AN4112 datasheet(14/17 Pages) STMICROELECTRONICS

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Brightness control using a light dependent resistor (LDR)

AN4112

14/17

Doc ID 023195 Rev 1

Brightness control using a light dependent resistor

(LDR)

In some battery operated applications, the microcontroller needs to be powered if the

environment is lit; otherwise, it must be kept powered off. For such applications, a light

dependent resistor (LDR), whose resistance depends on light intensity, is useful to control

the microcontroller state. Using an LDR sensor, the microcontroller can switch to/from low

power mode depending on the voltage provided by the LDR resistor. Figure 11 shows how

to connect an LDR resistor to an STM32F05xx device. The COMP2 non-inverting input can

be connected to an LDR resistor through a voltage divider (VIN). The threshold can be set

externally to the PA2 or PA5 pin or set internally to VREFINT, 3/4 VREFINT, 1/2 VREFINT, or 1/4

VREFINT. The COMP2 output (COMP2OUT) can be internally connected to EXTI line 22

which, when configured to detect both rising and falling edges on COMP2OUT, can be used

as an interrupt source to switch to/from low power mode.

Figure 11.

Connecting an LDR resistor to an STM32F05xx device

PA3 or PA1 can be used as COMP2 non-inverting input. Thus, VIN can be connected to PA3 or PA1.

The voltage VIN can be computed using Equation 5.

Equation 5

As the LDR resistance decreases with increasing light intensity, the voltage VIN decreases

as more light shines on the LDR.

MS30286V1

VREFINT

¼VREFINT

½VREFINT

¾VREFINT

Other possibilities

PA2/PA5

PA3/PA1

(1)

COMP2

EXTI line 22

COMP2OUT

Light

dependent

resistor

(LDR)

VDD

Resistor (R)

VIN

STM32F05xx

LDR

()

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