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Analog Fading LED Answered

Hello - I am brand new to electronics but am trying to learn about BEAM and other art + science circuits. I am looking to make a small analog circuit to fade in and out a LED. I have looked online but have found a bunch of ones using a 555 timer or a PWM and other chips. I saw a reference to doing this slow fade in and out technique using only capacitors and resistors but have been unable to find a diagram. Can someone help me out. FWIW - I am looking to charge this using solar and am only looking to drive one LED. Ideally a days charge would last a few hours of fade in and out cycling with the cycle time being slow - somewhat like the Mac "Sleeping" LED. Thank you in advance.


Hello,don't know if you managed to get it to work but here's a circuit from the BEAM group. ps:circuit courtesy of Ross.

astable pummer.JPG

Hey, this circuit looks great! I will try it as wanted to do same effect with some LEDs. Do you know where this circuit was found (book, website?)

Thanks Snailwatcher. Can you shoot me the link to the BEAM group? I appreciate it.

No problem,here it is:
Right now I'm building this circuit,so I'll let you know if this works for me.

hey snailwatcher - thanks for the link - btw, went looking for parts and could not find the VR1 47k. That is a 47k voltage regulator - and not a straight up resistor yes? Thanks again.

This one work ok.. with the fading was longer. I used a 3300cap 150 resistors on 12V


Isn't this missing something like a transistor? L

I don't believe it - how? You've got two separate 12V feeds through 600Ohm resistors (why 2?) How does the current stop flowing? L

ops.. i meant to say I wish the fading lasted longer

This can be done with just a few capacitors and a transistor.....I will have 1/2 of such a circuit published by the end of this week.....if I can find time to get back to making it look better.

PS: by 1/2 of the circuit, I mean that the switch turns the LED on, and it fades out......adjustable time with a potentiometer

It shouldn't be hard to create the fade in from this circuit too.

I think I'll stick with digital dimming.

I'll work on it, I'll post something if i get it to work

here's what I have so far, I know it won't work, but it's a start, I odn't have too much expierence with transistors, so othe electronic people help this circuit (unless you think it's doomed to start with).

led throb 1.0.bmp

I came up with a new thing, it might work, will post schematic soon (I have it on paper, I just have to use expressSCH), somebody please evaluate to see if there's any problems.

check this out, see any problems? the Vin charges both capacitors (the left one more fastly, and the one on the right slowly). Vin can charge the left capacitor (and power the led) through the PNP transistor. As the timing cap starts to charge the pnp becomes more resistive and the npn becomes more conductive. The led is now running off of the left capacitor. The npn transistor shorts out the timing cap and it starts over. I know it's not perfect, or if it will work so I'd like to here improvemnts. The only thing that might make it not work is that the npn's resitivness will match the rate that the capacitor charges, thus leaving the LED on a plain dim state.

led throb 2.0.bmp

how does the npn short the timing cap? it needs to be positively biased to go on. also, how does the pnp go on? where is it getting its negative biasement voltage from? i think it would work better if you changed the npn with a jfet. (jfets work in depletion mode.) ive seen led fader circuits that used a 555 and a 4017. is that good enough?


10 years ago

would throwing in a 555 or something along those lines be out of line with "BEAM" the main thing is I want to stay simplistic without going over to digital chips or having to learn or program chips ala the smart LED project. I am open to suggestions and to the best, most simplistic and elegant way of accomplishing this. Thanks everyone.

Well, for instance there are some "beam" pummer circuits that apparently fade an LED on and off by doing some sort of PWM-like things using 74x240-based "bicores" and such. Internal complexity is high, but external complexity is pretty low...


10 years ago

Transistors, caps, and resistors only? No mosfets, op-amps, or other ICs? Any limits on number of transistors? Real triangle waves are probably tough, since normally you'll get exponential waveforms. But that ought to be "close enough" for LED-fading purposes...

. This project has aroused my interest, so I took the time to search for an RC/LC/RLC circuit for sine/triangle waves - came up empty handed. :( . I just talked to my Dad, the Electrical Engineer, and he does not know of a way to build a sine/triangle wave generator with just Ls, Rs, and Cs. "There has to be a switching mechanism in there somewhere." With that in mind, there are many circuits out there for 555 and 741 based generators. . PS: keep in mind that Dad is used to working with KVs and MWs, so he's not an expert on electronics, but he is pretty knowledgeable. He's been working with this stuff since wayyyyyy before ICs came along.

Yeah, I just found my capacitor/neon blinky I made eons ago.....is looks like it is just a neon bulb and a few small capacitors, but then, it only blinks, and not very regularly either. But, like I said, it runs directly off of house current (it is completely encased in a big hot glue blob or I would reverse engineer what I did and post it; in case it gave anyone any ideas.


10 years ago

Thanks guys - The effect I am looking for is just a slow fade on and then a slow fade off - lather rinse repeat. I know that a square wave would not look right but a triangle one I think would. MAKE had something like this but they were using the arduino board and I was hoping to do this all analog. probably a bit ambitious for a first electronics project - but hey - only way to learn. Thank you again - and keep the suggestions flowing.

> probably a bit ambitious for a first electronics project - but hey - only way to learn. . Basically all you need is a signal generator and a transistor to take the generator output and drive the LED. If the generator will supply enough current (quite possible with only one LED), you won't even need the transistor.

Hmm, I have done something similar with "just" capacitors and neon bulbs, but that is from house current AC.....DC is a whole different animal :-)

I just looked up the apple patent - it is a sinusoidal wave

for this situation would you want a triangle wave or a sine wave?

. Probably wouldn't matter; I doubt the eye can tell the difference. Even a saw wave might be acceptable. Guess it depends on the effect he is looking for.

Keep me posted on what you make. I wanted to do the same thing with around 10-20 leds for my computer