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

1.4 TRANSISTOR BIASING

1. The transistor is biased using voltage-divider biasing.

This biasing has following advantages:

b. The circuit behaviour is independent of temperature changes etc.

c. The circuit gain is controlled

2. When the PWM signal is HIGH, the voltage drop across R17 is 0.83V

(10k *5V / (50k + 10k)).

Note: R15=100K is in parallel with R13 =100K.

(100k || 100k) = 50K.

The circuit analysis shows that this condition will force the transistor to go into the saturation

region. For saturation, the Vbe >= 0.7V approx.

3. When the PWM signal is LOW, the voltage drop across R17 is 0.41

(= 9.09 k *5V / (100k + 9.09k)).

Note: R13=100K is in parallel with R17 = 10K.

100k || 10k = 9.09K

The circuit analysis shows that this condition will force the transistor to go into the cutoff re-

gion. For cutoff, the Vbe <= 0.5V

4. Thus the microcontroller biasing signal (averaged by the RC circuit) can adjust the tran-

sistor biasing. The RC network is formed by R14 and C7 in Figure 2.

1.5 TYPICAL PIR DETECTOR (CONVENTIONAL)

In conventional detectors, the IR radiation is focused by the Fresnel lens on the PIR sensor

and then the output of PIR sensor (which is very small in amplitude) is amplified by an OP-

AMP based amplifier. The OP-AMP also works as low pass filter and rejects the high fre-

quency signals (more than 10Hz typically). The output of the amplifier is connected to the

comparator which compares the signal to the threshold level. An alarm is raised/light source is

powered if the reference (threshold) voltage is crossed.