Project uses LDR (Light dependent Resistor) to change the resistance across the circuit which can be used to control the behavior of different components.
In this project I am using LED and Speaker as the output components which detects the brightness in the room and alerts the user that Lights may be on. This can be extended to various applications like Home Automation, fun project to play different music when user passes by.

Step 1: Collect Arduino and Components

Components used:
• Arduino Uno
• Resistors(I am using 330 Ohms)
• Breadboard
• Speaker (EXULIT 1196)
• LED lights
• Connecting Wires

Step 2: Connect Them According to the Circuit Diagram

As shown in Circuit diagram, connect LDR to A0 and Resistor connected to GND and 5V
Output components LED and Speaker are connected to pin 10, pin 11.

Step 3: Write Code in Arduino and Connect to the USB.

Open Arduino IDE, Depending on the values you get in Serial(Depends on the brightness in the room)

int inputPin = 0;  //Pin for Photo resistor
int outputPin=11;     //Pin for LED
int outputPin1=10;

void setup()
  Serial.begin(9600);  //Begin serial communcation
  pinMode( outputPin, OUTPUT );

void loop()
  int reading = analogRead(inputPin);
  if(reading > 90) { //Value here depends on the brightness of the room and how you want to calibrate it.
    analogWrite(outputPin1, analogRead(inputPin));
    analogWrite(outputPin, analogRead(inputPin) );
  } else {
       analogWrite(outputPin1, 0);
       analogWrite(outputPin, 0);

Step 4: Write Python Code to Read Serial Data

To save serial data to file system using Python. Use below code:
# Python code starts.

import serial
ser = serial.Serial('COM3', 9600) //COM3 is the serial port name you connect your Arduino to, It might be different for differnt user

f = open("LDR.txt", "w")
while True:

#Python code ends

Step 5: Use Matplot Lib to Make a Graph Out of It.


import numpy as np
import matplotlib.pyplot as plt

with open("LDR.txt") as f:
    data = f.read()

data = data.split('\n')

x = [row.split(' ')[0] for row in data]

fig = plt.figure()

ax1 = fig.add_subplot(111)

ax1.set_title("Light brightness")   
lines = [i for i in open("LDR.txt") if i[:-1]]
ax1.plot(map(int, lines), 'b-')

<p>Hello, Do you have to use the Speaker (EXULIT 1196), or could any speaker substitute? </p>

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More by sbhat8:Simple brightness Indicator and alert using LDR, Arduino, Python. 
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