Many electronics circuits and projects for learning in simple ways.

Dual LED Flasher circuit using PNP transistors

I like this dual LED flasher circuit. 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 that triggers itself repeatedly. It also demonstrates a very important kind of digital logic circuit, the astable multivibrator.

To begin with, see in the circuit below. There are ten components only. I think you do not need to know its principles very very deep. I just want to see you use it in an easy way.

Dual LED Flasher using 2N2907

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.

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

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

And important, they increase the time required to charge capacitors and speed up the discharged of capacitors.

Final, 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 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 incorrect in full academic principles. I hope you understand it easily.

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

PNP transistor working looks like negative switches

You look at the circuit diagram below.

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

dual LED flasher working in first step

  • 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 a block diagram below.

Dual LED flasher working in second step

  • 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.

Next step in Figure below.

dual LED flasher working in third

  • 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.

Keep reading: ‘Two LED Flasher using Gate of IC 4011’ »

This circuit is working well on breadboard. below..

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 the good Hero(mine) in the electronics world.
His New project: Dual LED flasher

3V Dual LED flasher using PNP transistors

If you want to use this circuit with 3V battery or 3.7V cell phone battery. You can use this circuit below.

3V dual LED flasher using php transistor

It does not need to use the limited current resistor for LEDs.

Keep reading: ‘Blinking two LEDs using transistor BC548’ »

This Post Has 13 Comments

  1. thanks.i see your circuit

  2. Hi, farid.
    Thanks your comment.

  3. Great Circuits can Be Found here,,MANY THANKS FROM MR.OHM 1970!!!

  4. Hi, Mr.OHM 1970
    Many Thanks too,

  5. Bence 2N2907 li devrenin kondansatörleri ters bağlanmış.

  6. please sir, show the circuit for 24 led dancing light with bc 548 transistors…. am fed up searching for it. please help me sir…

  7. Hello sir. Can i use the 3 volt dc toy motor instead of the led ? I can use it for robot snake. pl reply me on my mail.

  8. Do you have a schematic for an AM Transmitter. Your web site is #1. Thank You for all your schematics’ You are #1 for all your information. JON.

  9. Can you make a circuit to get a 2 output LED bar / strip 12 volts of the following component list
    R 1 K ohm = 1
    R 10 K ohm =1
    R 330 ohm =2
    capasitor 10 nF =1
    capasitor 100 nF=1
    capasitor electrolit 10 uF/25v = 1
    capasitor electrolit 100uF/25v =1
    diode 1N4148 = 4
    transistor BC547 =1
    VR 10 K=1
    IC NE555=1
    IC CD4017 =1
    LED bar/strip =2

    help me please

  10. Please sir,can you explan the circuit of 2led flash using transistor which transistor r using is that pnp or npn can u please explan

  11. Your caps are in backwards.

    It takes a + voltage to turn a pnp transistor off.

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