This document describes how to measure light levels using an Arduino. It explains that a photoresistor (LDR) is used as a light sensor, whose resistance changes with brightness. The circuit connects the LDR to an LED and Arduino analog pin. Code samples show configuring the Arduino pin modes, reading the LDR value, and using it to set the LED brightness level. The result is the LED brightness changing in response to environmental light levels measured by the LDR.

1. 1
Arduino Tutorial #3
- Measuring the light -
Dong Ho Son
POSTECH
Pohang, Korea
donghoson@postech.ac.kr

2. 2
Measuring the light - Overview
 Sensor: Analog  Digital
• Analog : continuous
• Digital : discrete
 Photoresistor (or Light-Dependent Resistor, LDR)
• A light-controlled variable resistor
• The resistance increases/decreases according to the brightness
 Let’s configure the circuit
• The brightness of the LED is changed according to the brightness of the
environment
• If dark, the LED also would be dark
• If bright, the LED also would be bright
source: wikimedia.org

3. 3
Measure the light - Circuit
 Configure the circuit
+-
2kΩ
220Ω
Source: pixabay.com/photo-2168193

4. 4
Measuring the light - Sketch (1/3)
 setup() { }
• pinMode(10, OUTPUT);
− Prepare the mode of pin
− We will use pin #10
− We will order to LED  OUTPUT
• Serial.begin(9600);
− Prepare the serial communication to get
the value of light from the sensor
− 9600: serial communication speed
– 9600 bits per second is the default for the Arduino

5. 5
Measuring the light - Sketch (2/3)
 loop() { }
• int val = analogRead(A4);
− Read the analog value of light
• analogWrite(10, val/4);
− Turn on the LED
− Different from digitalWrite();
– digitalWrite(); just has HIGH or LOW
− Brightness of LED is controlled by ‘val/4’
− val  10 bit , val/4  8 bit
− analogWrite() uses 8 bit of digital pin

6. 6
Measuring the light - Sketch (3/3)
 loop() { }
• Serial.print(“ Brightness = ”);
− It prints the text “ Bightness = ”
to the serial console
• Serial.println(val/4);
− It prints the value of ‘val/4’ to the serial console
− Also changes the line (newline)
• Delay(10);
− Need 10ms to check step-by-step

7. 7
Measuring the light - Result
 The brightness of the LED is being changed!
+-
Brightness = 127
Brightness = 153
Brightness = 128
Brightness = 156
Brightness = 127
Brightness = 156
Brightness = 127
Brightness = 156
Brightness = 125
Brightness = 155
Brightness = 124
Brightness = 158
Brightness = 129
Brightness = 156
Brightness = 127
Brightness = 153
Brightness = 125
Source: pixabay
2kΩ
220Ω
Source: pixabay.com/photo-2168193