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Old 16th September 2009, 12:24 PM   #1
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Unhappy Help with LM3915 LED VU meter


This is my first post on diyAudio, because I couldn't figure things out by myself.

Here's the problem:
I've successfully built the LM3915 circuit as shown in the datasheet. Everything's the same except I used 10k instead of 8.06k for R2 and I've left out the 2.2 uF capacitor.
When I turn it on without an audio source, all LEDS light up, so I don't think I've made a mistake in the solder work.
When I plug in an audio source (headphone out from my stereo or computer), there are always 7 or 8 LEDs lit, regardless of the volume that is outputted by my stereo/computer. When I turn the volume up, LEDs 8,9 and sometimes 10 work as they should. So I conclude that my setup works? I just need to configure it so that it uses all the LEDs and when the volume is turned down, none of them are lit.

I'm not using a rectifier circuit, however I've built one along with a booster circuit but that didn't give a better result.
I'm thinking of building a new (better?) one from this site:
Would it help in any way?

I've tried using all kinds of different values for resistors but to no avail. I've also tried to work out the exact value I need according to Vref but I find the calculations in the datasheet somewhat confusing.

FYI: the signal on my headphone out fluctuates around 2.5V with peaks to 3V (is this correct?) and I have an adapter with adjustable voltage (3-12V) however when choosing different values, I don't really see a difference.

Hopefully someone can help me because I really want to finish this project.

Thanks in advance,
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Old 16th September 2009, 05:45 PM   #2
Nisbeth is offline Nisbeth  Denmark
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IIRC the LM3915 is not actually a VU meter, just a log scale bargraph driver. You need the LM3916 to build a "proper" VU meter for audio.

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Old 16th September 2009, 06:50 PM   #3
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Originally Posted by Nisbeth View Post
IIRC the LM3915 is not actually a VU meter, just a log scale bargraph driver. You need the LM3916 to build a "proper" VU meter for audio.

Thanks for the response, but that's not really an issue. I'm trying to make an ambient LED bar that lights up according to the beat of the music, so accuracy is no big deal.
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Old 17th September 2009, 07:15 AM   #4
dangus is offline dangus  Canada
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Here's one at Instructables... it probably works:

Maybe something is unstable... try adding the suggested cap. And try a resistor between the input pin and ground, like 47k or so, and then feed the signal in through a capacitor of 0.1 to 1 uF.
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Old 20th September 2009, 09:47 AM   #5
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Thanks for the suggestion, I added the resistor and it's much more responsive now !
I'll try building a rectifier now, I'll let you know how it turns out.
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