LED Flasher Circuit Using Transistors

I like this a dual-LED flasher circuit. Someone calls a blinking LED circuit. They have two LEDs flash alternately.

There are many ways to flash LEDs on and off using transistors. We may call it that Free Running Multivibrator.

It works like Flip Flop which triggers itself repeatedly. It also demonstrates a very important kind of digital logic circuit, the astable multivibrator.

There are 4 circuit ideas for you.

Dual LED flasher circuit using PNP transistor

To begin with, see the circuit below. There are ten components only. I think you do not need to know its principles very very deeply.

I just want to see you use it in an easy way.

Four important things! you should know

First, Both Q1 and Q2 are general PNP transistors such as 2N3906,2N2907, BC558, etc. They are like switches on-off to drive two LEDs.

Learn: How transistor circuits work

Second, R1 and R2 limit current to LEDs. If you use a 9V battery, you can use up to 1K, 3mm LEDs. In lower resistance will be high current and high lighting.

Third, R3 and R4 make a bias current to the base of transistors to conduct current. They control the gain of amplifiers.

And importantly, they increase the time required to charge capacitors and speed up the discharge of capacitors.

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Finally, the Flash rate is easily changed simply by changing one, or both, of the timing capacitors C1 and C2. Reducing them will fast the flash rate. When they have different values, one LED will stay on longer than the other.

How does a Dual LED flashing light circuit work?

This circuit is so easy, learning how it works is interesting. I will tell you the step of its working. Note: It may be incorrect in full academic principles. I hope you understand it easily.

First of all, suppose that Q1 works first. It is a PNP transistor that works as a switch. We need to feed a polarity voltage. It will work only on the negative voltage.

You look at the circuit diagram below.

LED1 turns on

• First, the positive current from B1-battery comes to the circuit. It is flowing to LED1 through R1 to the ground. So, LED1 grows up.

• Second, At the same time, some current also flows to charged C1 through R4 to ground.
• Third, since the voltage across R4 or Base of Q2 is positive. So Q2 does not work, LED2 goes out.

The current comes into C1 slowly until full.

After that, you look at the block diagram below.

LED 2 turns on

• First, no current flow through C1. It looks like an opened bridge.
• Second, a negative current from the ground comes to the base of Q2 through R4.
• Third, it makes the Q2 works. So, the current flows to R1, LED to ground. Now, LED2 grows up.
• Fourth, at the same time, some current charges to C2 through R3 slowly. No negative current flows to bias Q1. So it does not work, LED1 goes out.

The next step is in Figure below.

• First, at the same C2 is fully charged. So no current through it.
• Second, the negative voltage flows to the base of Q1.
• Third,  so Q1 works again. Then, the current flows to LED1 through the collector and emitter, R1 to ground. The LED1 grows up. But LED2 goes out. Why?
• Fourth, since now C1 is discharged to run out. It works like a closed bridge. Then, C2 passes some positive current to R4 instead of the base of Q2.   Thus, Q2 does not work, no current through R2 to LED2. LED2 goes out.

This process will work in the loop always follow the principles mentioned above.

Hope friends have fun LED Flasher. This circuit has plenty of fun and practical application.

This circuit is working well on a breadboard. below.

How to build it

PCB layout

My friend wants a PCB layout of this circuit. So I designed the PCB layout for him and you. It is so easy to builds as Figure below.

The components layout of Dual LED Flasher using BC556 or BC557 or BC558

And An actual-size of Single-sided Copper PCB layout.

Credit circuit by Forrest M. Mims III. He is a great Hero(mine) in the electronics world. This circuit is in The getting started in Electronics. I try teaching my children with his books. His book is translated into many languages including mine too. Thanks very much.

Last Update

Peter Notebaert shared: The polarity of C1 and C2 should be reversed. My daughter has retested It has the same effect. Both circuits have the same effect.

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Read Also: 1.5V LED Flasher circuit using transistors

Two LED Flasher Circuit using NPN transistors

We like two LED Flasher circuits using NPN transistors. There are many circuits here is one of the blinking circuits. We call it Astable Multivibrator. Often we know it 2 LED Flasher circuit, in this we use a PNP transistor.

But now we use 2 NPN transistors. We normally see it in a lot of circuits as oscillators. Both transistors work alternately like the automatic switches that are alternate working. The collector of Q1 or Q2 makes the frequency signal.

Operation of the circuit
To begin with, suppose that the Q1 run first. Then LED1 should grow more brightly.

It makes the current to flow through LED1, R1, and the collector-emitter of Q1. At the same time, some current will flow through LED2, R4, and to charge into C2 until full. The voltage BE is lower than 0.7V. So Q1 turns off, LED1 goes out.

