Brightness control using a light dependent resistor (LDR)

AN3248

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Doc ID 17758 Rev 3

6

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

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

1.

Equation 5

as more light shines on the LDR.

VDD

COMP2

Resistor (R)

VIN

Other possibilities

V

REFINT

PB3

¾ V

REFINT

½ V

REFINT

¼ V

REFINT

+

-

EXTI line 22

ai18728

CMP2OUT

Light dependent

resistor (LDR)

STM32L1 device

V

IN

LDR

LDR

R

+

()

----------------------------

V

DD

×

=