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
GianniM5 months ago

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.

90alx9 months ago

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.

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

tqnata_14 years ago
[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

check the images

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.
lukeD4 years ago
that's cool!!!
mania1234564 years ago
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)  mania1234564 years ago
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.
mania1234564 years ago
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.
robot13984 years ago
which transistor did you use @metsfan1938
mcmahanly4 years ago
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!
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.
godofal4 years ago
would it be possible to expand this to create a knight rider circuit??
Wyle_E godofal4 years ago
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 Wyle_E4 years ago
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 godofal4 years ago
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.
Do you have a circuit diagram of this ? That would be really helpful, thanks.
metsfan1938 (author)  scraptopower4 years ago
Posted a diagram:)
i agree