Random LED fade on/off

Discussion in 'The Projects Forum' started by offsky, Dec 16, 2012.

  1. offsky

    Thread Starter New Member

    Dec 16, 2012
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    I need some help with a circuit. Im trying to make a circuit that will fade in and then fade out an LED, and then stay off for several seconds before repeating. I'm essentially making a fancy LED Throwie that blinks intermittently with a fade.

    I thought of starting with a solar engine (http://www.beam-online.com/Robots/Circuits/1381.html) which I should be able to tune to get the intermittent blinking, but I can't figure out how to modify this to make the LED fade in/out. I need the circuit to be small and cheap, so Id like to avoid using any sort of programmable logic.

    Any ideas would be appreciated. I'm not very good at electronics.
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,132
    267
    Hmmm.

    Digital / Programmable = Cheap and small

    Analog = large complex circuit.

    If you are building a Throwie, the analog version will be more like a Brickie.

    Consider the PIC10F series micro-controller, tiny 6 pin device can do it all.
     
  3. offsky

    Thread Starter New Member

    Dec 16, 2012
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    Thanks for the feedback. You may be right, but building dozens of these, I think it would be easier/quicker to use a dozen analog components than to have to program an IC. For me that would involve getting a lot of equipment and learning how to program an IC, which is more time that I want to invest. I know how to solder and am ok with the extra bulk of extra analog components. Can any recommend a circuit for this?
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    The part that makes it difficult is the randomness, this is not a usual feature of analog. Fading is easy.

    LEDs, 555s, Flashers, and Light Chasers

    Chaper 12 - Special Effects

    With digital you have memory, Sensacell is right, if you are serious about this project a small programmable device is the way to go. It will be much, much cheaper, smaller. The hard work is in the programming.
     
  5. tshuck

    Well-Known Member

    Oct 18, 2012
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    I have to agree with Sensacell, the 10F family of PICs are ~$0.50 per unit in small quantities, you probably won't meet that price point with analog components. with the PIC, you could do PWM to dim the LEDs, (pseudo)random number generation for the randomness.

    That being said, some 555s could do what you are suggesting with additional components. You may need larger magnets for this to work as the additional components will mean added weight, though, if you're using neodymium magnets, you shouldn't have a problem.
     
  6. offsky

    Thread Starter New Member

    Dec 16, 2012
    3
    0
    Bulk isn't really a concern. The reason I said "small" was to avoid getting an answer that required an Arduino or something overkill. Im not really making a Throwie, that was just the closest analogy that I could think of. Going the PIC route would probably be the smallest and cheapest, but it has a steeper learning curve than I can handle right now. I was hoping to just throw together some transistors and capacitors and have something quick that I can experiment with.

    It doesn't need to be random intervals. I was thinking on for 2 seconds, off for 10. Repeat.
     
  7. tshuck

    Well-Known Member

    Oct 18, 2012
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    In that case, you could simply use an astable multivibrator

    You should also check out Bill's post from a thread on astable multivibrators....

    I'm not sure if the component requirements would be feasible, though, for your time intervals...
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
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    Believe it can all be found in Bill's Blog: A one-shot,555, with a darlington transistor as an emitter follower driving LED, base to timing cap. Falling output triggers a second 555 one-shot, 10 sec,on falling edge of output retriggers first OS. Not self starting.
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
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    This may work for your need. Transistor, MPSA 14, darlington presents a high impedance so as to not load timing RC, C 1, R 4. Can also be made from 2 NPN's. With 5 V supply, sawtooth V at C1 goes from ground to about 3.3 V, so LED is RED, fV around 2V,; for other colors supply needs to be increased. R 1 depends on LED & supply V. Switch ,SW, starts the chain reaction. All C's are μF's.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I would use a hex schmidt inverter IC like a 74ACT14 or 74HC14, it can make 6 independent astable oscillators and each just needs 1 resistor and 1 cap.

    The INPUTS of the inverters (where the cap is) will ramp up and down as the astables oscillate, so i would couple the 6 inputs through resistors etc to the base of a transistor. So the transistor base will see a "random" analogue signal made from 6 analog ramping signals at different frequencies mixed together.

    That should do it, one 14pin IC and a transistor and a few discretes.
     
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