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By: Anonymous: xDGx () on Wednesday, March 02 2011 @ 03:15 AM PST (Read 7988 times)  
Anonymous: xDGx

Hello,
I've made an LED pulse using a 555. It's a very popular circuit, specially because it's the same effect macbooks have. It uses a cap, a transistor, and a pot to control the speed of the pulse.
Here's the tutorial for those interested: http://www.instructables.com/id/ThrobbingFading-LED-with-555-Timer/step2/Breadboard-it/
I'm using a 5k pot, tip41c npn transistor and a 220uF cap.
And then it works with one LED, but it doesn't work if I plug 15 LEDs in parallel. I think 15 LED's too much current for the 555 to handle, but there's the transistor feeding the led and my power source has 1200mAh so it should work; and then I got all confused... What am I doing wrong ? do i need another transistor to light more LEDs using this circuit ? Thanks





       
   
By: Windell (offline) on Wednesday, March 02 2011 @ 04:07 PM PST  
Windell

I didn't find a clear circuit diagram of what's in that circuit. But driving 15 LEDs in parallel can take a lot of current, so your result is not surprising... depending on how it's wired. If you can show me how it's wired up, I might be able to make some suggestions.


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By: Anonymous: xDGx () on Wednesday, March 02 2011 @ 11:57 PM PST  
Anonymous: xDGx

Here it is, breadboard view and schematic by fritzing. Sorry about the messy autoroute schematic, I'm still learning eagle/proteus...




I thought the npn would "use current" from the power supply and not the 555's..? Does that even make sense ? lol
The 555 can sink/source 200ma but my power supply has 1200mah, so I thought the NPN would get the signal from 555's pin 3 and 'switch' the power supply.. but er. lost again. it didnt work so i added another npn to the emmiter. that didnt work either
Thanks for all the advice Windell Smile





       
   
By: Windell (offline) on Thursday, March 03 2011 @ 12:16 AM PST  
Windell

>I thought the npn would "use current" from the power supply and not the 555's..?

Looks like it will. One problem may be the resistor in series with the LED-- the value there is chosen to *limit the total current.* It may also be better to put the LED on the high side of the transistor, right where it says "Q1", rather than on the "low" side where it is.


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By: Anonymous: xDGx () on Thursday, March 03 2011 @ 08:40 AM PST  
Anonymous: xDGx

It may also be better to put the LED on the high side of the transistor, right where it says "Q1", rather than on the "low" side where it is.



That would be the transistor's second pin instead of the 3rd pin where it is now ? and then let the 3rd pin empty ?





       
   
By: Windell (offline) on Thursday, March 03 2011 @ 10:27 AM PST  
Windell

Leave the transistor exactly where it is. Replace the resistor+LED with a wire. Replace the wire segment labeled Q1 with the resistor+LED. Should still work.

Then, figure out what the right resistor configuration should be for driving lots of LEDs in parallel.


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