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5x10 VU meter

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randominality

New Member
OK, first post - So I am going to be doing a bit of case modding and thought it would be cool to incorporate a VU meter / spectrum analyser into the mod, however my GCSE knowledge of electronics has not enabled me to work out the whole circuit. Thus I come to you!

Having had a good look through the serious amounts of threads on this type of project yesterday, I think I finally managed to find a nice schematic for an lm3915 VU meter:

From This site.
One thing that puzzled me however, is the resistor marked 390E in this diagram - What does that mean?

I then incorporated this into my overall design (IC1 is an lm3915):
8948-15jed4.jpg

Now as you may be able to see, I have attempted to create a band pass filter in order to only input part of the audio frequency spectrum into the VU meter circuit, this is becuase I plan to have 5 of these circuits to make a spectrum analyser or w/e its called. I have not been able to work out the value for the capacitor marked C2 in the diagram because I dont know what resistance the lm3915 circuit will produce and therefore I cannot use the formula of 1/(2piRC) = cuttoff frequency.

Also note that I may well be in slightly over my head with this since I have not touched electronics since about two and a half years ago when I did electronics GCSE.
 

randominality

New Member
ok thanks for clearing up the resistor business.
However, I dont know much about filters so could you suggest a filter that would be better? I do however want it to be fairly simple since i need to build 5 of these circuits.
 

bountyhunter

Well-Known Member
ok thanks for clearing up the resistor business.
However, I dont know much about filters so could you suggest a filter that would be better?
I will attach the schematic of the spec analyzer I designed and built for my stereo. You can use the filter section values. Each one is a bandpass filter centered on the frequency listed by it. The filters are shown on the page 2 schematic below.
 

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  • Spectrum Analyzer page 2.pdf
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randominality

New Member
I will attach the schematic of the spec analyzer I designed and built for my stereo. You can use the filter section values. Each one is a bandpass filter centered on the frequency listed by it. The filters are shown on the page 2 schematic below.

Ok, that is extremely helpful and the filters arent particularly complex.
I am only splitting the signal into 5 bands however, so I dont think I can use those values, how do I go about calculating my own.
Now one last thing. I see that you have spaced the bands logarithmicly, why is that?
 

bountyhunter

Well-Known Member
Ok, that is extremely helpful and the filters arent particularly complex.
I am only splitting the signal into 5 bands however, so I dont think I can use those values, how do I go about calculating my own.
I believe on page 1, I have the explanation and formulas for calculating the frequency for the bandpass center frequency (Fo). Also "Ao" is the midband gain of the filter.

Now one last thing. I see that you have spaced the bands logarithmicly, why is that?
That's pretty much standard for audio use.
 
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randominality

New Member
I believe on page 1, I have the explanation and formulas for calculating the frequency for the bandpass center frequency (Fo). Also "Ao" is the midband gain of the filter.

That's pretty much standard for audio use.

Oh I got ya, Fo is center frequency. But I have no idea what midband gain is:confused:.

So if I wanted to split it into 5 bands i would just use filters with Fo of 62, 250, 1K, 4K and 16K. But how do I make the bands wider so that I am not missing chunks of the spectrum? Or is that where the midband gain comes into it?
 

bountyhunter

Well-Known Member
Oh I got ya, Fo is center frequency. But I have no idea what midband gain is:confused:.
These are not unity gain filters, they have voltage gain. take the 1K Hz filter: midband gain means if you put a 10mV (1 KHz) sine wave into it, you will get out a 1Khz sine wave whose amplitude is larger by the midband gain.

So if I wanted to split it into 5 bands i would just use filters with Fo of 62, 250, 1K, 4K and 16K. But how do I make the bands wider so that I am not missing chunks of the spectrum? Or is that where the midband gain comes into it?
(TO WIDEN PASSBAND, SEE POST BELOW)
 
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bountyhunter

Well-Known Member
I researched it back to the book I used to design the bandpass filters. If you want the filters to have a wider passband, it means lowering the "peaking" of the filter which is termed "Q" in the design equations. I copied the sections of the book related to the bandpass filter and attached them. Follow the equations and they should work. You don't have to use an LM387 op amp, I used LF356 and TL084 FET op amps. Any good bandwidth op amp should work.
 

