is generated indicating the presence of a human being. Earlier
versions of a PIR sensor had only one such element and hence the radiation generated by the sun was also being
detected. This drawback was overcome using two sensing elements and reducing the range of wavelengths to be
detected. A layer of Fresnel lenswhich consists of unprocessed silicon wafer or polyethylene covers the sensing
elements. The radiation of human beings falls in the range which is detected by the sensor. But the cost of using two
sensing elements was that stationery human detection was not possible. A human being had to pass through both the
range of both elements in order to generate a signal. This drawback is overcome by rotating the sensor through 180
degrees clockwise and anti-clockwise. The result is that within the complete range of the sensor’s radiation detection, a
human being, whether he is standstill or moving, a signal will be generated by the sensor indicating the presence of a
human being.
A servomotor is used to rotate the PIR sensor. Servomotors are easy to interface with a controller and are cheap. They
are preferred when a controlled movement is desired. In this system a 180 degree movement was desired. Hence a
servomotor was desirable.
The servomotor and the PIR sensor attached to it are both interfaced with a micro controller. This micro controller on a
general purpose board is used to generate the appropriate pulses required for the rotation of the motor [1].