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Getting an LED to fade on and off Answered

I plan on making an LED sign and I would it to fade on and off continuously. How would I go about doing that?? I don't have much experience with electronics so pardon my ignorance.



I have a question for anybody who wouldn't mind helping me out;

I need a circuit design that includes 19 fading blue LEDs (3V fwd voltage, 20mA fwd current). I can use a phone charger or any other similar household transformer to supply power from an outlet. How should I make the circuit for the fading LEDs? I don't care if they fade synchronously or not - either way is just as satisfactory for their purpose. I've never designed a circuit from the bottom-up before, so I could use some help figuring out how to get the basic circuit structure to make this work. Thanks,


6 years ago

I have just completed a circuit that fades leds, you would just need to hook it up to a 555 timer or a capacitor timer.

my circuit is here:


the 555 is grounded in pin 1. grounded meaning connected to the negative of the battery

fading led.jpg


Thank you.

how come the 555's pin one is not connected to the ground of the circuit?

. Googling LED fade worked for me. Doesn't look like a beginner's project. :(

Pardon my ignorance as well, but I've always wondered why can't we just we use a simple variable resistor when fading an LED? Battery + LED + Variable Resistor.

. You can, but then you have to sit there and twist the knob back and forth. :) . I didn't look at any of the search hits, but, judging by the titles, they probably use counters and D/As (Digital to Analog) to do the fading - which I suppose you can look at as a computer-controlled pot.

. Oh! Since you're pleading ignorance, pot = potentiometer = variable resistor, not marijuana. heehee

Awweessommeee!!!!! Hahahahahahahahah!!!!! It took me a few seconds to understand the pot function... :P

LOL with that pot you can see the LED doing a whole lot of other things than just fade :)

I would use an ardunio board. They are really, really easy to use.


10 years ago

Most common way is to use PWM. Modern microcontrollers have hardware support for this, and lots of cut-n-paste code to support it.

Easy? Difficult? Not too tough, but if you've never done much with electronics, it can be intimidating. Search for AVR or PIC instructables. But it's a whole level of commitment to learning beyond a simple LED circuit, clearly.

(I've used AVRs. Extensive opensource software is available. No programmer?--make a parport version for about 5 bucks...)

How about playing around with this? No chips in it. Leave out one LED or they will go back and forth. More of a fade effect with lower (like 5K) base resistors. Not too critical, any transistors should work.


Hi Viron, I built one of these the other day and noticed that the fade cycle dims OK, but that the brightening happens very quickly. Sort of a slow dimming, but a rapid brighten, if you follow me. I've played with the base resistor values but can't seem to change this behavior. Any advice? Thanks! Otaku

And bigger caps (like 470uF) make it go slower.

Yeah, what he said, nice circuit. But MrTiki should have indicated how many LEDs are to be used. I assume a sign requires dozens. Scaling up a discrete cap charge/discharge solution will be much tougher, as the analog fade also must be applied to the 'power' transistors used to handle the current for the multiple LEDs. PWM (with micro or 555) approach is digital, tho, and the transistors will simply be on or off... You could try and scale this analog circuit without power drivers (a separate resistor/transistor for each LED), but the parts count would rise fast, as westfw says. And since the design relies on the interaction between a pair of transistors and Res/Cap charging I think it would be a nightmare.

Thanks for the replies. I think I'll just scrath the pulsating/fading till I get better acquainted with electronics. Seems a bit to complicated for my newbie self.

It's actually not that hard. Use a 555 timer to create a square wave (ON-OFF) of the right frequency. This is a very basic element of many electronic circuits, and you should be able to find plenty of info online to accomplish that part. Then you add a resistor in series with the LEDs, and a capacitor in parallel with the LEDs - that creates a low-pass filter, turning the square ON-OFF wave into a triangular FADE IN-OUT wave. How long it takes for the voltage to go up and down is proportional to the product of the resistor and capacitor. So pick a resistor which is appropriate for the voltage and LEDs you are using, and play around with different capacitor values until you get the desired effect. It's a nice little trial-and-error project. Electronics 101, really...

Um. That works in theory, but let's attach some numbers. The fundamental time constant of an RC network is R*C, and we'd like that to be, say 2 seconds. R is going to get fixed by the current we want through our LED, say 100 ohms for a 5V supply and a 20mA 3V LED. So we have 100*C = 2, or C = 0.02 F
That's 0.02 FARADS, or 20,000 microfarads, a size that would have been unthinkable for such a circuit not too many years ago. But it's not unthinkable anymore; 0.022F capacitors are widely available on the surplus market or from real dealers, and they're not prohibitively expensive, either. I've got some like that lying around, I'll have to give it a try.

(I should point out that by the time you build your 555 circuit and attach a 0.02F capacitor to it, you'll have exceeded the cost AND the complexity of a micro-controller PWM-based fader. If you assume the software is free and trivial.) (This is exactly the sort of observation that led me from electronics to computer science. I realized that nearly all of the electronics projects I'd ever built could just be PROGRAMMED into a suitable micro...)

Not to nitpick, but at most you need a tiny 555 IC, 3 resistors and two capacitors, and you could probably design it with just one or two resistors and one capacitor - most of which you should be able to scrounge out of a piece of old electronics junk. That's what, 1 or 2 dollars? Let's make it $7 if you splurge on a breadboard. That's got to beat a micro-controller plus cables etc. Not to mention the hurdle of learning how a micro-controller works, etc.

A micro-controller really seems overkill in this case.

The minimal microcontroller solution is a sub-$1 6 to 8 pin micro and a bypass cap; call it $0.75; probably equivalent to the cost of the 555 and associated passives EXCEPT for the 0.022F supercap, which will add another $1 or so. The "hurdle" is a separate issue, thus the tongue-in-cheek reference to free and trivial software... (gotta keep an eye on those microcontrollers. They keep changing the rules and making it "reasonable" to use a micro in places where it was "overkill" before. Atmel was selling ATtiny11s for $0.25 each, and Microchip has a 40pin PIC16F59 that sells for less than $1 in modest quantities, for example.)

25 cents? Dang! I guess that's what happens when you haven't touched a soldering iron for a couple of years. :D Of course, if he's planning on doing a larger LED sign, he would need more current, and thus a smaller resistor and an even larger cap. But I didn't want to scare Mr. Tiki off by talking about transistors or opamps;)

What about a hooking it up the a capacitor? Time how long it takes to charge/release, and just either use a 555 timer or a microcontroller?

Gee, go for the obviously EASY answer *LOL* where is M|Y head at ... A 555 timer would do quite well.

Hey guys I was checking out the UTUBES and in a video some guy mentioned that adding a flashing led ( in the #1 position) in a series would make the whole series flash. I think this would be the simplest way for me to go.

Oh I will try not to be ;-) I am gettin a little slow in my old age.....now where did I put that cane....