Tutorial 8: Dual LED Flasher

Created on: 2 August 2012
Updated on: 14 January 2023

A dual LED flasher circuit that uses two transistors to flash or blink two LEDs on and off alternately. This tutorial shows beginners in electronics how to build an easy transistor circuit on an electronic breadboard.

The video below shows what the dual LED flasher circuit does. Two LEDs blink on and off alternately.

Prerequisites

Complete tutorial two - Transistor Timer before attempting this tutorial.

Dual LED Flasher Circuit Components

In addition to an electronic breadboard, wire links, a 9V battery and a battery clip, you will need:

Qty Part Designator Notes Type
2 22k resistors (red - red - orange) R2, R3 1/4W, 5% or better Resistors
2 470 ohm resistors (yellow - violet - brown) R1, R4
2 100uF C1, C2 Electrolytic capacitors, 16V or more

Try using 47uF or 33uF capacitors to speed up the rate at which the LEDs flash
Capacitor
2 KSP2222 or PN2222 Q1, Q2 NPN transistors

2N2222, BC107, BC108 or BC109 transistors can be used in the circuit, but have a different case to the KSP2222 / 2N2222
Semiconductor
2 5mm red LED D1, D2 Or similar LED

The required parts for the two transistor LED flasher circuit are shown below:

Components for the dual LED flasher circuit
Two LED Flasher Circuit Components

Dual LED Flasher Circuit Diagram

The dual LED flasher circuit diagram, or two LED flasher circuit diagram is shown below. As can be seen in the circuit, it uses two NPN transistors, two LEDs, four resistors and two capacitors.

Dual LED flasher circuit diagram
Two LED Flasher Circuit Diagram

Build the Dual LED Flasher Circuit

Follow the instructions below to build the dual LED flasher circuit on an electronic breadboard.

1. Insert the Transistors into the Breadboard

To start building the dual LED flasher circuit, first insert the two transistors into the breadboard with the flat face pointing down as shown in the photo below. These can be PN2222, KSP2222, plastic 2N2222 or similar transistors. Use wire links to connect the emitter of each transistor to the bottom breadboard rail. Get the two LEDs and the two 470 ohm resistors ready.

Step1: insert transistors and wire the emitters on the breadboard
Step 1: Insert the Transistors and Wire the Emitters

2. Connect the LEDs and 470Ω Resistors

Connect one end of a 470 ohm resistor to the collector of each transistor. Connect the other end of the resistor to a tie point on the other side of the breadboard middle insulating channel, as shown in the photo below.

Connect an LED to each resistor as shown below. To do this, connect the anode of each LED to the top rail of the breadboard. Connect the cathode of each LED to its corresponding 470 ohm resistor.

Get the two 22k ohm resistors and two wire links ready.

Step2: connect LEDs and resistors to breadboard circuit
Step 2: Insert LEDs and 470Ω Resistors

3. Connect the two 22k Resistors

Connect the two 22k resistors across the middle channel of the breadboard. Connect the top lead of each 22k resistor to the top rail of the breadboard using a wire link, as shown in the photo below.

Get the two 100uF capacitors ready.

Step3: wire 22k resistors to breadboard top rail
Step 3: Insert the 22kΩ Resistors and Wire to Top Rail

4. Connect the two 100uF Capacitors

Insert the capacitors with the positive lead of each capacitor on the collector of its corresponding transistor. Connect the negative lead of each capacitor to its corresponding 22k resistor.

Get two wire links ready.

Step4: connect the capacitors to the breadboard circuit
Step 4: Connect the Capacitors

5. Cross Wire the Transistor Bases

Use wire links to connect the base (middle pin) of each transistor to the negative lead of each opposite capacitor. The completed circuit is shown in the image below.

Power Up the Dual LED Flasher Circuit

Power up the dual LED flasher circuit using a 9V battery. Connect the black negative lead of the 9V battery clip to the bottom horizontal breadboard rail. Finally connect the red positive lead of the 9V battery clip to the top horizontal breadboard rail.

The following photo shows the completed dual LED flasher circuit. If the circuit was built correctly, the LEDs will start flashing on and off alternately after the battery is connected.

Step5: connect the battery to the dual LED flasher breadboard circuit
Connect the Battery to the Dual LED Flasher Circuit