AN4368

How does the PIR sensor work

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3/18

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How does the PIR sensor work

The aim of the PIR sensor is to detect motion in a specific area. As seen in Figure 1: "PIR

sensor", these sensors contain two parts that are sensitive to IR. Since it is motion that is

important, the signal delivered by the sensor reflects different amounts of IR detected by

each part. If both parts "see" the same amount of IR, the sensor detects nothing. But, if one

of the two parts "sees" more or less IR than the other part, the sensor detects something

and the output voltage of the sensor varies.

Figure 2 shows how the output voltage varies when a heat source moves in or out of the

area protected by a sensor.

Figure 2: Principle of the PIR sensor

The area of

the two rectangles that is IR sensitive is small (≈2 mm² for each rectangle).

Clearly, it would be better to increase the IR sensitive area. This can be done in two ways:

by increasing the number of sensors

by using a Fresnel lens

The latter method is highly recommended as it is inexpensive and improves IR reception by

the sensor. It multiplies the area visible to the sensor and it focuses reception of IR by the

sensor.