[moved] Query about VU meters (led bargraph)

the 20k pot on the opamp isnt that the gain control for the opamp?

I increased the gain of the opamp then you can use its 20k volume control as its gain control.
Your last schematic is missing an important capacitor on the opamp.
I looked at the simple peak detector circuit and now I think it will not work. The datasheet for the LM3915 says "DC couple its input" but you can't because it has a DC blocking capacitor that will get charged non-symmetrically because the base-collector junction of the transistor will conduct when the input signal swings negatively.

ok the this revised version with the lm358 should work then?

based on my observation your eagle has more libraries and seems to do certain things differently like labeling, where can I get some good libraries I dont actually have a pot that I can use the the one I use in the schematics I sent is the dummy symbol wires aren't actually connected to it.

We usually use a VU meter as a level indicator so that the input to the power amplifier is not going to be excessive. Audio power amplifiers are designed to be as linear (const gain in the given freq range) as possible and such amplifiers, if run with a low signal, will waste energy and if run with a high signal will produce distortion. Music signals have wide frequency range and also have large range of amplitude. It is therefore useful to know the actual level of signal being fed into the power amplifier. Automatic level control circuits, like automatic gain controls, have limited use because they have a short time constant. The signal level indicators (VU meters) are commonly part of the pre-amplifier systems and often the power amplifier has practically no display worth the name. The bass and treble are processed in the beginning. And, of course, you will adjust the volume of the amplifier output with your ears and not with your eyes.

Enzy said:

ok the this revised version with the lm358 should work then?

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Yes.
Do you want the LED VU meter connected to the main 5k volume control then when it is turned down most LEDs will be turned off or do you want it connected to the input of the 5k volume control so it always shows the levels of the input signal? Or do you want to attenuate the speaker signal and connect it to the LED VU meter to show amplifier output power? The LM3915 has the LEDs 3dB apart so the 10th can show 400W, the 9th would show 200W, the 8th 100W, the 7th 50W etc.

based on my observation your eagle has more libraries and seems to do certain things differently like labeling, where can I get some good libraries I dont actually have a pot that I can use the the one I use in the schematics I sent is the dummy symbol wires aren't actually connected to it.

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I don't use Eagle to do schematics, I use Microsoft Paint program instead where I can copy and paste parts from datasheets, from your PDF schematics or from any other parts library.
Your Eagle schematic is missing a dot where there is a connection in most places so it is confusing when a line crosses over or connects to another line.
I copied and pasted your pot into Paint then turned it 90 degrees so it is standing up awake instead of lying down sleeping. But instead I could have copied and pasted a pot from another schematic.

Audioguru said:

Yes.
Do you want the LED VU meter connected to the main 5k volume control then when it is turned down most LEDs will be turned off or do you want it connected to the input of the 5k volume control so it always shows the levels of the input signal? Or do you want to attenuate the speaker signal and connect it to the LED VU meter to show amplifier output power? The LM3915 has the LEDs 3dB apart so the 10th can show 400W, the 9th would show 200W, the 8th 100W, the 7th 50W etc.


I don't use Eagle to do schematics, I use Microsoft Paint program instead where I can copy and paste parts from datasheets, from your PDF schematics or from any other parts library.
Your Eagle schematic is missing a dot where there is a connection in most places so it is confusing when a line crosses over or connects to another line.
I copied and pasted your pot into Paint then turned it 90 degrees so it is standing up awake instead of lying down sleeping. But instead I could have copied and pasted a pot from another schematic.

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Ok so do you actually make pcb and things like that if so what would you use? I am trying to use eagle to do it now but have some issues as I go along.


"I would want to want to attenuate the speaker signal and connect the vu to it so I can see the amplifier output."

I have made many single custom-made audio circuits during my career. For most of them my prototype was built on stripboard (Veroboard) and was the item that was sold but a few had my schematic and stripboard prototype sent to a local pcb design, manufacturer and component installation place who made tens of thousands of my project. I never used Eagle.

To have the LED VU meter accurately show amplifier output power you should have an adjustment on the attenuator so that you can set the LED level for the amplifier's maximum output power. To set the level you see on an oscilloscope or hear the amplifier's clipping distortion.

ok so for one of the resistors attenuator I should use a pot. I have a hand held scope dso nano v3 that I could use to check waveform for clipping.

I will do a final drawing soon and then if all is well i will see how soon i can build it.

One more thing: you need to ensure that the lowest voltage you are using will light up one LED. The driver is a log scale driver and the 10 LEDs represent a dynamic range of 2^10=1024 (fold)- in other words the top LED (10th position) indicates about 1000 time more than the lowest level. Human ear is more sensitive than that and you may decide to set the highest level in that case.

I do not remember whether this chip has the peak hold option, but the input must be high frequency filtered (only for this display driver) so that high frequency high intensity signals are not seen - they may cause a visual distraction- the display will follow a more familiar needle meter style motion...

Of course these are matters of personal choice...

About 52 years ago National Semi made a module with this LM3915 IC as a blob-on-board and a 10 LED strip. I used two in series for a 60dB range and connected it to the output of my stereo amp that produced 18W into 8 ohms per channel. I did not use a peak detector then peaks and short duration sounds were a dim blur but the average levels were fairly bright.

The 52 years old LEDs were not good so about 10 years ago I made it with modern LEDs and an LM3915 IC and added a peak detector similar to this one. It holds peak levels for about 30ms which is long enough to see at full brightness. I increased the range of the IC from its original 30dB to 50dB. Its input is a microphone.

Audioguru said:

Its input is a microphone.

