Automatic Night Light Detector

Here, it is shown how to build an Automatic Night Light Detector circuit using a simple setup involving a BC547 transistor, an LDR (Light Dependent Resistor), a 100 kΩ resistor, an LED, and a 3.7V battery. This circuit can automatically turn on an LED at night and turn it off during the day. Let’s dive into how it works and the role of each component in the circuit.

Components Needed

BC547 Transistor
LDR (Light Dependent Resistor)
100 kΩ Resistor
LED
3.7V Battery
Connecting wires and a breadboard (optional for prototyping)

Circuit Diagram

The following shows the circuit diagram of the automatic night light detector.

Below is the description of the circuit:

The LDR and the 100 kΩ resistor form a voltage divider.
The junction of the LDR and the 100 kΩ resistor is connected to the base of the BC547 transistor.
The emitter of the transistor is connected to the negative terminal of the battery.
An LED is connected to the collector of the transistor, with its cathode connected to the collector and its anode connected to the positive terminal of the battery.
The other end of the 100 kΩ resistor is also connected to the positive terminal of the battery.

How the Circuit Works

The transistor (BC547) in this circuit acts as a voltage comparator, and the behavior of the circuit depends on the light intensity falling on the LDR:

1. At Night (Dark Conditions):

The LDR’s resistance becomes very high (in the range of megaohms).
In the voltage divider, most of the battery voltage is dropped across the LDR, increasing the voltage at the transistor’s base.
When this base voltage exceeds the transistor’s base-emitter threshold voltage (≈ 0.6V to 0.7V), the transistor turns on.
Current flows through the LED, from the battery’s positive terminal, through the transistor’s collector-emitter path, and back to the negative terminal. This lights up the LED.

2. During the Day (Bright Conditions):

The LDR’s resistance becomes very low (a few hundred ohms or less).
In the voltage divider, most of the battery voltage is dropped across the 100 kΩ resistor, reducing the voltage at the transistor’s base.
When this base voltage falls below the transistor’s base-emitter threshold voltage, the transistor remains off.
No current flows through the LED, so it stays off.

These claims are demonstrated in the following video.

The Role of the LDR

The LDR is a crucial component of this circuit, as it is responsible for detecting changes in light intensity:

In Darkness:
- The LDR’s resistance is high (typically in the range of megaohms), leading to a higher voltage at the transistor’s base. This turns the transistor on and lights up the LED.
In Light:
- The LDR’s resistance is low (a few hundred ohms or less), leading to a lower voltage at the transistor’s base. This keeps the transistor off and the LED unlit.

The voltage at the base of the transistor is determined by the voltage divider equation:

V_{\text{base}} = V_{\text{cc}} \cdot \frac{R_{\text{LDR}}}{R_{\text{LDR}} + R_{100k}

Where:

$V_{\text{base}}$ is the voltage at the base of the transistor.
$V_{\text{cc}}$ is the supply voltage (3.7V in this case).
$R_{LDR}$ is the resistance of the LDR (varies with light intensity).
$R_{100k}$ is the fixed 100 kΩ resistor.

Finding $R_{\text{LDR}}$ at Night and During the Day

At Night (Measured Base Voltage = 0.726V):

We calculated the LDR resistance using the voltage divider equation:

R_{\text{LDR}} = 100,000 \cdot \frac{0.726}{3.7 - 0.726} \approx 24,400 \, \Omega

Thus, the LDR resistance at night is approximately 24.4 kΩ.

During the Day (Measured Base Voltage = 0.0184V):

Similarly, for the day, when the base voltage drops to 0.0184V, the LDR resistance is calculated as:

R_{\text{LDR}} = 100,000 \cdot \frac{0.0184}{3.7 - 0.0184} \approx 499 \, \Omega

So, the LDR resistance during the day is approximately 499 Ω.

Advantages of the Circuit

Automatic Operation: The LED turns on and off automatically based on the light intensity, requiring no manual intervention.
Low Power Consumption: Uses a single 3.7V battery, making it energy-efficient.
Simple and Inexpensive: Requires only a few commonly available components.

Applications

Night Lamps: Automatically turn on a lamp when it gets dark.
Outdoor Lighting: Use in solar-powered lights for gardens or pathways.
Security Systems: Indicate low-light conditions in certain areas.

For more projects and detailed designs involving light sensors, check out our Light and Darkness Sensor Circuit with LDR for a deeper understanding of how LDRs work in detecting light.

By following this guide, you can easily build an Automatic Night Light Detector circuit that serves as a practical and educational project. It demonstrates the basics of transistors, voltage dividers, and the functionality of an LDR in a simple yet effective application.

If you're interested in learning more about how LDRs are used in other electronic projects, be sure to explore our post on Light Dependent Resistor (LDR) and Its Applications. Additionally, for more advanced applications like alarm systems, our Laser Diode LDR-Based Alarm System could be of interest.