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

1 SENSOR OVERVIEW

The human body radiates infrared waves with wavelengths of 8 to 12 micrometers. Any move-

ment by a person leads to a change in the amount of infrared energy which a sensor can de-

tect within its range. The PIR sensor reacts to this change in infrared energy and provides a

low-frequency, small amplitude signal. This signal can be amplified and decoded using a

ST7Lite05 microcontroller (as explained in Section 1.2).

1.1 INFRARED FOCUSING BY FRESNEL LENS

The sensor can sense the change in the amount of infrared energy within small distances, ap-

proximately up to 10 inches. For detecting movements at longer distance, infrared radiation

has to be focused. This focusing is done by a Fresnel lens. A Fresnel lens divides the whole

area into different zones. Any movement between zones leads to a change in the IR (infrared)

energy received by the sensor. There are different types of Fresnel lenses depending on the

range (distance) and coverage angle. For example, volumetric lenses and curtain lenses etc.

1.2 PIR DETECTOR USING ST7FLITE05 MICROCONTROLLER

A PIR detector can be made easily with ST7FLITE05 using the circuit shown in Figure 2. The

sensor interfacing circuit (shown on the left side of the microcontroller in Figure 2) can be di-

vided into the following modules:

1.Transistor circuit used as an amplifier.

2.Transistor biasing controlled through the microcontroller.

3. Software-controlled transistor output.

Figure 1. Block Diagram

PIR sensor

RC

Integrator

Transistor

Amplifier

ST7 Micro-

controller

Alarm

Biasing Signal

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