The Fading RGB LED

Discussion in 'The Projects Forum' started by campeck, Sep 22, 2009.

  1. campeck

    Thread Starter Active Member

    Sep 5, 2009
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    Hello again!

    Even though I haven't quite figured out my last LED thingy I thought I would ask this. I am trying to get an RGB LED to randomly fade in and out of all colors with minimum board real estate available.

    [​IMG]

    I have seen this circuit by Audioguru and was wondering first if someone could walk me through it's operation. I have a basic understanding of feedback and opamps but I would leave it at basic.

    Secondly could you use comparators instead of opamps? If so I have a LM339.

    Thanks!

    I was thinking there is another easy way to fade the LED. using a capacitor in parallel with the LED but I have noticed it only fades one way and either turns on or off abruptly. How to make it fade in AND out like in this video. I thought I saw the connections correctly but My circuit didn't work like his.
    http://www.youtube.com/watch?v=_Blg8T0DWtU
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    There are several ways to do this. The circuit your showing is a triangle wave generator, the triangle wave is fed to the LED, which causes the effect.

    I've built a bunch of simple 555 timers that light up a pair of LEDs for a couple of seconds, then go out for slightly longer. I built over 30 of the suckers, adding wire with clips so they can chain together. The idea is to put them in a bush with one 9V battery powering them all. I call them bush goblens, and the effect is excellent.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    An opamp has an output that goes both high and low. It has frequency compensation inside so it can use negative feedback to control its gain.

    A comparator has an open collector at its output that goes low. A pullup resistor is needed at its output to make it go high.
    It does not have frequency compensation so cannot have negative feedback to control its gain.
     
  4. Wendy

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    For the record, the LM339 is a comparitor. It can not be used.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    There is a schematic in one of the datasheets that shows a comparator with a pile of parts added (a huge capacitor at its output to ground) that kills its oscillation so it can be used as a very slow amplifier.
     
  6. Wendy

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    Mar 24, 2008
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    Another possible way...

    [​IMG]

    I haven't built it, but just finished drawing it for another thread. These projects seem to come in groups.

    R1 is the cycle rate (how many seconds a flash lasts), and R2 is the fade rate. If R3 is adjusted to 91Ω it can go down to 6V with no problem, it may even manage 5V (R3=39Ω). It will go to 3 seconds per flash to faster than the eye can see, Set both pots for max before powering up, and don't take R1 to it's minimum value or the 555 will get hot.

    This is a variation of the 555 Hysteretic Oscillator, R2 and C2 both slowly charge and discharge, Q1 and Q2 are basic transistors, 2N2907 or 2N3904 (almost any PNP transistor) will work.
     
  7. campeck

    Thread Starter Active Member

    Sep 5, 2009
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    I will build it tomorrow!
    what do you recommend for Q1 and 2?
    I have numerous 2n3904 type PNP and NPN transistors and a darlington TIP120

    I also have a NE555 and CMOS versions.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Q1 and Q2 are PNP transistors. You could use 2N2907, 2N3906, 2N4403, and many other general-purpose PNP transistors.

    You could also use TIP Darlington transistors, as long as the part number ends in 5, 6, or 7, which are PNP.
    TIP Darlington transistors that end in 0, 1, or 2 are NPN, and are not suitable for this project.

    R1 needs to have at least a 150 Ohm resistor in series with it. Otherwise, if R1 is set near to 0 Ohms, it will probably get fried.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Sorry Wook, the TIP series won't work, they need to be descrete transistors. This is because the TIP105 etc. have internal resistors. Could be wrong about this, but don't think I am. I'm not where I can check at the moment.

    Since the rate of flash will be slow I elected not to put the resistor in series with R1. By the time you hit 150Ω the sucker is flashing so fast you can't see anything, and with a fresh 9V the 555 gets hot, but it won't blow. Found this out the hard way. If the battery is the least bit used the 555 won't even get warm. With a serious power supply you'd probably pop the top off. BTW, the same logic also applies with R2.

    OK, I was able to look up fairchild's data sheet. Check it out.

    http://www.fairchildsemi.com/pf/TI/TIP105.html
     
    Last edited: Sep 23, 2009
  10. campeck

    Thread Starter Active Member

    Sep 5, 2009
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    alright,
    I didn't get around to building it today, But I went to radio shack and picked up some goodies and got a box of samples from stmicro. It's like Christmas!
     
