As everyone surely knows the entire foundation of modern technology, nay the very bedrock of our advanced civilization rests on the humble blinking LED. Nothing says high tech quite like a flashing LED. Now I know they make LEDs that blink all on their own. But what if you don't quite like how they blink?

Also what about all of the blink challenged LEDs we already have in our possession? What is a poor person to do? Should we invest in integrated circuitry in order to upgrade our plain LEDs to blinking LEDs? Do we really need the power of an advanced microprocessor just to get a light to flash? I say no!

In this article I would like to present a classic circuit called an astable multivibrator, also known as a free running multivibrator. Then with a pair of transistors, a pair of capacitors, and a few resistors you can make your LEDs blink.

## Step 1: The Schematic

Other than the parts this is all anyone really needs in order to make an electronic circuit so lets just have at it shall we?

I guess I forgot to mention this circuit blinks two LEDs at once! Although my favorite way of building it is to blink one bi-color LED. I use 3 legged ones. If your LED is a common anode you can even change the transistors to make this circuit work with those. But I'll leave how to do that as an exercise for the reader.

In order to change the rate your LEDs blink change the values of C1 and C2 and R2 and R3.

R1 and R4 are current limiting resistors to protect the LEDs. I built mine so it runs off a 9 volt battery. If you supply different voltage, or use different LEDs you may have to calculate new values for those resistors. I usually run LEDs at about 12ma but some LEDs today can handle much more current than that. Maybe even more than a small transistor can supply, so be careful not going beyond that.

Looking at the data sheet it appears a 2N3906 can handle 200ma. I don't think I'd want to try to touch it if it was doing that though. I can't believe those little buggers can handle a half a Watt! Transistors are great aren't they?

http://www.fairchildsemi.com/ds/2N/2N3906.pdf

## Step 2: The Build

I usually build these as hanging gardens. Although I have built this circuit on perfboard too. In the attached image is one I made using the hanging garden method.

## Step 3: What Are They Good For?

I don't know. Amaze your friends? I've used this as indicator lamps too. Here is one I made on perfboard and used as an indicator lamp on a power supply I made. This circuit can be made pretty compact.

I laid out a printed circuit board but I've never made one like it yet. But I'll include the image so you've some idea what that might be like.

Remember it just isn't high tech if it doesn't have flashing LEDs!
All hail the blinking LED! This is a fantastic guide. <br> <br>I remember my first introduction to electronics was a 555 timer IC and a little 20 page booklet from radio shack with all the things you could make with it... Including the blinking LED.
This is really cool, I never thought about hanging these on gardens, it is a nice way to decorate it. :)
Thanks.
So I have this circuit but I replaced one led with a piezo buzzer and took out the resistor ( for protecting the led ) It used to work but I got new batteries and only the led goes on... HELP!
And I made a bit different circuit but it is a multivibrator if you look up &quot;flip flop&quot; in the instructables search bar. Thanks <br>Clayton
Please forgive an elementary question. What does &quot;SL1&quot; in your schematic represent? I've done some searching, and I don't find &quot;SL&quot; in lists of standard abbreviations such as <a href="http://en.wikipedia.org/wiki/Electronic_symbol#Reference_designations" rel="nofollow">http://en.wikipedia.org/wiki/Electronic_symbol#Reference_designations</a>.
Power connector. The name is the default for the component in the software I used to draw the schematic. If there were two connectors in the circuit the next one would be named SL2. You can rename them, but I didn't. I think it stands for Single in Line.
These are fascinating (and confusing, but I am dyslexic) circuits. I like this wikipedia article for a breakdown of how they work: http://en.wikipedia.org/wiki/Multivibrator#Astable_multivibrator<br><br>Nice instructable. I don't agree with acmefixer about the Vbe though as the transistor junction itself will limit the voltage to around 0.7 volts, and the highest current it will see is given by nine volts, minus LED voltage and Vbe in series, divided by 560 ohms, and then only momentarily when one of the capacitors is fully discharged, since as it charges the voltage and hence the current will reduce.
I can't say as I agree with them either. This is a common and well known circuit designed by someone with far more experience and knowledge than every joker on this site combined possesses. That someone ain't me!
I would hope it isn't you, cause if that someone had more experience then everyone on this site... and your on this site... that would mean that overall everyone else in this site had a negative amount of knowledge and experience 8-0.<br><br>Great instructable! I'm probably going to make it!
Lol, that was intended to be funny, and not meant to elicit an actual response... woops :)
No it wasn't me. I don't know if anyone knows who developed this circuit originally. It might be a hold over from the days of vacuum tubes or something. It is an all time classic. What I do know is built it works, and has always worked well for me.<br><br>Sometimes I think this site does have an overall negative amount of knowledge and experience on it. Then I post articles on it like this one to try to abate that thought. There is a lot of pretty lame stuff here.<br><br>Make one of these circuits. There isn't much to it really.
I totally concur on the subject of the importance of LEDs. However, I am confused about the statement &quot;R1 and R2 are current limiting resistors to protect the LEDs.&quot; I would think that R2 and R3 were the ones protecting the LEDs. In a sense, I guess they all are protecting the circuit, but generally, I thought that one would put the resistor between the LED and the anode of the power supply. Granted, I'm kind of a noob on the electronics side of things.
You're right, I must have been looking at another schematic when I wrote my text. I will fix that now. Thank you for pointing it out.
Nice instructable, Fred. Electronics is my pending matter. I've done some very simple things, like a small power supply or similar things, but I dare not undertake projects more interesting.