Let's measure the brightness.

1. 1
Raspberry Pi Tutorial #2
- Measuring the light -
Dong Ho Son
POSTECH
Pohang, Korea
donghoson@postech.ac.kr

2. 2
Measuring the light - Overview
 Let’s use the sensor
• Sensor : Analog  Digital
− Analog : continuous
− Digital : discrete
• Photoresistor 3 pin (or light-dependent resistor, LDR)
− A light-controlled variable resistor
− The resistance increases/decreases according to the brightness of light
• Let’s configure the circuit
− The brightness of the LED is changed according to the brightness of the environment
− If dark, the LED is turned on
− If bright, the LED is turned off

3. 3
Measuring the light - Overview
 Measure the brightness of light and turn on/off the LED
 Necessary Components
• 1 x Raspberry Pi
• 1 x Breadboard
• 1 x red LED
• 1 x Resistor
• 1 x Photoresistor 3 pin
• Jumper cables
Anode
Cathode
GND
OUT
VCC

4. 4
Measuring the light – Circuit configuration
 Ground pin  the cathode of the LED
 The anode of the LED  a ballast resistor
 The ballast resistor  the pin GPIO 18
 VCC of photoresistor  3.3V
 OUT of photoresistor  the pin GPIO 17
 GND of photoresistor  Ground pin
Cathode Anode

5. 5
Measuring the light - Python
 Import RPi.GPIO as GPIO
• Import Python GPIO module
 GPIO.setmode(GPIO.BCM)
• Use Broadcom chip number
 GPIO.setup(17, GPIO.IN)
• Select GPIO #17 as input pin
• Measure the brightness using sensor
 GPIO.setup(18, GPIO.OUT)
• Select GPIO #18 as output pin
• Turn on/off the LED

6. 6
Measuring the light - Python
 GPIO.input(17)
• Return 0(bright) or 1(dark)
 GPIO.output(18, GPIO.LOW)
• Turn off the LED
 GPIO.output(18, GPIO.HIGH)
• Turn on the LED
 time.sleep(1)
• Delay for 1 second

7. 7
Measuring the light - Result
 If bright, the LED is turned off
 If dark, the LED is turned on
Cathode Anode