---=== LM3915 Questions ===--- - diyAudio
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Old 29th September 2004, 10:39 AM   #1
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Question ---=== LM3915 Questions ===---

Hi to all !

I have one small problem with my vu meter built on my LM3915 IC chip.

I set current values for all all leds and the input signal but i'm confused with setting of gain

How can i make the IC to show at pin1 -18db or any other value that i want.

I want a scale from -18db to 9db in 3 db steps.
and another scale from --12db ---> +12db for other vu meter

Please help
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Old 29th September 2004, 11:07 AM   #2
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Please check the datasheet first.
http://cache.national.com/ds/LM/LM3915.pdf

Secondly, the "gain" is set by the reference voltage, meaning the DC voltage for obtaining 0dB (all LED's on). You must also have a rectifier circuit with average or peak function. All this is in the datasheet.
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Old 30th September 2004, 10:07 AM   #3
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I don't understand what is the need of an rectifier circuit in a vu-meter. I read LM3915 datasheet but is too heavy to understand.

Please help

All i want to do is to make a vu-meter with a log scale :

-24, -21, -18, -15, -12, -9, -6, -3, 0, +3 dB

Difference between LM3915 & LM3916 ....
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Old 30th September 2004, 12:13 PM   #4
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The 3915 has equal steps of 3dB each.

The 3916 is non-linear (designed for VU meters) with steps at -20, -10, -7, -5, -3, -1, 0, +1, +2, +3 dB.
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Old 30th September 2004, 12:21 PM   #5
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I think you have to read the datasheet, look at the pages at the end where you have very practical advices.

A rectifier with averaging or peak function will create a more "steady" light (looks nicer) but as I understand the LM391x can handle pure AC signals also.

You must adjust the reference voltage so it corresponds to the voltage or your +3dB, 1.4 volts maybe?
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Old 30th September 2004, 12:32 PM   #6
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What about this circuit:

http://users.otenet.gr/~athsam/vu_meter_3.htm

What are the dB stages on each led ?

If I load a signal between 60mV - 1.25V all leds will be lit to 0dB ?
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Old 5th October 2004, 10:53 AM   #7
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I have a soundcard Creative PCI 128 2 channels

I don't have a multimeter to check the output power...

What is the power in mV on spk output .

Anybody knows ?????
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Old 25th October 2005, 08:34 AM   #8
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Where I can find a vu-meter based on this IC but with peak hold function or a max hold function.

How can I add such a funtion. I did not find any documentation on this in the datasheet

thx
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Old 25th October 2005, 08:42 AM   #9
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I'm afraid that you'll have to read the datasheet in order to get a working VU-meter. If you check the "Application hints" you'll find a couple of rectifier circuits, check especially page 11, figure 4.

http://www.national.com/ds/LM/LM3875.pdf
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