AN3248

Brightness control using a light dependent resistor (LDR)

Doc ID 17758 Rev 3

15/17

selected threshold (COMP2 inverting input) defines the limit of dark/light. The bottom part of

on light intensity. Using EXTI line 22 (which is internally connected to CMP2OUT), the

microcontroller can detect the CMP2OUT level switch (from a high level to a low level or vice

versa).

Figure 13.

Comparator output behavior versus light intensity

The LDR resistor can be used in other microcontroller-based applications rather than

darkness control (for example: DC relay control and AutoFocus).

For other uses of analog comparators using LDR resistors, refer to the STM32L152-EVAL

(for STM32L1xx Ultra Low Power Medium-density devices) or to STM32L152D-EVAL (for

STM32L1xx Ultra Low Power High-density devices) demonstration firmware. In this

demonstration, the LCD glass contrast is adjusted according to the luminosity detected

using the LDR resistor.

Threshold

Time

Time

Low level

High level

Dark

Light

Light

VIN > threshold

VIN < threshold

VIN

Microcontroller enters

Low power mode

Microcontroller exits

Low power mode

CMP2OUT

EXTI22

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