AN3248 datasheet(14/17 Pages) STMICROELECTRONICS

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Brightness control using a light dependent resistor (LDR)

AN3248

14/17

Doc ID 17758 Rev 3

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 12 shows how

to connect an LDR resistor to an STM32L1 device. COMP2 non-inverting input can be

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

externally to PB3 or set internally to VREFINT, 3/4 VREFINT, 1/2 VREFINT, or 1/4 VREFINT. The

COMP2 output (CMP2OUT) can be internally connected to EXTI line 22 which, when

configured to detect both rising and falling edges on CMP2OUT, can be used as an interrupt

source to switch to/from Low power mode.

Figure 12.

Connecting an LDR resistor to an STM32L1 device

PB4 or PB5 can be used as COMP2 non-inverting input. Thus, VIN can be connected to PB4 or PB5.

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.

VDD

COMP2

Resistor (R)

VIN

Other possibilities

REFINT

PB3

¾ V

REFINT

½ V

REFINT

¼ V

REFINT

PB4/PB5 (1)

EXTI line 22

ai18728

CMP2OUT

Light dependent

resistor (LDR)

STM32L1 device

LDR

()

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