# Super Simple LED Strobe / Flasher



## corey872 (Jan 10, 2010)

Thought I would throw this up in case anyone needs a last minute project to add a little 'flash' to a dark corner of the haunt. This is essentially the simplest LED flasher you can have - barring of course, an integrated flashing LED. To give full credit, I originally found this circuit at :

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

It wasn't exactly what I was looking for, but in throwing it together quickly, I realized it could be a pretty neat LED strobe circuit. I put together a schematic of the values I used. Though with the simplicity comes some tinkering. It uses a transistor in a somewhat unconventional 'avalanche' mode which means the transistor is actually in the circuit backwards. The capacitor is charged through the resistor and when the voltage reaches some point, the transistor breaks down and allows power to flow through the LEDs until the capacitor is discharged. The transistor resets and the cycle repeats.

So what you actually need to build the circuit - minimum:

~12VDC power supply
~1Kohm resistor, 1/4 watt or higher
2N2222 transistor (or equivalent)
~470uF electrolytic capacitor (16V or higher)
20mA LEDs

(I used the approximately '~' a lot because this circuit is really tolerant of the components used...just change the values to get the effect you want)










Some key notes:

- A smaller R1 or higher voltage will charge the capacitor faster - hence faster repeating flashes.
- A resistor added to the ground of the LEDs limits current, but also slows the discharge of the capacitor making a longer 'on' cycle. For 'strobe' effect, I did not use any current limiting resistors.
- Power supply voltage needs to be at least 9+ volts to generate the 'avalanche' effect, and makes a pretty nice 'strobe' effect from 12-16V...maybe even higher if your capacitor/LEDs can take it.
- I ran up to four 2.2V / 20mA LEDs in this configuration with no current limiting resistors. Since the pulse is such a short transient, I'm sure the current spikes up (likely well over the 20mA rating) - so start with conservative values and work up. Personally, I don't mind going a bit over the rated current, especially flashing the LED. They can generally handle it when the duty cycle is short, plus even if it shortens the rated life from 100,000 hours, to 5,000 hours, that is still enough to last all the Halloween's for the rest of my life.
- A larger capacitor would likely support more / higher current LEDs.
- I didn't really take any current measurements, though I used my trained eyeball to gauge how bright the LEDs were at the rated 20mA, then crank the circuit up till they seemed to be that brightness or a little brighter.
- I ran off a 12V power supply for testing, but this could probably run for hours on a pair of 9V batteries hooked in series. (18V into the 1K resistor is 18mA. 9V batteries can easily do 300-400+mAh in capacity, so 300/18 = 16hours, 400/18 = 22 hours - ie several nights of flash when used conservatively)
- You could also likely run multiple 'strobes' off one power supply - handy if you want light coming from a couple sources or to throw some contrasting colors from a couple different angles.

Anyway, try it, tweak it, make it act like you want. This isn't going to be a blazing light at a dance party, but it's a very simple way to throw some flashy accent at a skull or other prop.


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## pshort (May 6, 2008)

Nice idea.


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## pshort (May 6, 2008)

I wonder what would happen if you tweaked the circuit so that the 1K resistor is replaced with some sort of photo-sensor? You could have several copies of this circuit that are close enough together that each one could sense the light from others. The idea is that they would behave like fireflies - initially out of sync, but gradually coming into sync. There have been articles in hackaday.com over the years trying to come up with this sort of behavior, except that they use either microcontrollers or 555 circuits instead of a simple relaxation oscillator circuit like this one.


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## corey872 (Jan 10, 2010)

I suppose anything replacing the resistor which lowered it's resistance with incident light would cause the circuit to charge the capacitor sooner and make the flash sooner.

With a bit of tweaking, you can get the flash rate down to one 'blip' every 3-4 seconds...probably even slower than that. So even a slight lowering of resistance would make the time interval a bit shorter. So if you had several 'identical' circuits, they might tend to synch up. Though in the real world, there are enough differences they may not ie- 5% resistors, 10 or 20% capacitors, the exact point each transistor avalanches at, etc.


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