How to Build a Simple LED Night Light Circuit Using Just 5 Common Components

Building a simple LED night light circuit is an easy and fun electronics project. With just a few common components, you can make a useful night light that turns on automatically when it gets dark. Here's how to build an LED night light circuit in 9 simple steps.

What You'll Need

To build the LED night light circuit, you'll need the following components:

1x 5mm white LED - This provides the light for the night light. Choose a bright LED for maximum illumination.
1x 220 ohm resistor - This limits the current through the LED to prevent it from burning out.
1x photoresistor (LDR) - This light-dependent resistor senses ambient light levels. It has high resistance in the dark and low resistance in bright light.
1x BC547 NPN transistor - This acts as a switch to turn the LED on when it gets dark.
1x CR2032 3V lithium coin cell battery - This provides power for the circuit. CR2032 batteries are commonly used in key fobs, watches, and other small devices.

You'll also need some solid core hookup wire to connect the components. 22 AWG wire works well.

How the Circuit Works

The photoresistor and transistor form a simple switching circuit to turn the LED on and off based on ambient light levels.

Here's how it works:

When it's bright, the photoresistor has low resistance, which turns the transistor OFF. No current flows to the LED, so it's OFF.
In the dark, the photoresistor has high resistance. This turns the transistor ON, allowing current to flow through the LED, lighting it up.

So in daylight, the LED is off and doesn't waste battery power. At night, the LED automatically turns on to provide lighting.

The resistor protects the LED by limiting current through it.

Construction Steps

Follow these steps to build the circuit:

Step 1: Cut and strip wires

Cut three 2-3 inch lengths of hookup wire and strip about 1/4 inch of insulation off each end. These will be used to connect the components.

Step 2: Solder photoresistor leads

The photoresistor has no wire leads, just bare metal pads. Solder two of the wires onto the photoresistor pads to extend the leads.

Step 3: Solder LED and resistor

Solder the resistor and LED in series, connecting the resistor to the positive (longer) LED lead. Solder the third wire to the negative (shorter) LED lead.

Step 4: Solder transistor

Solder one photoresistor wire to the collector pin on the transistor. Solder the LED negative wire to the emitter pin. Leave the base pin for now.

Step 5: Solder battery holder

Solder the loose photoresistor wire to one battery terminal. Solder the loose resistor wire to the other terminal. Use the positive (+) and negative (-) marks on the holder to orient the wires correctly.

Step 6: Stick components on protoboard

Use a small piece of protoboard or perfboard to mount the components. Stick on the photoresistor, transistor, and battery holder. Leave wires loose.

Step 7: Connect loose wires

Connect the loose transistor base wire to one photoresistor terminal. Connect the other photoresistor terminal to the positive battery wire.

Step 8: Install battery

Install a CR2032 coin cell battery in the holder, orienting it positive (+) side up. Make sure the holder clamps down on the battery securely.

Step 9: Test circuit

The LED should turn on when you cover the photoresistor with your hand. Remove your hand and the LED turns off. Try it in a dark room or closet and it will light up. Your night light circuit is complete!

The LED will provide a soft night light that turns on automatically at night. For best performance, avoid aiming the photoresistor directly at the LED, or it may not turn off properly during the day.

You can tweak the light sensitivity by adjusting the photoresistor - LED distance, or substituting a lower/higher resistance photoresistor. Have fun experimenting! Let me know if you have any other questions.