Is it possible to detect human motion by using a single pyroelectric sensor? According to me we can't do so using a single sensor.
To detect the motion of a human or other heat source, the sensing element has to distinguish between general background heat radiation and that given by a moving heat source. A single pyroelectric sensor would not be capable of this and so a dual element is used.
One form has the sensing element with a single front electrode but two, separated, back electrodes. The result is two sensors which can be connected so that when both receive the same heat signal their outputs cancel. When a heat source moves so that the heat radiation moves from one of the sensing elements to the other, then the resulting current through the resistor alternates from being first in one direction and then reversed to the other direction. Typically a moving human gives an alternating current of the order of 10-12 A. The resistance R has thus to be very high to give a significant voltage. For example, 50 GV with such a current gives 50 mV. For this reason a transistor is included in the circuit as a voltage follower to bring the output impedance down to a few kilo-ohms. A focusing device is needed to direct the infrared radiation onto the sensor. While parabolic mirrors can be used, a more commonly used method is a Fresnel plastic lens. Such a lens also protects the front surface of the sensor and is the form commonly used for sensors to trigger intruder alarms or switch on a light when someone approaches.