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PIR (PASSIVE INFRARED) DETECTOR USING ST7FLITE05/09/SUPERLITE

1.3 SENSOR CIRCUIT DESCRIPTION

Transistor Q3 is biased in the active region and amplifies the signal from the PIR sensor. The

microcontroller provides a biasing signal, which is connected to the biasing network on the

transistor. This biasing signal is integrated through a capacitor (C7) and resistor (R14). The

bias signal at 0 level (LOW) for a long time (greater than the discharging time of the capacitor)

puts the transistor into the cut-off region, making it OFF. The HIGH from the microcontroller for

a long time (greater than the charging time of the capacitor) causes saturation of the tran-

sistor. Thus the transistor gain is controlled by the micro-controller and it can shift the bias

from cut-off to saturation region and vice-versa.

The software adjusts the biasing signal to keep the output of transistor amplifier at constant

threshold level which is checked by continuously reading the value using the A/D Converter

(ADC) of the microcontroller. The application uses a software counter to track the number of

times the biasing signal was changed to LOW or HIGH to maintain the threshold.

The C7 capacitor at base of transistor causes integration of the biasing signal generated by

the microcontroller, causing the voltage at base of transistor to be approximately constant.

This voltage causes the transistor to operate within the active region.

1.3.1 Detecting signal variations

If there is no signal from the PIR sensor, the software counter (which tracks the number of oc-

currence of LOW / HIGH in a particular time) variation remains within in a small range. In case

of motion and hence a signal from the PIR, this software counter will change beyond the limits

set by the software.

1.3.2 Filtering signal variations due to changes in surroundings

Slow changes because of temperature, ambient light etc. are compensated by software by

keeping the transistor output/feedback signal at constant level. The effect of these parameters

may cause changes in the threshold voltage. But by maintaining the threshold signal at con-

stant level these effects are nullified. This changes the value of the software counter slightly,

which can easily be distinguished from human movement (which also causes changes in the