LED Pulsating\Dimming with 555 Timer

Discussion in 'General Electronics Chat' started by Rudster816, Aug 18, 2009.

Aug 18, 2009
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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:

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!

2. Wendy Moderator

Mar 24, 2008
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Aug 18, 2009
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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.

4. Wendy Moderator

Mar 24, 2008
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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.

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.

Aug 18, 2009
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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.

6. Wendy Moderator

Mar 24, 2008
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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?

Aug 18, 2009
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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'.

8. Wendy Moderator

Mar 24, 2008
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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.

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.

Last edited: Aug 18, 2009
9. Fenris Active Member

Oct 21, 2007
288
2
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

10. Wendy Moderator

Mar 24, 2008
20,765
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Tired computing bit me again, I repeated myself.

11. Fenris Active Member

Oct 21, 2007
288
2
It's well worth repeating . The world needs more LED's!!

regards

Fenris

Aug 18, 2009
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How hot should the 555 get? Im use to dealing with hot chips (Ima major PC geek) but I didnt think it would get as hot at all being such a simple circuit.

Is it beacuse im using a 12v input? It says its rated for a maximum of a 18v input, and a max operating temp of 70c. Its just barely too hot to touch so its probably around 55c on the surface of the IC.

Is there something that I could be doing wrong thats heating it up so much? My LED's are behaving just like they should be though....

Last edited: Aug 19, 2009
13. Wendy Moderator

Mar 24, 2008
20,765
2,535
Show us your circuit schematic. The 555 should not get hot at all, so you may have blown some parts. You may have too large a capacitance, or swapped your collector/ emitter on your transistor. I'll repeat, it should not get hot, so there is something off.

Aug 18, 2009
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Do you think it might be because im using a Computer PSU to power the circuit. Its a 12V DC output but it can give off a crazy amount of amps.

Im going to try re wiring my circuit first. Its put toghther by a bunch of wires and totally ghetto, so posting a schematic of how it actually looks would be difficult.

EDIT:

I connected just the Vcc and the ground and it didnt get hot at all so I dont think its the supply voltage.....

Aug 18, 2009
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Ok I re did my circuit and this time I had organaztion in mind. Although I got rid of the 555 heating up, im having another problem.

Anytime I connect it to the power the LED lights up, turns off and than never turns back on. Im pretty sure my timing circuit is right, any other 'obvious' problem that it could be. Ima get some sleep and look at it in fresh eyes in the morning.

16. Wendy Moderator

Mar 24, 2008
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The first step with any project is the schematic. A little digital noise won't affect the 555 much (some, but it would be hard to notice). Let's see your schematic, and we can help.

Second question is the power supply, what is it. If you look at my 555 projects in my tutorials a 9V battery is a favorite, with 2X AAA being runner ups.

Aug 18, 2009
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My circuit looks like this

I switched to a 9v battery and am getting the same thing. It has something to do with pin 7 on the 555 (discharge) it doesnt change anything at all if i disconnect it. Im positive its in the right place.

Is there any way that I could have broken my 555?

18. Wendy Moderator

Mar 24, 2008
20,765
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If it isn't flashing then the 555 is likely toast. I'm betting on a wiring error somewhere, as that schematic is sound. The original power supply was 12V? My calculations show 1/5 of a hz, or one flash every 5 seconds.

19. darenw5 Active Member

Feb 2, 2008
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Possibly, you could use a PNP transistor to pick up the signal at the 555's timing capacitor. This is a (nonlinear) sawtooth starting low and rising. That would turn on the PNP and slowly turn off. The PNP may actually need to be a darlington arrangement to supply enough current for the LED but not load the timing cap. You may need a diodes to level-shift, since the sawtooth never reaches the positive supply rail.

Perhaps I'll have time later to upload a schematic and breadboard the circuit.