  11. campeck

    Thread Starter Active Member

    Sep 5, 2009
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    Hey Bill,

    Your circuit works great on 5v. I am wondering what determines the duty cycle here? It looked like it or the frequency was varying.
    So the timing capacitor charges and discharges through R1 and the output pin and C2 through R2 and the output?

    There is a downside to this circuit with all the big capacitors needed to drive three colors.
    Although I will build it anyway for practice and show you guys.

    The next circuit I try will be audiogurus.
    And since I have a quad op-amp it should keep parts down.

    Thanks!
     
  12. campeck

    Thread Starter Active Member

    Sep 5, 2009
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    Also.

    Using 2N3904's for the pair, What is the max current I can sink? Or is that on the datasheet?

    thanks again
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    I've posted this on several threads, started building one myself. There are some alternate ways to build it without too much heartburn. Use a CMOS 555, up R1 and R to 100KΩ, and downsize C1 and C2 to 22µF. If you can't get the flash rate slow enough (it should be identical) then use 47µF.
     
  14. campeck

    Thread Starter Active Member

    Sep 5, 2009
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  15. campeck

    Thread Starter Active Member

    Sep 5, 2009
    194
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    I just saw this.
    What do you make of it Bill?
    It does what I need with barely any parts.
     
  16. Wendy

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    Mar 24, 2008
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    Don't know, it looks like it would work, but differently. I've never seen that configuration before, pin 4 is usually not part of the oscillator. It will flash much faster, and the LED will be a bit dimmer at it's maximum brightness. My circuit aimed for 20ma, this one goes for 13ma. Nice thing about protoboards though, the stuff is 100% resuable, build one and see.

    Another variation on the design, Wookie suggested it at one point. I rejected it because while it would work it dosn't have the same controlability. You seem to indicate you don't like the parts count, which I do empathise with.

    [​IMG]

    I transistioned to the larger resistors and smaller caps I mentioned earlier that could be used in the previous design. Again, a CMOS 555 would probably work, and overall the circuit would draw much less current. This design, as with the previous, is capabile of going much slower, faster is usually not a problem.

    BTW, if you use RGB LEDs, you can reconfigure this pretty easily for common anode or common cathode. Common cathode would require NPN transitors though.
     
  17. campeck

    Thread Starter Active Member

    Sep 5, 2009
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    That last photo was incorrect. He drew the circuit wrong.
    This is how to configure it.
    Is there any way to change duty cycle here?
    I like the parts count of this circuit. But the LED does look pretty dim...
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Use a pot connected between pin 5 and ground, resistance between 5k (min) and perhaps 50k.
    With nothing connected to pin 5, default threshold/trigger levels are 1/3 Vcc and 2/3 Vcc. The lower the resistance between pin 5 and ground, the lower both thresholds become.
    Decrease R2 somewhat.
    With a bjt 555, you'd be better off using a PNP transistor with the LED & resistor connected between the emitter and Vcc.
     
  19. campeck

    Thread Starter Active Member

    Sep 5, 2009
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    I just realized the fixed version of the youtube circuit is basically that last circuit you have there Bill.
    I made the changes to the previous circuit you gave with 555cmos and 100k and 22uF
    But to change the duty cycle I put a diode in series with the 100k for discharge and a diode in series with a 47k for charge. (those both in parallel at R1)

    I will build your second circuit after fiddling with this one.

    SGTWOOKIE

    what do you mean by bjt?

    I will try the 5 pin way since I have yet to mess with this feature of the 555.
    Do the threshold and trigger go down while keeping the same ratio? or do they go down linearly?
     
  20. Wendy

    Moderator

    Mar 24, 2008
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    BJT = Bipolar Junction Transistor, a normal transistor.

    Pin 5 was originally meant to allow noise imunity for a 555, but it can also be used to set one of the trip points. A 555 has trip point 1/3 and 2/3 Vcc, one of them can be adjusted.

    I've shown the link, but if you want to understand how the 555 Hysteretic Oscillator article also has a theory of operation.
     
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