Page 225 - ARM Based Microcontroller Projects Using MBED

8.24 PROJECT 20—LIGHT LEVEL METER 211
8.24 PROJECT 20—LIGHT LEVEL METER

8.24.1 Description
This is a light level meter which measures the ambient light level (illumination) in lux and
displays the readings on the PC screen. A light-dependent resistor (LDR) is used in this project.

8.24.2 Aim
The aim of this project is to show how a LDR can be used in a project to measure and dis-
play the ambient light level in lux.

8.24.3 Block Diagram

In this project, the NORPS-12-type LDR is used (see Fig. 8.77). This device is based on CdS
photoconductive cell and is sensitive to visible light in the region of 550nm. The resistance of
the device increases as the light level is reduced. As shown in Fig. 8.78, the resistance is greater
than 1 mega-ohm in the dark, and it falls to around 1K in the light. The block diagram of the
project is as in Fig. 8.79.

8.24.4 Circuit Diagram
The circuit diagram of the project is shown in Fig. 8.80. A resistive voltage divider circuit is
formed by connecting the LDR to the power supply through a 10K resistor. The voltage across
the LDR is connected to ADC1/0 (GPIO port PA_0, pin 28 of connector CN7) pin of the
Nucleo-F411RE development board. The program measures the voltage across the LDR, cal-
culates the resistance of the LDR, and displays it on the PC screen.

FIG. 8.77 The NORPS-12 LDR.