2.2.1 Photoresistor

Introduction

Photoresistor is a commonly used component of ambient light intensity in life. It helps the controller to recognize day and night and realize light control functions such as night lamp. This project is very similar to potentiometer, and you might think it changing the voltage to sensing light.

Components

_images/list_2.2.1_photoresistor.png

A photoresistor or photocell is a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits photo conductivity. A photoresistor can be applied in light-sensitive detector circuits, and light- and darkness-activated switching circuits.

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What is ADS7830?

The ADS7830 is a special chip that helps your Raspberry Pi read analog signals. Think of it as a translator - it converts the continuous voltage signals from the joystick into digital numbers that your computer can understand.

Key features for beginners: - It can read 8 different analog inputs - It uses I2C communication (a simple 2-wire protocol) - It provides 8-bit resolution (values from 0 to 255)

_images/ADS7830_Module.png

Understanding ADC (Analog-to-Digital Converter)

What does ADC do? An ADC converts analog signals (like the smooth movement of a joystick) into digital numbers that computers can work with.

How does it work? Our ADC has 8-bit resolution, which means it can produce 256 different values (2^8 = 256). It takes the 3.3V input range and divides it into 256 equal parts:

_images/ADC_S.png

Simple breakdown: - Range 1: 0V to 3.3/256V = Digital value 0 - Range 2: 3.3/256V to 2×3.3/256V = Digital value 1 - Range 3: 2×3.3/256V to 3×3.3/256V = Digital value 2 - And so on…

Why does this matter? The more bits an ADC has, the more precise it becomes. With 8 bits, we get 256 different positions - that’s pretty good for detecting joystick movement!

Connect

_images/2.2.1.png

Code

For C Language User

Go to the code folder compile and run.

cd ~/Basic-Starter-Kit-for-Raspberry-Pi/c/2.2.1/

g++ 2.2.1_Photoresistor.cpp -lwiringPi -lADCDevice

sudo ./a.out

The code run, the brightness of LED will vary depending on the intensity of light that the photoresistor senses.

This is the complete code

#include <wiringPi.h>
#include <stdio.h>
#include <softPwm.h>
#include <ADCDevice.hpp>

#define ledPin 0

ADCDevice *adc;  // Define an ADC Device class object

int main(void){
   adc = new ADCDevice();
   printf("Program is starting ... \n");

   if(adc->detectI2C(0x48)){// Detect the ads7830
      delete adc;               // Free previously pointed memory
      adc = new ADS7830();      // If detected, create an instance of ADS7830.
   }
   else{
      printf("No correct I2C address found, \n"
      "Please use command 'i2cdetect -y 1' to check the I2C address! \n"
      "Program Exit. \n");
      return -1;
   }
   wiringPiSetup();
   softPwmCreate(ledPin,0,100);
   while(1){
      int value = adc->analogRead(0);  //read analog value of A0 pin
      softPwmWrite(ledPin,value*100/255);
      float voltage = (float)value / 255.0 * 3.3;  // calculate voltage
      printf("ADC value : %d  ,\tVoltage : %.2fV\n",value,voltage);
      delay(100);
   }
   return 0;
}

For Python Language User

Go to the code folder and run.

cd ~/Basic-Starter-Kit-for-Raspberry-Pi/python

python 2.2.1_Photoresistor.py

The code run, the brightness of LED will vary depending on the intensity of light that the photoresistor senses.

This is the complete code

#!/usr/bin/env python3
import RPi.GPIO as GPIO
import time
from ADCDevice import *

ledPin = 11 # define ledPin
adc = ADCDevice() # Define an ADCDevice class object

def setup():
   global adc
   if(adc.detectI2C(0x48)): # Detect the ads7830
      adc = ADS7830()
   else:
      print("No correct I2C address found, \n"
      "Please use command 'i2cdetect -y 1' to check the I2C address! \n"
      "Program Exit. \n");
      exit(-1)
   global p
   GPIO.setmode(GPIO.BOARD)
   GPIO.setup(ledPin,GPIO.OUT)   # set ledPin to OUTPUT mode
   GPIO.output(ledPin,GPIO.LOW)

   p = GPIO.PWM(ledPin,1000) # set PWM Frequence to 1kHz
   p.start(0)

def loop():
   while True:
      value = adc.analogRead(0)    # read the ADC value of channel 0
      p.ChangeDutyCycle(value*100/255)
      voltage = value / 255.0 * 3.3
      print ('ADC Value : %d, Voltage : %.2f'%(value,voltage))
      time.sleep(0.01)

def destroy():
   adc.close()
   p.stop()  # stop PWM
   GPIO.cleanup()

if __name__ == '__main__':   # Program entrance
   print ('Program is starting ... ')
   setup()
   try:
      loop()
   except KeyboardInterrupt:  # Press ctrl-c to end the program.
      destroy()

Phenomenon

_images/221.jpg