# Band Pass for musical notes

Discussion in 'General Electronics Discussion' started by jshoe, Apr 21, 2011.

1. ### jshoe

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Apr 21, 2011
I'm making a circuit which takes the audio out from my laptop, and uses it as an AC power source. It then passes only frequencies corresponding to the notes A3 to G4, after which it goes through a diode rectifying bridge, and is sent to specific LEDs depending on the frequency. The function of this device should be so when a note (or it's sharp) is played, it activates a certain combination of 3 colors of lights (x, y, z, xy, yz, zx, xyz). So far this only exists on paper, but first I want to know two things:

1: Are there any fundamental flaws in my idea?

2: How do I make a simple, non-amplifying bandpass filter, and what is the equation to calculate the values of the parts when all I know is the frequency?

Thanks.

Last edited: Apr 22, 2011
2. ### Resqueline

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Jul 31, 2009
1: No. I'd call it a 7 channel binary spectrum analyzer. There will be a challenge in powering it entirely from the audio though.

2: It sounds like you need a simple high-Q L-C filter for each note. Fo=1/(2*pi*sqr(L*C))

Last edited: Apr 22, 2011
3. ### jshoe

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Apr 21, 2011
Then where should I put a supply voltage?

And what's Fo?

And what does this have to do with binary?

Last edited: Apr 22, 2011
4. ### Resqueline

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Jul 31, 2009
The filters may be unpowered, but the decoding and driving of the LED's may require external power.
It depends on the audio power level available from the computer. It's usually no more than 0.3V while the IC & LED's require over 3V, so some multiplication must be done.

Fo = Center frequency

I'm not musically literate so I may be mistaken here but afaIk there are 7 notes (as follows) in the interval you stated.

A3=220.0001Hz

B3=246.9417Hz
C4=261.6256Hz

D4=293.6649Hz

E4=329.6277Hz
F4=349.2283Hz

G4=391.9955Hz

You have only 3 lamps to represent these 7. Also, you state that your lamps x, y, z are to light in a x, y, z, xy, yz, zx, xyz pattern. This is called a binary representation.

After rectification/detection you would use an octal to binary converter IC before driving the LED's. (Octal = 8 = numbers of states including zero.)

5. ### jshoe

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Apr 21, 2011
Why do I need an ic? I don't have one in my plans. I go from the rectifier all to the anode, then from the cathode to the rectifier (with a resistor between of course).

6. ### davennModerator

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Sep 5, 2009
and how are you going to detect the different frequencies with a basic rectifier diode?? It is unable to perform that function

you need IC's maybe several because you need to separate out (detect) the various frequencies you want to use to light the leds. This, as Res has hinted at, will require some sort of spectrum analysis/multiband filter system.
Think of a graphic equaliser section of a stereo system. Say one that has 7 channels( frequencies) the 7 that you are interested in. but instead of the outputs of each stage of the equaliser being recombined and fed to the audio amplifier, you will need to keep them separate where their signals can be fed into the following stages and to finally drive the LEDs. I would be suprised if you could get away with anything less than ~ 6 IC's and you are going to need an external power source to power all that electronics ... there isnt enough voltage available in the audio signal to do all that
There will be multiple Op-Amps in the equaliser/analyser stage alone.

its going to be a pretty serious project. Not impossible, just quite complex.

cheers
Dave

Last edited: Apr 22, 2011
7. ### jshoe

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Apr 21, 2011
I was going to have it split, and go through band pass filters, then go through a rectifier bridge, then to the LEDs. What more do I need?

8. ### Resqueline

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Jul 31, 2009
You need to define what you want to do. For example: do you want to detect all 7 notes, or just 3, or 3 groups of those 7? It's called communication, and is quite vital.

9. ### davennModerator

13,785
1,936
Sep 5, 2009
honestly its really not that easy. how are you intending to split all the various frequencies ? have you got circuits for band pass filters that have the narrowness to let the desired freqs through and not the others ?
and as we have already said... there is not going to be enough power to drive the LEDs

Dave

Last edited: Apr 22, 2011
10. ### jshoe

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Apr 21, 2011
I'm going to split it then filter it.

_______Filter
|______Filter
|______Filter
__|______Filter
|______Filter
|______Filter
|______Filter

11. ### Resqueline

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Jul 31, 2009
From that I deduce that you want to detect all 7 notes with 7 filters. Now, what does your plan say about encoding those 7 signals into 3 signals?
Additional question: will there ever be more than one note present at a time?

12. ### jshoe

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Apr 21, 2011
No only one note, and depending on that note, different combinations happen. Only one filter is active at a time, which leads to one active rectifier and leads to anywhere from 1x to 3x LEDs.

Last edited: Apr 22, 2011
13. ### Resqueline

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Jul 31, 2009
Ok, that's a relief to hear. But, since you don't seem to relate to anything outside your plans (or more than one thing at a time for that matter) I'll just have to ask again: