Fading LEDs

Discussion in 'General Electronics Chat' started by M1K3Y, Mar 25, 2006.

  1. M1K3Y

    Thread Starter New Member

    Mar 25, 2006
    Hi y'all. My name is Mike and I'm brand new to this forum. This looks like a high quality, professional forum and I'm hoping that this will be a great source of information. Anyway, about myself, I'm a sophmore in college majoring in Electrical Engineering. I'm a good student, but I find myself lacking in knowing what is out there and what can be done with it; basically, the applications of things. This is what I'm hoping reading and posting questions here will help me with. I hope to broaden my knowledge of how to build circuits and apply them to useful applications. ANYWAY, to my question:

    I am currently in possession of a piece of simple circuity that makes three seperate LEDs fade in and out in a long, but discernable pattern. However, there happens to be a drop of some black stuff that I'm not exactly sure what it is used for (maybe they didn't want me to know what the circuit was? lol) over the 'brain' of the circuit. The whole circuit is on a 1x1 inch square and the drop is only about 1/4 in diameter and is probably 5mm high. This absolutely amazes me because the pattern isn't just the LEDs fading in and out over time...they start off by fading in and out, but then change to just one LED fading in and out to two at the same time and more. I've been doing some searching on how to make LEDs fade in and out, and as I suspected I saw a lot of things using capacitors and 555 timers...but this circuity most definitely does NOT have a 555 timer or large capacitors (because the drop is so small) and for the life of me I can't figure out how it is done. The circuit runs off of 4.5 volts and the only other things I can tell about the circuit is that is has the 3 LEDs and 2 resistors - a 22 Ω and 150k Ω .

    My question then is, quite simply, how? LOL. I would like to know because I want to build a circuit that is able to have 3 LEDs fade in and out at different intervals to get different mixes of the colors. I'm sure I could do it, but knowing that it can be done so simply and on so small a scale, this information would be very helpful. Thanks a lot (:D)
  2. hgmjr


    Jan 28, 2005
    There are several ways to fade an LED.

    You can vary the current drive to the LED using an analog ramp circuit, you vary the current drive to the LED using a digital-to-analog convertor or you can use Pulse-Width modulation.

    If I had to guess the method being used with the circuit you have, I would guess that if is using a digital-to-analog convertor. I say that because you indicated that the only external components you can see are resistors.

    You would most likely see an external capacitor if the circuit were using an analog ramp or pulse-width modulation (PWM).

    Welcome by the way to the wonderful world of electonics. You are in for a real treat as an EE.

  3. pebe

    AAC Fanatic!

    Oct 11, 2004
    Hi Mike. Welcome.
    I've got one of those devices you are talking about. It is in the base of a glass figurine to give it a sort of 'internal lighting' effect. I think the black blob contains a microcontroller programmed to give the sequence of fades. You won't be able to make anything as small because the micro isn't in its usual pinned IC. You could make a similar thing using discrete parts but it would be quite large (and complex). The better way would be to program one of the PIC micros to do the job.

    Talking of these things, it's amazing just what they get into 'black blobs' now. Last Xmas we got a toy for a relative's child. It was a singing dog that sang a tune and had fully articulated body swaying in step with the music. I couldn't resist looking inside. Yes, it used a 'blob'.
  4. M1K3Y

    Thread Starter New Member

    Mar 25, 2006
    Wow, thanks for the input guys. I'll look into Digital to Analog converters and see if I can get a circuit like that working. I really appreciate it. I think I'm gonna like it here (:D)
  5. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    What the black blob actually is is the internals if an intergrated circuit without the case. The manufacturers the silicon die and place it directly to the PCB. It is cheaper and smaller. Also stops one knowing what it is. And also makes it a throw away item as you can't replace the chip.