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!