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Thread: I need LED VU Meter Resistor Help

  1. #1
    Constant Bitrate
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    I need LED VU Meter Resistor Help

    OK, i am gonna make a VU meter using 10 blue leds for each channel, here is the diagram i am using


    now, my question is, will i need to put a resistor in the power line for the LEDs, my guess is yes, since the leds only need like 3.5 volts or so. What size resistor do i need? Thanks.

  2. #2
    FLAC
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    Re: I need LED VU Meter Resistor Help

    no you dont, since the LM3915 IC will have internal resistors for the task..... so you should be good to go..... if this is for the car, I would have some sort of regulator on the voltage input, since you may get the LED's flickering a bit with changes in your cars voltage system...
    Project - GAME OVER :(

  3. #3
    Constant Bitrate
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    Really, i was just going by where it says 'VLED must be kept below 7V or dropping resistor should be used to limit IC power
    dissipation.'

    Now that i read that somemore, it probably isn't critical.

    Good point on the regulator, do you think if i regulated the 12.6V non running and 14.4V running down to like 5V would be a bad idea as far as heat?

    Thanks.

  4. #4
    FLAC
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    well since the circuit says that i requires a 12V - 20V input, u dont wanna regulate down to 5V!

    I would say a nice and simple 7812 regulator/heatsink and a few smoothing caps should do the trick.... keep the voltage at a fairly even 12V....
    Project - GAME OVER :(

  5. #5
    Constant Bitrate
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    hehe, i wasn't thinking.

    I am just curiuos though, how well is this gonna work with the car not running (bat should be about 12.6V) and it says 'Input Voltage Min (Volt) 13.70'

    would i even need a heatsink, LEDs don't take much power, right?

  6. #6
    Rob
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    From reading the chip's spec on National's web site.

    http://www.national.com/pf/LM/LM3915.html

    Operates with single supply of 3V to 25V
    So why do you need to us a regulated 12v source, surely a lower voltage supply would work better due to the voltage drop?

    Output current programmable from 1 mA to 30 mA
    This negates the need for current limiting resistors, however when setting the current level rember that blue LEDs require a fair bit more than other colors, approx 20mA max current.

    N.B. I'm still waiting for the full pdf to download so I'll put a futher update when I get more info.
    And you say people actually pay money for M$ Windows?
    www.mp3mini.co.uk (Does what it says on the URL) www.openclassic.co.uk (The new car, with zero rust!) www.rob-web.co.uk (My other site)
    Total re-design underway: on the whole progress is very slow as the car is taking up too much time :)

  7. #7
    Rob
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    It might be worth replacing the 2.7k resistor with a 5k pot so you can varry the current in the LEDs and hence their brightness.
    And you say people actually pay money for M$ Windows?
    www.mp3mini.co.uk (Does what it says on the URL) www.openclassic.co.uk (The new car, with zero rust!) www.rob-web.co.uk (My other site)
    Total re-design underway: on the whole progress is very slow as the car is taking up too much time :)

  8. #8
    Constant Bitrate
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    Thanks for the tip on the Pot, probably a good idea.

  9. #9
    Variable Bitrate
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    Just interested as to where the original circuit came from?

    Cheers

    Dave

  10. #10
    Constant Bitrate
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    It came right out of the LM3915 datasheet, check out http://www.national.com/pf/LM/LM3915.html

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