Next Q2 start working, LED2 also grows more brightly. The C1 begins to charge until full, VBE of Q2 is lower than 0.7V. It turns off, LED2 goes out.

After that Q1 will turn on and LED1 to grow up again. the circuit working will repeat loop in like this.

The bicycle tail light for my son.
The principle of this circuit, I built the bicycle tail light flashing for my son. At first, he was very appreciated. But he embarrassed every time, friends to tease. Because it’s funny.

I used a hot food box because preventing water heat resistant and very economical. And more importantly, it increases the safety of the children. When riding a bicycle in the evening.

3V Dual LED flasher circuit

If you want to use this circuit with a 3V battery or a 3.7V cell phone battery. It’s almost like the circuits above. You can use these circuits below.

PNP transistor version

In this case, we use a white LED that requires about a 3V supply. So, it does not need to use the limited current resistor for LEDs.

Next, If you have an NPN transistor. It also can be made the 3V two LED Flasher circuit. Both circuits have a few parts. It may be ideas for showing the light of a bicycle or others. Sure, it uses a long battery lasts.

We can assemble all components on a breadboard. We use BC548 transistors and do not need a Resistor limit current to LEDs.

In the circuit below.

and He tests it as the video below.

10 LED flasher using multivibrator transistor

This 10 LED flasher is a Kind of flashing light circuit which is ideal for use as string light that looks so beautiful. And To add to the atmosphere at night. Or other parties as well. Because it is a circuit that is cheap. The construction is very simple.

You can also adjust the speed of the flashes as well. For flashing it will flash on and off. Alternately each 5 LEDs continuously.

Working of LED flasher multivibrator

The circuit is a low-frequency oscillator circuit. Which we call that a multivibrator, it uses dual transistors to work alternating. Each transistor will connect current to one LED set. They have 5 LEDs for 2 sets or include are 10 LEDs.

The potentiometer VR = 10K is the speed flashing controller. Which can be slow for the first time about one second. Until quite fast.

The creation and use

This LED flasher beauty is where to place lamps LED. Here we set the LED alternately in a circle. and Switch bulbs use colors such as green and red. This causes the selected color. Because it is easier to buy colors and bright colors than others.

Assembly, such devices should be R, C, and TR, before, so gradually put the LED. And be careful with the polarity LED. The long leg is the positive leg to turn off the PCB layout.

This circuit is normally set to use the power supply 9V. It can even be used to run low power down to 3V. The brightness is reduced only slightly. The PCB layout can be seen in Figure 2.

Components list

Q1,Q2: C1815 or C828, 45V 100mA NPN Transistor
C1,C2: 33uF 16V, Electrolytic capacitors
R1,R2: 330 ohms 1/4W 5% resistors
R3,R4: 3.3K 1/4W 5% resistors
LED1-LED10: LED as you need.
VR1: 10K, potentiometer

Super 12V Light Flasher Circuit

This is a Super Two LED Flasher circuit. It has high power out to load (12V light bulb), with source voltage from a 12V battery or DC supply, at current 2A up.

We use an Astable Multivibrator with transistors and other parts. And two transistors-2SC1061 or TIP41 for drive load up to 10 watts.

This circuit is easy and cheap. We hope you like it.

How it works

There are a lot of ways to flash dual LEDs ON and OFF. The two transistors are cross-coupled in such a way that the circuit switches back and forth between two states using transistors.

This circuit is one of the most stable of LED flashers. The values in this circuit, two LEDs will alternately flash ON and OFF about once per second.

The Q1, Q2, R1, R2, R3, R4, C1, C2 are a Free running astable multivibrator or Oscillator Generator circuit in a square waveform.

The Flash rate or speed is easily changed simply by changing one, or both of the timing capacitors (C1 and C2).

The Q3, Q4 are power transistors 2SC1061 or TIP31, to drive high up current to dual LEDs includes a series resistor(R5, R6) to limit current.

If we want to use load is the 12V light bulb. We remove R5, R6, and the dual LEDs to use multiple LED light by the circuit.
But you need to use 12V at a current minimum of approximately 0.5A.

For other details see the circuit.

How to build this circuit

Parts will you need

Q1, Q2: BC549, 45V 100mA NPN Transistor
Q3, Q4: TIP41 or 2SC1061, 40V 4A NPN Transistor
0.25W Resistors, tolerance: 5%
R1, R4: 10K
R2, R3: 47K
R5, R6: 470
C1, C2: 10µF 25V Electrolytic Capacitors
LED1, LED2, As you like

How to builds
This circuit is small, we can solder them on the perforated PC board to save money and time. But you can use the PCB layout below for a good project.

Actual size of Single-sided Copper PCB layout

Component layout for the PCB

Download This Post as a PDF and all PCB layouts

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