Python Program 2: Interfacing LDR Sensor with Raspberry Pi

OBJECTIVE:

To interface an LDR sensor with the Raspberry Pi and write a Python program to turn ON an LED based on the ambient light level sensed by the LDR.

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RESOURCES REQUIRED:

• Raspberry Pi (any model with GPIO support)
• 330Ω Resistor
• LED (Light Emitting Diode)
• LDR (Light Dependent Resistor)
• 10kΩ Resistor (pull-down for LDR)
• Breadboard
• Jumper wires

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THEORY:

LDR (Light Dependent Resistor) Basics:

• An LDR is a light-sensitive device whose resistance decreases as the light intensity increases.

• In bright light, the LDR conducts more current; in darkness, its resistance increases, limiting current.

• By connecting the LDR in a voltage divider with a resistor, we can read the light level as a digital HIGH or LOW using the Raspberry Pi’s GPIO pin.

Current Limiting Resistor for LED:

• Just like with any LED circuit, a 330Ω resistor is used in series with the LED to prevent excess current and protect the LED.

GPIO Pins on Raspberry Pi:

• The Raspberry Pi features GPIO pins that can be set as input or output in software.

• We’ll use one GPIO pin as an input (to read from LDR) and another as an output (to control the LED).

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Circuit Design:

• LDR and 10kΩ form a voltage divider.

• GPIO 26 reads the divided voltage and determines light intensity (1 = dark, 0 = light).

• GPIO 2 controls the LED based on the LDR input.

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Python Code: Switching LED Based on LDR Detection

import RPi.GPIO as GPIO
import time

# Define pins
LDR_PIN = 26          # Input from LDR
LED_PIN = 2           # Output to LED

# Setup
GPIO.setwarnings(False)
GPIO.setmode(GPIO.BCM)
GPIO.setup(LDR_PIN, GPIO.IN)
GPIO.setup(LED_PIN, GPIO.OUT)

try:
    while True:
        if GPIO.input(LDR_PIN):  # If it's dark
            GPIO.output(LED_PIN, True)  # Turn ON LED
        else:
            GPIO.output(LED_PIN, False) # Turn OFF LED
        time.sleep(0.5)
except KeyboardInterrupt:
    GPIO.cleanup()
Explanation of the Program:

Import Libraries:

• RPi.GPIO → To control GPIO pins
• time → To allow delay (debounce and loop control)

Pin Setup:

• LDR_PIN = 26 → Connects to the voltage divider from LDR
• LED_PIN = 2 → Connects to the LED circuit
• GPIO is set to BCM mode
• Pins are set as input (LDR) and output (LED)

Main Loop:

• If the LDR detects darkness (logic HIGH), the LED is turned ON.
• If light is detected, the LED is turned OFF.
• A small delay helps smooth the readings and prevent rapid flickering.

Exit Handling:

• On pressing Ctrl + C, the loop exits, and GPIO.cleanup() resets the pins to a safe state.

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🔁 Assignment Time!

📘 Assignment 1: Switching Multiple LEDs Based on LDR Reading

Objective: Extend the above program to control three LEDs (GPIO 2, 3, 4).
Behavior: When LDR detects darkness, turn all LEDs ON. Otherwise, turn them all OFF.

📘 Assignment 2: Switching Multiple LEDs With Different Delays Based on LDR Reading

Objective: Use 3 LEDs on GPIO pins (2, 3, 4).
Behavior:

• When darkness is detected:

  • Turn ON LED 1 (GPIO 2) immediately.

  • Turn ON LED 2 (GPIO 3) after 1 second.

  • Turn ON LED 3 (GPIO 4) after 2 seconds.

• When light is detected, turn OFF all LEDs at once.

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🚀 Ready to Build?

💬 Drop your solution in the comments — show off your GPIO magic!
🔔 Check back tomorrow to see if your answer matches the solution!