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  #1  
Old 08-18-2009, 06:50 AM
Rudster816 Rudster816 is offline
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Default LED Pulsating\Dimming with 555 Timer

Ok, I have some experience with electronic circuits and for the life of me cant figure out how to get the effect I want. I want to make a pulsating LED. I want it to blink but at the end slowly fade, than come back on fully and fade again.

I have a circuit that blinks the LED at the speed I want, but cant figure out how to make it fade. Ive tried google and cant find anything.

The effect can be seen on a PC motherboard here:
http://www.youtube.com/watch?v=x97tVW8t1vs

The LED is near the bottom left corner of the heatsink.

My current circuit and graph looks like this, you can see what I want to do with the voltage with the little green line that I made in paint. Its supposed to be exponential decay if you cant tell



Ive tried playing with capacitors but They always just make it blink like normal. Im now pretty sure because the current isnt big enough to charge the cap and power the LED at the same time.

Any way I can get around this?

Thanks!
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  #2  
Old 08-18-2009, 06:56 AM
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Bill_Marsden Bill_Marsden is offline
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Fade In/Fade Out is easy enough, it was discussed in this thread.

http://forum.allaboutcircuits.com/showthread.php?t=2389

Another thread is closer to what you want, though it didn't use 555's. It was how to delay a LED on time after the input had gone low, very close to what you want.

http://forum.allaboutcircuits.com/sh...ad.php?t=26443

Have you seen this? It has quite few 555 projects and a bit more.

Bill's Index
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Old 08-18-2009, 08:39 AM
Rudster816 Rudster816 is offline
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Ok I think I got it right. The graph looks good but the program (Yenka, random I know) doesnt show the LED dimming.

Am I missing something important here or will this produce an effect close to what im trying to get

Blue is supply voltage, Red is the actual voltage the LED is getting.

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Old 08-18-2009, 09:37 AM
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Bill_Marsden Bill_Marsden is offline
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The problem with your concept is LED drop a relatively constant voltage (Vf). You're going to have to have some resistance in there so the capacitor can have more voltage than the LED. Read the second link I suggested, this was discussed in depth.

http://forum.allaboutcircuits.com/sh...ad.php?t=26443

Just connect these circuits to the output of the timer. During experiments the OP of that thread decided only one transistor was needed.

.................................................. ..... or
......

The first will have more resistance, the second will probably last longer for the same capacitor due to the larger voltage swing. I'd go with the second myself.
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Old 08-18-2009, 10:36 AM
Rudster816 Rudster816 is offline
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Thanks! I get a constant voltage on the LED but its pulsing exactly how I want it too. I guess thats because of the current

I got my parts list and im heading to "The Shack" AKA Raido Shack tommorow to see if they have everything I need.

555 Timer
5k Ω Resistor
100k Resistor
10μf Cap
20μf Cap
.01μf Cap
NPN Transistor
680 Ω Resistor(s)
Red LED(s)

I will probably also pick up different size Caps to play with the Pulse too. This circuit was a lot more complicated than I though it was going to be.
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Old 08-18-2009, 12:35 PM
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Bill_Marsden Bill_Marsden is offline
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Cool, check out my index, I'm one of the project guys around here (hence the E-book Developer). My LEDs, 555s, Flashers, and Light Chasers has a lot of other project ideas, and some of the recent queries about fading LEDs (you're not the only one) have me seriously thinking of updating it.

What part of the country are you, if I might ask?
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Old 08-18-2009, 03:10 PM
Rudster816 Rudster816 is offline
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Im from Eastern Washington. Why do you ask?

It wouldnt be a bad thing to add. I thought I could do this without help but after thinking about I had no idea how to properly use the capacitor to power the LED after the 555 shut off

Here is a video of the effect I created. Hopefully it looks just as good in real life as it does on 'paper'.

http://www.youtube.com/watch?v=-K4DdzACbIc
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Old 08-18-2009, 06:37 PM
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There are some good parts houses up that way, but basically I was just being nosy.

You could even eliminate the transistor, it acts as a capacitance multiplier. I'd keep it though, just talking theory.

Have you read this?

LEDs, 555s, Flashers, and Light Chasers

I've been known to use the constant voltage across LEDs as voltage references in some of my designs. They are almost as good as zener diodes, though can't take much current. If you look at the voltage across the resistor/LED you'll see the variation in voltage.
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Last edited by Bill_Marsden; 08-18-2009 at 06:56 PM.
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  #9  
Old 08-18-2009, 06:46 PM
Fenris Fenris is offline
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That looks familiar Nicely done there. I'm the 'other chap' Bill has helped out with the fade effect. I'm doing a 15 segment knight rider type circuit with 3 super bright LED's per segment with transistor and cap for fade. I tested Bills schematics out using a 555 timer on my breadboard. It works a treat you wont be disappointed! Checkout Bills index theres some great stuff in there as well as a dearth of knowledge.

regards

Fenris
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  #10  
Old 08-18-2009, 07:04 PM
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Bill_Marsden Bill_Marsden is offline
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Tired computing bit me again, I repeated myself.
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General info:
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