Blink LEDs with Discrete Parts

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

Comments

[yoyology]

Please forgive an elementary question. What does "SL1" in your schematic represent? I've done some searching, and I don't find "SL" in lists of standard abbreviations such as http://en.wikipedia.org/wiki/Electronic_symbol#Reference_designations.

[pfred2]

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.

[throbscottle]

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

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.

[pfred2]

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!

[schumi23]

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.

Great instructable! I'm probably going to make it!

[schumi23]

Lol, that was intended to be funny, and not meant to elicit an actual response... woops :)

[pfred2]

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.

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.

Make one of these circuits. There isn't much to it really.

[acmefixer]

I think it needs to be stated what the operating voltage range is.  The minimum voltage is a few volts above the LED forward voltage, which is 2V for red and 3.2 volts for blue or white.  I'm not sure if it will work reliably below 5 volts.  I think it would be better at 6 volts.  But I've got them to run better by putting a 10k across each of the LEDs.  Even if it runs reliably at low voltages, there can be a problem with batteries if the voltage changes with the battery's condition.  This can cause a fairly noticeable change in the flash rate.

[pfred2]

I didn't say I ran mine at 9 volts in my article? I could have sworn I did. Ah yes here it is,

"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."

Mine always flashed OK until the LEDs wouldn't even light up anymore. Mine flash at a very high rate though. I used mine as a party favor toy of sorts.

[acmefixer]

I was talking about minimum voltage. One thing I forgot to mention is that there is a negative voltage from base to emitter when the transistor switches off. . It is very important that this does not exceed the maximum rated E to B voltage, which is 5 volts for most silicon transistors. If you use a supply voltage above 5 volts, there should be a diode in series with the base to protect it from excessive voltage. Some designs put a diode across the emitter to base, cathode to base. But this messes with the timing.

[pfred2]

If timing accuracy is critical to you then this probably isn't the best circuit. All this one does is flash LEDs. This is a simple configuration of the circuit. I'm sure any number of modifications can, and have been done on it by many. It is somewhere for someone to start. Enjoy it.

[acmefixer]

I'll explain why it's important. If the voltage from emitter to base goes above 5 volts, the emitter to base junction will break down like a zener diode.  This causes immediate and permanent damage to the transistor.  The current gain drops dramatically, and the transistor can't amplify, and the circuit no longer works.  The transistor then has to be replaced.

I would say this is important, because this circuit will not blink if the transistors are not working.

[pfred2]

I made one of these in 1983 and it still works today. You were saying?

[acmefixer]

You're lucky. I checked the circuit and found that the LEDs subtract 2V (for red LEDs) or 3V (for blue or white) from the 9V. If you made it that long ago your LEDs are most likely red or orange. The maximum voltage across the emitter to base junctions is 9V minus 2V or about 7 volts. This is still 40 percent higher than the maximum 5V allowed by most transistor specifications.

If you look at the actual emitter to base breakdown voltage, all of the transistors are tested and must pass 5V. As the voltage rises above 5V, at first a small percentage, and then at higher voltages a larger and larger percentage of the transistors start to break down. I would guess all break down somewhere in the 6 to 9V range. What happened was you played Russian Roulette with the transistors you chose and were lucky you hit empty cylinders - the transistors had a breakdown voltage of more than 7V.

This risky design method might work for home projects, but if it happened on a production line, there would be some equipment that would fail when tested and would have to be sent back for repair.

[pfred2]

I'm charmed. As are everyone else who uses this circuit. Contact them to find out why this works so well. All I know is it does.

[The Green Gentleman]

I totally concur on the subject of the importance of LEDs. However, I am confused about the statement "R1 and R2 are current limiting resistors to protect the LEDs." 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.

[pfred2]

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.

[rimar2000]

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.