VU illumination using LED with AC

Discussion in 'General Electronics Chat' started by mrstan, Mar 7, 2014.

  1. mrstan

    Thread Starter New Member

    Mar 7, 2014
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    Hi guys,

    I am working on the same kind of project here with a VU meter that is illuminated by a tiny tiny incandescent bulb powered by a 2.2V AC circuit. The amperage has to be tiny taking the size of the lead wires for hte bulb are smaller than a hair. I am going to try the led with a 1k resister to see what comes from it. I do not know the frequency of the AC though, and I have to presume it is the 60 cycles from the mains power.. dunno. This would make the LED light at 30hz then? I think this is indiscernable.. maybe (blinking). Does anyone have any suggestions for this line of thought? I will post to tell what I see from the output on the 1K and led. I think the led that is in my kit
    is rated at 20 or 30ma, and not sure the reverse though. The incandescent draw is not much since it is such a tiny bulb in the VU meter backlighting.

    Comments or suggestions? I am trying to get light without too many components to mess with as a goal. I figure an LED and resistor.. and hopefully not a cap to calm the ac waves :).
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You hijacked someone else's thread.

    This thread was moved from here.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Motion pictures have a frame rate of 24Hz. So the book says you are fine.

    Some people (me included) are particularly sensitive to blink rates and can see this or higher blink rates under many conditions so personally I say go higher... if possible. You're pretty stuck with 30Hz so stick with it.

    LEDs have a terrible reverse breakdown on the order of 5-6 volts, but that should be acceptable for this. If you need to add a diode it should be a Shockley type to leave any voltage to light the LED.
     
  4. #12

    Expert

    Nov 30, 2010
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    Did you mean, "Shottky" diode?
     
    ErnieM likes this.
  5. KJ6EAD

    Senior Member

    Apr 30, 2011
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    That 2.2V AC supply seems extremely low for a bulb circuit. Is the bulb in series with other illumination or low because of a series resistance, fixed or variable? I'd have expected an AC illumination secondary from the transformer to be in the range of 6.3V to 18V.
     
    Last edited: Mar 8, 2014
  6. #12

    Expert

    Nov 30, 2010
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    I remember now. They were called, "grain of wheat" bulbs. Had some in a Radio Shack radio. I was able to buy a pair 30 years ago. Maybe I still have one in the junk pile. I'll go look.
     
  7. Metalmann

    Active Member

    Dec 8, 2012
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    Yeah, I have quite a few Grain of wheat, Grain of rice, bulbs.
    Some only handle 3-5 volts and Poof.:D
     
  8. #12

    Expert

    Nov 30, 2010
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    Mine are labeled 12V 60ma. Quite a lot of power compared to an LED. That's why we all expect that an LED can be used to replace a tiny light bulb.
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Or maybe even a Schottky type.
     
  10. #12

    Expert

    Nov 30, 2010
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    I don't mind about the extra, "C" but Shockly was a famous semiconductor guy, too. Schottky would get a bit crazy if you gave Shockley the credit. :D
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    What about "Shonky"?

    He's the famous semiconductor designer than makes those cheap Chinese semis.
    ;)
     
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