Introduction: Two Transistor LED Flasher Circuit

Picture of Two Transistor LED Flasher Circuit

How to make a circuit to make two LEDs flash back and forth.  This is also called a free running multivibrator.

Step 1: The Parts

Picture of The Parts

For this project you'll need the following:

-PNP transistor x2 Q1, Q2
-4.7-22 uf capacitor x2 C1, C2
-470 ohm resistor x2 R1, R2
-100k ohm resistor x2 R3, R4
-5mm LED (any color) x2 LED1, LED2
-Solid core wire, 22 gauge
-Prototyping Breadboard
-4 AA holder x1

Step 2: Insert the Transistors

Picture of Insert the Transistors

The transistors leads were to close together, so using needle nose pliers I bent them to fit in the breadboard.  Insert the transistors two rows away from each other.  Insert it so the rounded side is facing outwards and the side with the letters is facing towards the breadboard. 

Step 3: Circuit Diagram

Picture of Circuit Diagram

Step 4: Assemble the Circuit

Picture of Assemble the Circuit

Step 5: Troubleshooting

Picture of Troubleshooting

If the circuit fails to operate:

-Check if the transistor is in backwards
-Make sure that they are both PNP transistors
-Make sure that the LEDs and capacitors are not in backwards
-Be sure that none of the bare leads are coming in contact with one another


Cannonball (author)2017-04-13

In your schematic the caps are backwards. A charged cap in series with the power supply is the only way to reverse the bias on these transistors. When the capacitor discharges the transistor will turn on and place the charged capacitor in series with the power supply turning off the other transistor.

GianniM (author)2015-03-13

It works because two transistors are not identical, due to imperfections and manufacturing issues, thus one of them starts conducting current befoer the other, and the cycle begins.

botdemy (author)GianniM2016-12-22

That explains why I cannot get this to work in :) In, transistors are perfectly identical so they both conduct at the same time.

AtulV8 (author)2016-06-17

Made it! But LEDs are blinking too much fast.

TylerH66 (author)2016-05-18

Are these really suppose to be PNP transistors? All the other tutorials I've seen use NPN (

hassaank98 (author)2016-02-08

you rocked dude

SHOE0007 (author)2015-11-26

I did this and with the proper functioning transistors I got it to work. Then I replaced the diodes with a speaker and replaced the values for C1 to 4.7 uf and C2 to 10 uf.

This made a simple buzzer. I am thinking of adding a 555 timer to it to adjust the buzzer.

90alx (author)2014-11-13

Can you explain me please why the LED's are flashing one each at a time and not both of them in the same time ?I don't understand why the capacitor are not charging and discharging in the same time.

marcell.veszpremi (author)2014-08-27

both of my leds are on, what should I do?

tqnata_1 (author)2011-06-28

[sory for the english] i want to know these things because in the electonics shop want this parametres PNP transistor -model and evretyng for them
-4.7-22 uf capacitor-V=?
-470 ohm resistor -W=?they works
-100k ohm resistor -W=?they works

sghosh21 (author)tqnata_12014-04-15

check the images

Adambowker98 (author)2012-10-21

How fast do they flash? Like how long are they on before it switches?

That depends on how large the capacitors are. The larger the capacitor, the slower they switch.

lukeD (author)2011-06-23

that's cool!!!

mania123456 (author)2011-04-20

I want to make sure are the values of the two capacitors are 22 microFarad and 4.7 microFarad or not? Also these are npn transistors right or pnp?
Please reply soon and I really appreciate your efforts and creating and uploading such helpful ideas.

metsfan1938 (author)mania1234562011-05-04

what i meant is the values of the capacitors can be any where from 4.7 to 22. they can probably be any value but that is what i experimented with. the transistors are pnp.

mania123456 (author)2011-04-22

I connected everything perfectly but as soon as I am connectingthe circuit to power supply, both the LEDs are ltting up at once and not blinking at all. I have rewired everything gazillions of times but I thin that there is some major thing that I am not able to see. I am in desperate need of help.

robot1398 (author)2011-02-15

which transistor did you use @metsfan1938

mcmahanly (author)2011-01-18

Is anybody else having trouble with this? I used (2) 2n3906 transistors and said caps and resistors. Only one led lights up. No shorts. I've rewired it numerous times. Same prob. Could it be my capacitors? Im using (2) 22uf capacitors rated for up to 50 volts...not sure if my lingo is correct here. I also tried a similar circuit from Forrest Mims... no dice! Somebody pleas help!

mcmahanly (author)mcmahanly2011-01-21

Problem solved: my breadboard was faulty! I had put this circuit together with multiple sets of new components. I even bought new batteries and a charger. It didn't work until the whole circuit was moved to one side on one section with the battery leads close to the emitters/LEDs/resistors. or maybe there is something i'm missing... i need a class and teacher.

godofal (author)2010-11-02

would it be possible to expand this to create a knight rider circuit??

Wyle_E (author)godofal2010-11-03

This simple circuit can only flash one or two LEDs. The Knight Rider effect can be done with a few inexpensive integrated circuits. The old Radio Shack Engineer's Handbook had one version, but that book is long out of print.

IIRC, it used a 1-to-16 demultiplexer chip driven by a 4-bit up-down counter. A 555 timer served as the counter's clock, and a flipflop switched the counter's direction pin between 'up' and 'down' when the LED at either end of the row lit. The 16 LEDs probably cost more than the three integrated circuits.

godofal (author)Wyle_E2010-11-03

i know those circuits, but i was just curious if this was possible
nice instructable though, might make a nice use as a fake car alarm xD

Wyle_E (author)godofal2010-11-05

I just remembered an old class of circuits called "ring counters". A ring counter is sort of like a multivibrator with multiple stages, with the last stage connected around to the trigger input of the first stage. Triggering one stage starts a short delay, after which that stage shuts off, triggering the next stage. They can be implemented with transistors or silicon controlled rectifiers. I haven't seen one in decades, because it's so much easier and cheaper to use the counter-decoder method. If you're curious, Google could probably find some schematics.

scraptopower (author)2010-11-02

Do you have a circuit diagram of this ? That would be really helpful, thanks.

metsfan1938 (author)scraptopower2010-11-02

Posted a diagram:)

Sandisk1duo (author)scraptopower2010-11-02

i agree

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