Lesson 20 Photoresistor Module

Introduction

A photoresistor is a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity.

Components

– 1 * Raspberry Pi

– 1 * Breadboard

– 1 * Network cable (or USB wireless network adapter)

– 1 * PCF8591

– 1 * Photoresistor module

– 1 * 3-Pin anti-reverse cable

– Several Jumper wires (M to F)

Experimental Principle

With light intensity increasing, the resistance of a photoresistor will decrease. Thus the output voltage changes. Analog signals collected by the photoresistor are converted to digital signals through PCF8591. Then these digital signals are transmitted to Raspberry Pi and printed on the screen. The schematic diagram:

Experimental Procedures

Step 1: Build the circuit

Raspberry Pi	PCF8591 Module	Photoresistor
SDA	SDA	*
SCL	SCL	*
3V3	VCC	VCC
GND	GND	GND
*	AIN0	SIG

For C language users:

Step 2: Change directory

 cd /home/pi/SunFounder_SensorKit_for_RPi2/C/20_photoresistor/

Step 3: Compile

gcc photoresistor.c –lwiringPi

Step 4: Run

sudo ./a.out

For Python users:

Step 2: Change directory

 cd /home/pi/SunFounder_SensorKit_for_RPi2/Python/

Step 3: Run

sudo python 20_photoresistor.py

Now, change light intensity (e.g. cover the module with a pad), and the value printed on the screen will change accordingly.

C Code

#include <stdio.h>
#include <wiringPi.h>
#include <pcf8591.h>
#include <math.h>

#define		PCF     120
#define		DOpin	0

int main()
{
	int analogVal;
	
	if(wiringPiSetup() == -1){
		printf("setup wiringPi failed !");
		return 1;
	}
	// Setup pcf8591 on base pin 120, and address 0x48
	pcf8591Setup(PCF, 0x48);

	while(1) // loop forever
	{
		analogVal = analogRead(PCF + 0);
		printf("Value: %d\n", analogVal);

		delay (200);
	}
	return 0;
}

Python Code

#!/usr/bin/env python
import PCF8591 as ADC
import RPi.GPIO as GPIO
import time

DO = 17
GPIO.setmode(GPIO.BCM)

def setup():
	ADC.setup(0x48)
	GPIO.setup(DO, GPIO.IN)


def loop():
	status = 1
	while True:
		print 'Value: ', ADC.read(0)
		
		time.sleep(0.2)

if __name__ == '__main__':
	try:
		setup()
		loop()
	except KeyboardInterrupt: 
		pass