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audioguru

Well-Known Member
Most Helpful Member
I think that an audio level meter needs to have a peak detector circuit (as shown in the datasheet for the LM3915) for it to clearly show peak levels since without it the peaks are a dim blur.
 

randominality

New Member
Well all that value selection stuff seemed a bit too involved for me since I didnt understand what half of the symbols stood for and I didnt really know what I should set the bandwidth at since it is a logarithmic scale. However I did find this, which I should just be able to use the first half of and use the output of each filter seperately. However since this circuit is obviously designed with high output quality in mind which I really dont need, could someone help me trim it down to make it a bit simpler.

The document also has useful information on the calculation of the values and puts it in a simpler way than the various other documents I have found.
 
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bountyhunter

Well-Known Member
Well all that value selection stuff seemed a bit too involved for me since I didnt understand what half of the symbols stood for and I didnt really know what I should set the bandwidth at since it is a logarithmic scale. However I did find this, which I should just be able to use the first half of and use the output of each filter seperately. However since this circuit is obviously designed with high output quality in mind which I really dont need, could someone help me trim it down to make it a bit simpler.

The document also has useful information on the calculation of the values and puts it in a simpler way than the various other documents I have found.
The bandpass filters shown using a single op-amp per band are as simple as it gets for a bandpass filter.

Well all that value selection stuff seemed a bit too involved for me since I didnt understand what half of the symbols stood for and I didnt really know what I should set the bandwidth at since it is a logarithmic scale.
If you know the center frequencies (Fo) you are going for on each filter (you listed them above) just set the bandwidth of each so it goes about 1/2 way to the next higher filter center frequency (Fo).
 
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bountyhunter

Well-Known Member
I think that an audio level meter needs to have a peak detector circuit (as shown in the datasheet for the LM3915) for it to clearly show peak levels since without it the peaks are a dim blur.
I agree with that.

But as to the OP's first post:

OK, first post - So I am going to be doing a bit of case modding and thought it would be cool to incorporate a VU meter / spectrum analyser into the mod,
I should have probably pointed out that a VU meter and spectrum analyzer are two entirely different things. I thought you were building both, and the info I posted was related to the spectrum analyzer.
 

randominality

New Member
I thought you were building both, and the info I posted was related to the spectrum analyzer.

Well I dont really know the difference so sorry for all the confusion, but isnt a spectrum analyser just a load of VU meters that each measure a part of the spectrum (or a VU meter hooked up to some sort of variable filter that can sweep through the spectrum).

Also, with the simplification, I was thinking more about the bit before the filters (the input buffer I think its called) and if it is strictly necessary.
 

audioguru

Well-Known Member
Most Helpful Member
Each filter circuit must be fed from a very low impedance. The output of the input buffer opamp has a very low impedance.
 

bountyhunter

Well-Known Member
Well I dont really know the difference so sorry for all the confusion, but isnt a spectrum analyser just a load of VU meters that each measure a part of the spectrum (or a VU meter hooked up to some sort of variable filter that can sweep through the spectrum).
No, a VU meter actually measures and displays what is called "volume units", hence the name VU. It's supposed to be a representation of the signal level's loudness. To do that, the whole bandwidth of the audio signal is processed and "weighted" so that that the meter reading will reflect perceived loudness to the ear.

A spectrum analyzer just separates out the signal into frequency bands and displays the level, usually no weighting is applied it's just the voltage level of the signal. Both a VU meter and spec analyzer display information related to the signal level, but not really the same things.
 
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randominality

New Member
Ok so how does this look:
Basically I will use the first half of that equaliser circuit so it will look like this:

8985-291jaj8.jpg


Then the ouput of each filter will go into this circuit:

8986-2r5gup5.jpg


So, is there anything I am missing now and what improvements can I make?
 

randominality

New Member
Ok so I guess this should work:

Firstly I'll just use the first part of that equaliser circuit since it has an input buffer and the right filters, which will therfore look like this:
8995-291jaj8.jpg


Then each output from the filters will go into this circuit:
8996-2r5gup5.jpg


Is there anything that I am missing now or that should be changed?
 

audioguru

Well-Known Member
Most Helpful Member
You need to add a coupling capacitor from the +6V output of each filter opamp to the 0V input of the peak detector transistor.

You have the display IC set so that 9.9V on its input pin 5 lights the top LED and you have a voltage divider made with two 10k resistors feeding it. Then it needs 19.8V from the peak detector transistor.

But your peak detector will have a max output of only 3.6V.

To fix it, remove the two 10k resistors at pin5 and change the 2.7k resistor to 390 ohms.
 
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