lm 3915 vu meter problem.

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Mooly

Mooly

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Yes, the top wire is the positive rail. That goes to pin 7 of the opamp. Pin 4 of the opamp connects to the negative rail (which is really ground or 0 volts here). Its confusing because we have the pins on the LM3915 as well which happen to be similar.

The blue triangle is a just a ground symbol that the circuit simulator needs to work with.
 
Mooly

Mooly

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Michael Bean said:
But if the supply voltage isn't high enough for the designed circuit, the drive current for the LEDs won't be high enough without adjusting resistor values.

Mike
Click to expand...

The data sheet reads that the outputs are current limited and programmable and so supply variation shouldn't alter things so much :) That's how I take it. 9volts, 12 volts, 15 volts, all are plenty high enough for this. The calculated current should be independent of supply voltage because it uses the internal bandgap reference as its starting point.
 
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lukutis

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i think u didint understand my question. I mean the top wire of the opamp which is positive so i can simply connect this to my positive rail? i mean this http://d32zx1or0t1x0y.cloudfront.net/2009/06/breadboard_3_lrg.jpg
And the bottom wire of the opamp in this circuit is connected to the pin 2 and pin 7 . so can i connect to the negative rail in the photo i shown you?
and about that triangle - should i connect it with something or i just leave it alone?
 
Mooly

Mooly

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Yes, that is exactly what you do. Pin 7 of the opamp goes to the positive rail. The bottom wire of the opamp that goes to ground is pin 4 of the opamp. That connects to your ground rails. The triangle is just a symbol for the simulator program so ignore it.

That's it for today :)
 
Mooly

Mooly

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Yes, that's it. Remember that diagram didn't scale correctly and the vertical traces from pin 7 and pin 4 are missing. Pin 7 to supply, pin 4 to ground.

For the opamp just get the cheapest one. They are just different manufacturer codes. uA used to be Fairchild and LM National Semiconductor.
 
Mooly

Mooly

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Like that :)

The striped end of the diode is the cathode and goes to C2 etc. I always found that very confusing when I first started out with electronics because a certain magazine (Practical Wireless) always drew diodes with a plus symbol on the cathode, which I found very confusing.
 
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lukutis

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nevermind. decided to build everything from zero. so first i will try building that opamp.
http://www.diyaudio.com/forums/atta...20987547-lm-3915-vu-meter-problem-circuit.png
i will go for this one because it seems easiest but i got some questions:
1.That capacitor C1 positive connects to pin 3 of the opamp and the negative goes to positive side of battery?? I dont get it becuase in both sides are those 2 circles with + and - signs which confuses me
 
Mooly

Mooly

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I'd forgotten where we were up to on this :)

The easiest way to test any circuitry is with an oscilloscope, it really is an essential tool of the trade.

Just looking at the circuit again and you could try this. You could try making C2 larger, say 10uf and C3 quite a lot bigger to pass the 'slow moving peaks' to the LM3915.

You can do a basic test of the opamp by measuring the DC voltages. You should have around half the supply voltage on the output pin.
 
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