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3 Channel Spectrum Analyzer - question


Andrei_kiss
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      I am new to electronics and I'm also in first degree at a engineering university. I have been requested to do a project of my choice for a class that I study - Electronic circuits and components.  I was very, very happy when I found a schematic on exactly the thing I desire to do - the 3 channel spectrum analyzer. I don't have any technical background or technical experience but I'm willing to learn some of the basics, and not only, in electronics through this project.
      Now...I have found certain schematics for a single VU meter but I thought that by making individual Vu meters for every channel I also had to make filters to separate the frequencies of the audio signal. But the most easiest to do project involved using a LM3915 for the VU meter, and I couldn't find any on the local market. Anyway...


        My only clear questions , for the moment, are :
1. The  U1A, U1B, U1C, U1D ,in the schematic, are they all LM3900  ?
2. Can I get intouch with the person who posted this project, since I am almost sure I will come up with more question related to the construction of the project?

That was all , thanks for the atention and I'm sorry for the spelling mistakes but english is still my second language and if I said something that was totally rubish, technicaly speaking, please let me know and please bear with me .

Andrei Kiss

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1. The  U1A, U1B, U1C, U1D ,in the schematic, are they all LM3900  ?

One LM3900 has all four opamps inside. They are different from "ordinary" opamps. If ordinary opamps are used then the circuit will need to have some small changes.

2. Can I get intouch with the person who posted this project, since I am almost sure I will come up with more question related to the construction of the project?

We can answer your questions here.

The circuit's schematic has errors. The errors will cause mal-function and might cause damage to its parts:
1) The middle frequency filter's output is connected to the wrong place on the schematic.
2) The middle LED driver should not have pin 8 connected to ground. Pin 8 is its input.
3) Pin 5 on all 3 LED drivers and the resistor and capacitor connected to them should be connected to ground.

I didn't fix the schematic because the ICs are odd and are difficult to get.
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http://www.electronics-lab.com/projects/audio/037/index.html
- here is the project.
It really hit me when I read your reply and fopund out about the problem with the schematic. I have another question related to this project since I am still willing to go on constructing something like this :

1. Can  the circuit be modified so that it will work properly ?
2. Can this be donne without using a LM3900 ? The AN6884 could remain for ease of construction but I'm having a hard time finding a LM3900 on the local market, so I would have to order it if I use it wich involves quite a lot of lost time waiting.
3. If all the above, the proper schematic  and elimination of the LM3900, would be made , the power suply used could still be 12 V ?

Thank you for your time and if somebody if willing to help me build this thing I will forever be greatfull as it would be start of my education in engineering.

Andrei 

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1. Can  the circuit be modified so that it will work properly?

Yes.

2. Can this be donne without using a LM3900?

The circuit can be modified so it can use ordinary opamps. But I looked for the old Panasonic LED driver ICs in two of the largest American electronic parts distributors and they don't have any. It is a big job to redesign the circuit to use ordinary opamps and common LM3915 LED driver ICs.

The AN6884 could remain for ease of construction

It is difficult to get and might be obsolete. The LM3915 would also be easy to use.

3. If all the above, the proper schematic  and elimination of the LM3900, would be made , the power suply used could still be 12 V?

A 9V supply would be better because 12V causes the LED driver IC to heat up a lot.
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      Ok...I think I should start with this : Thanks for looking up for that opamps, audioguru, but even there is a big problem and that is that I don't live anywhere near America. I live in Europe,Romania. And from what I've seen by checking a few electronics shops in the city, what you said about the AN6884 does not seem to be true. I had no problems finding a AN6884 but about the LM3915 and LM3900 I can't say the same thing. The only posibily of purchasing them is by making a special order and therefore the components would be imported from ...I don't know, I think Germany or maybe Hungary as being more closer to Romania.

  So my question this time is :

1. Using a LM3900 and a AN6884, for led control, would make the project to be put in action as in realised ? I don't think that finding the rezistors and capacitors would be a problem.
2. By using a LM3915 would I have to use the AN6884 too ?

Could you clear these things to my knowledge please,because right now I am kind of confused and because of the fact that I would have to order a LM39xx as I said before, if I don't get the right one things could go off the line.
Thaks again for all this info and I hope that my future questions will find their answers with you

Andrei

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After a few hours of searching the internet I have discovered that this BM3900 has no datasheet or its datasheet can't be found with the most know search engines. All I could find, relevant, is this :

http://www.inginerie.protectia-mediului.ro/engineering/electronics/sale/ic.htm

IPRS Baneasa is a company from Romania. I hope I will find some info about this BM3900 and hopefully that it's as good as the original thing.
I'll be back with more questions after I solve this!

Thanks for all until now,
Andrei

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I got the sockets for the 2 CI's. Now another question before I can peacefuly begin asembling...

Can I use the power suply of my computer to power the circuit? 

I mean the cable which powers the harddisk, that type of cable. I have read the 4 "pins" are 5V Null Null 12V . But somebody told me that the voltage is not quite 12V and keeps fluctuating through 13 V .

Is that a problem for the op amps ???

Andrei

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A computer power supply won't hurt the circuit if you connect it with the correct polarity.
The circuit is missing any kind of filtering for the supply so the output might have buzzing from the computer. Add a 100 ohm resistor from the +12V input to the positive supply to the opamps and a 470uF capacitor at the opamps to ground as a filter if it is needed.

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