Throbbing LED Question

Discussion in 'General Electronics Chat' started by ShockBoy, Nov 27, 2009.

  1. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Hi. Upon success of this circuit, I went ahead and added a PNP for an alternating LED. My question is why is the second LED (on the PNP) High-Low cycle different than the first LED on the NPN? The first is Perfect. My addition is ON longer then off.
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    What do you mean by different?

    Where did you find this oscillator circuit? It does not seem reasonable to me.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    It is an old standard, I think it started of at Bill Bowden's website. AudioGuru has posted it several times.

    Basically it is a variation of a Hysteresis Oscillator. If the output of the oscillator doesn't go rail to rail (power supplies) it isn't symmetrical. I've written about it here...

    555 Hysteretic Oscillator

    Bill's Index

    The output of the LM324 is seriously lacking. I've come up with several variations on LED throbbers, this is the closest to what you have.

    [​IMG]

    The output voltage only goes between 1/3 and 2/3 of the power supply, it is a fundamental limitation.

    Another variation, which I have adapted for power outputs, has the advantage of going PWM, so the LED is instantaneously on or off, but the end effect is still throbbing.

    [​IMG]

    May not be exactly what you asked for, but I hope it helps.
     
  4. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Thank You So much. I am curious though... Mathmatics is precision, components have characteristics: The first LED running off the NPN is flawless in its action i.e. 50-50 ( using the c1815). Why then is there not a way to capture the opposite signal exactly?
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    If you really look at the schematic, and analyze it, you will find that it isn't as symmetrical as you think. For example, you didn't mention colors on the LEDs, but I bet one side is different than the other. The color of an LED is important, it dictates how much voltage is dropped across it.

    The transistors themselves also introduce an offset, in the form of the BE voltage drop.

    The PWM version will compensate for most of that because the LED signal is digital.

    I've written several articles on these subjects.

    LEDs, 555s, Flashers, and Light Chasers
     
  6. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Understood, not symmetrical. Both LED's are blue high-intensity. With different LED's, there is different LED requirements that change the visual effect. Why is there not a simple solution to solve this issue in it's current form? (Closest desired result was high-intensity LED on the PNP and lower value on the NPN.)
     
  7. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    I figure that the only way to get an absolute equal blink rate is to duplicate the circuit.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    It's a hobbyist circuit, not a professional circuit.

    Why don't you experiment with different values of resistance in it?

    For starters (and where I would start) is replacing the two 47k resistors on the left with a 100k pot, the wiper being the junction of the two resistors.
     
  9. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Thank You, beginner hobbyist is what I currently am, and I really appreciate all of the patience given in regards to dealing with folks like me.
     
  10. mik3

    Senior Member

    Feb 4, 2008
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    If you just want to flash the LEDs use a 555 timer IC as an unstable multivibrator. Google '555 unstable' and you will get many results.
     
  11. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    I like the fade in and fade out effect. This is good for displaying art, gemstones, etc. I've tried the 555 timer and had issues with duty cycle not being close to 50-50 without additional transistors. I've just purchased the 4017 for some additional experiments. Down the road, looking for sound activated LED with electret mic.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    You can get a near perfect 50% duty cycle with with a 7555 or the second schematic. A human can not see 0.3%.

    [​IMG]

    My articles cover all this territory (this is an illustration from LEDs, 555s, Flashers, and Light Chasers ).
     
  13. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Thanks Bill! I see that pin 5 is optional?
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    It is for noisy environments. For a single 555 on a battery it is not needed.

    Pin 5 has lot of other uses for other designs though, it can make a dandy VCO input.
     
  15. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Hey Bill, Could you tell me a couple of applications for pin 5? With the VCO input, from what I understand works with wireless apps. Just wondering how the 555 works into such remote/wireless apps. With the high/low signals I mean?
     
  16. Wendy

    Moderator

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    Funny you should ask that, check this thread out...

    http://forum.allaboutcircuits.com/showthread.php?t=28802&highlight=LED

    The design on this is still pretty rough, McGuffin hasn't really told us how he modified the schematic. What I gave him was pretty bare bones. Many cases you need to experiment to polish an effect you want, we can guide you but you have to do the work.
     
  17. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    I've seen that thread, from my understanding you need to convert the square wave from the 555 to a pure sine wave to get sound. Is this not correct?
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    Yes, this is not correct.

    A square wave has a different timber, a lot like bag pipes, while a sine wave is a pure tone, like a flute, but both are sounds and have notes. As I have mentioned in the other threads, a 555 can not drive a speaker directly, it will burn up. A simple resistor and capacitor will work to connect it to a speaker, and if you want louder there are lots of options.
     
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