Light Sensor Circuit using LM3909 and LDR

Introduction

A light sensor circuit is an essential component in many automatic lighting systems, streetlights, and smart home devices. This particular circuit uses a Light Dependent Resistor (LDR) and an LM3909 IC to detect ambient light levels and turn ON an LED when darkness is detected. The LM3909 is commonly used for timing and blinking circuits but can also be configured as a comparator in light-sensitive applications. This project is ideal for beginners in electronics and is powered by a simple 1.5V to 5V battery, making it a great low-voltage, low-power solution.

Component Details

LM3909 Pinout

Circuit Diagram

Circuit Explanation

Working Principle:

This light sensor circuit works based on the resistance change of the LDR when exposed to light. In bright conditions, the resistance of the LDR is low, while in darkness, the resistance increases significantly.

Power Supply:

• The circuit operates on a battery ranging from 1.5V to 5V, making it compatible with AA cells or small Li-ion batteries.

LM3909 Configuration:

• Pin 5 is connected to the positive rail (Vcc).
• Pin 2 and Pin 3 are used to connect the timing capacitor (100µF) and the LDR in parallel.
• The LDR and capacitor combination affects the charging/discharging cycle at these pins, altering how the LM3909 behaves.

LED Control:

• The LED is connected to Pin 8, which is the output of the LM3909.
• When the ambient light falls below a certain level (darkness), the LDR’s resistance increases, triggering the LM3909 to activate the LED.
• In bright conditions, the low resistance of the LDR prevents the LED from lighting up.

Capacitor Role:

• The 100µF capacitor is used to introduce delay and timing effects, helping the LM3909 detect gradual changes in light.

Applications