# Simplest LED flasher



## Otaku (Dec 3, 2004)

Hi all,
I ran across this while looking up some op amp circuits:

http://www.cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.html

I don't think you get to adjust the rate, though. I'm gonna build one tomorrow and test different voltages as 12VDC is not really portable.


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## halstaff (Nov 18, 2009)

I'm looking forward to seeing the results as that's as easy as it gets.


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## Otaku (Dec 3, 2004)

Yeah, I'm going to play with the resistor values and see if I can get it to run on 6V or 9V. More to come.


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## pshort (May 6, 2008)

Another possibility might be to use a PUJT, such as the 2N6028. Not as common a part as the 2N2222, though.

P.S. According to the article, you can change the flash rate by changing the value of the cap.


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## Otaku (Dec 3, 2004)

I spotted that, too. I was thinking more about changing the rate on the fly.


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## TheOneAndOnlyKelly (May 7, 2009)

I _think_ you could also change the rate by swapping out the resistor with a potentometer, since a "pot" is just a variable resistor...


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## Otaku (Dec 3, 2004)

OK, I built one of these with the following alterations - I used a 220 uF cap (couldn't find a 330) and as MacabreRob suggested, I spliced in a 10K pot in front of the 1K resistor. I get an adjustable flash rate from 2X a second to once every 2 secs. Still need to use 12VDC, though. I'm working on that part.


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## corey872 (Jan 10, 2010)

Interesting circuit and novel use of a transistor. Definitely interested to see how the experiments turn out.

Also thought I would pass this along:

http://media.digikey.com/pdf/Data Sheets/Fairchild PDFs/FAN5645.pdf

Your post sparked my memory about a flasher chip and I found this. I just had a second to scan it, but if I'm reading correctly it's a LED flasher which can be programmed for 3 on / 3 off events (of different durations if needed), on a single chip, for about a buck. It's sister chip the FAN5646 can do 'breathing' or 'throbbing' LED cycles.

This doesn't approach the novelty of your circuit, but depending on your definition of 'simple', the single programmable chip might be a contender...at least if you only consider parts count and not internal complexity!


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## HomeyDaClown (Oct 3, 2009)

Ah, the ol tunnel diode effect. Should not be hard to get it up on 12 volts.....here's one at 9 volts:










here's another at 35 volts:


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## Otaku (Dec 3, 2004)

Hey Homey. The one I built won't flash on 9VDC, it needs 12 volts. All parts are the same as you show in the schematic except for the cap - I used a 220 uF. I'll try changing the cap and see if it will run on 9 volts.


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## HomeyDaClown (Oct 3, 2009)

Make sure the voltage rating on the cap is not too high, you want it to dump it's charge as soon as it reaches max.


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## hpropman (Jul 27, 2008)

why not just use a picaxe depending on which chip you choose you can hang a led off all of the chips outputs and have them flash or pattern any way you like. If you use high brightness leds then will need to add the transistor to the I/O pin to handle the current. It works on 5 volts and you can have multiple leds hang off each transistor. The possibilities are are almost endless with this setup. The chip is so easy to program and use my 11 year old scouts are having no trouble with it. Worst case I can write the program for you. If you building a circuit anyway why not use this or you can use one of the picaxe PC boards like VLC controller that Fritz wrote up for us.


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## Otaku (Dec 3, 2004)

Actually, I don't plan to use this flasher - I'm just playing with a new toy. Most people would find it easier to use blinking LEDs for this effect, although the circuit does let you adjust the rate. For high-power LEDs I typically use a DS3658 quad driver. The chip can sink 600mA per output, more than enough for plenty of LEDs.

Homey, I'll check the voltage on the 220uF cap - I think it's 35V. Thanks for the tip.


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## pshort (May 6, 2008)

Otaku - 

I think that this circuit relies on the transistor going into reverse collector-emitter breakdown v(br)eco. The battery voltage must be greater than the v(br)eco of the transistor, which evidently exceeds the 9V that you're looking for. There may be other transistors that have a lower reverse breakdown voltage, although that parameter isn't always present in the datasheets.

I've got some 2N6028 PUJT's that I will experiment with soon. The circuit would require two addition resistors for programming the part, but should work at a lower voltage. The biggest issue, though, is that most people don't have these in their spare parts bin, nor is it carried by Radio Shack.


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## pshort (May 6, 2008)

Incidently, I was looking at the PUJT as an alternative to this approach (which, in turn, was an alternative to using a micro-controller). All just for fun...


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## Otaku (Dec 3, 2004)

Cool. Man, there's a ton of stuff out there. Sometimes the Web is a curse - there's just too much that I want to do!


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## pshort (May 6, 2008)

FWIW, I've got a 2N6028 design working with a 9V battery, with a range of flashing similar to what Otaku noted with the 2N2222 circuit. The schematic is similar to the relaxation oscillator give here. Since I wanted a fairly bright flash, I had to change some of the values. I was using R=24KΩ, C=47μF, R1=1KΩ and R2=680Ω to lengthen the amount of time that the LED was on (cycle time of . With these values the cycle time was 400 ms (i.e. 2.5 flashes/second) with an LED on-time of 64 ms.

These values worked with the transistor that I was using and the temperature/voltage conditions at the time, but I haven't done a worst case analysis for these values. Also, I think that it would have worked better with a 2N6027 instead of the 2N6028, because I could have increased the values of R1 and R2 and thus decreased the power draw (which is significantly affected by the current through R1 and R2.


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## Otaku (Dec 3, 2004)

Hmmm...I gotta order some parts next week. I tried changing the cap to a 100uF 16V, but I still can't get the 2N2222 circuit to work with a 9 volt battery. It does give you a nice strobe effect with the lower value, though. Thanks!


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## pshort (May 6, 2008)

I added some 2N6027 parts to my Digikey order, and I'll see if they allow a lower current consumption than with the 2N6028.


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## Stinky Pete (May 2, 2011)

I've been looking for a simple flasher circuit to make embers look a little more lively. I found this circuit http://www.kpsec.freeuk.com/projects/random.htm that can drive 8 LED's with very few components. I tested it tonight and it works pretty well.


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## ouizul1 (May 22, 2010)

I like this little circuit, thanks for sharing it! And it's already setup for 12 vdc, that's perfect.

But I might argue a little for the power of 12 vdc. While it is certainly not as portable as 9 vdc, especially in weight and size of the battery, it does have it's advantages. 

I have one pop-up prop under construction that uses the following 12 vdc components:
- low voltage landscape lights
- pneumatic control valve
- pair of relays to control lights and fog machines
- several sets of LEDs (automotive accent lights, actually)

It all runs off one car battery, so it's all self-contained and can be placed just about anywhere in the yard. Even the fog machine is powered from the battery through a power converter.


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