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With a microphone as input, and with some calibration, it will be an effective sound pressure meter...

c_mitra said:

With a microphone as input, and with some calibration, it will be an effective sound pressure meter...

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That is why my project is also powered from a rechargeable 9V Ni-MH battery.
Yesterday a local grocery store celebrated 35 years and had a live band. I measured peaks of 120dB about 5m in front of them. My electret mic can from a cell phone and probably is overloaded above 120dB. I never tried it inside a drum or piano.

sorry for the late reply these are my final drawings.

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sorry for the late reply these are my final drawings.

The input of the LM358 peak detector will blow up to kingdom come because your circuit is missing an attenuator to reduce the 55V peak of the amplifier's output to 12V when the amplifier is powered by your dual 40VAC transformer. R1 is not needed.

Why do you think the amplifier has two outputs? Since the amplifier is Chinese then there is no schematic or connections diagram for it. But the IRS2092 datasheet shows only one output.

What will you do with the unused opamp in the TL072? Instead you can use a single opamp which is a TL071.

the first schematic on this topic I showed had a 10k resistor going to pin 5 of the Lm3915 and another 10k resistor from the same pin 5 going to GND and you said that was the attenuator so I assumed putting back those components in the circuit would be the attenuator if not then what is it that I should do to reduce the 56v. ( So what I did was the place them at the start of the peak detector circuit, and it seems that was wrong so how would I do it.)

You asked why do I think the amplifier has two output, it has 2 screw terminals just like any other amplifier I have ever seen in my life that you connect 2 speaker wires to, so that connector with 2 screw terminals is where I planned to connect the vu meter and also the 2 wires from the speaker.

I keep using tl072 because I already have the chip if its a case where there will be a problem using it in the way I have it here then I would have to search for a tl071 as you said. ( so is it ok to use the tl072 or do I have to use a single opamp?)

Enzy said:

the first schematic on this topic I showed had a 10k resistor going to pin 5 of the Lm3915 and another 10k resistor from the same pin 5 going to GND and you said that was the attenuator so I assumed putting back those components in the circuit would be the attenuator if not then what is it that I should do to reduce the 56v. ( So what I did was the place them at the start of the peak detector circuit, and it seems that was wrong so how would I do it.)

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An attenuator is a resistor in series with the input (that you did not have) that feeds a resistor from the input to ground, making a voltage divider. The first schematic shows a series 10k feeding a 10k to ground that reduces the input voltage to half. If it did not attenuate enough it does not matter because the datasheet for the LM3915 shows a maximum allowed input of 35V. The maximum allowed input of the LM358 is only its supply voltage which will be 12V in your circuit.
You need to reduce 56V to 12V so the input resistor must be (56V - 12V)/12V= 3.67 times the value of the resistor to ground parallel with the input 100k resistor. Then the series input resistor can be 33k and the resistor to ground can be 10k (10k parallel with 100k is 9.09K).

You asked why do I think the amplifier has two output, it has 2 screw terminals just like any other amplifier I have ever seen in my life that you connect 2 speaker wires to, so that connector with 2 screw terminals is where I planned to connect the vu meter and also the 2 wires from the speaker.

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This amplifler like many amplifiers has one of the two screw terminals as the amplifier's ground connection to the speaker. You must measure it to find which is the audio output terminal and which is the ground terminal. Some amplifiers have two signal outputs and I looked at the datasheet for the chip on the Chinese amplifier to see that it has only one output, not two.

I keep using tl072 because I already have the chip if its a case where there will be a problem using it in the way I have it here then I would have to search for a tl071 as you said. ( so is it ok to use the tl072 or do I have to use a single opamp?)

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Like the LM358 dual opamp, the unused opamp must be properly disabled to prevent it from oscillating or overheating. Connect the output to the (-) input so it has a voltage gain of 1 then connect the (+) input to 0V.

When you are making something for yourself only, it is usually very tempting to make a one-time stuff that only you will know the details. I do not like this view and I always suggest that you follow some standards and recommendations and conventions. It is always good because your stuff will now work with any other that follows the same standards or conventions. When you make your power supply, it will be convenient to keep a 12V and a 5V outputs in the back of your mind even if you do not need them right now. When you plan for the input of the filter, think of the input at the line level - all audio inputs will (the plugs that take RCA type connectors) and outputs (not power) will then work with any standard audio instruments. It was a pleasant surprise when I found that my phono input has equalization for both american and european standards.

Yes, it will mean a couple of more components on board and a few more simulations but you will know that your device is now fully compliant and you will be sure that it will work even long after you have forgotten what it is all about. It is simply worth it; it is a good habit.

I think you keep forgetting that i'm not a pro at this so the fact that you asked me why its 2 output wires should be obvious from my schematic, because 1 wire is always GND and the other is signal so it couldn't mean for it to be any other way, If I refered to 2 outputs its because I am using 2 amplifiers which would make it 2 terminal blocks with 4 screw terminals.

To test which is ground how would I do that, I a figuring I could test between pin 2 of the IRS2092 and the speaker terminals.

For the Lm358 in my case I would connect pin 7 to pin 2 and then connect pin 3 to pin 4.

for the tl072 pin 7 to pin 2 and pin 3 to pin 4.

Measure the resistance between an amplifier output screw and the input ground on the amplifier to find which output screw is ground (a very low resistance, less than 1 ohm). The amplifier ground must be connected to the ground of your circuit.
Connect a 33k resistor from the signal output of each amplifier to the input of the LM358 circuit (not a pin on the LM358) that has the 10k resistor to ground.
You do not have the datasheets or you do not understand them. For the LM358 and the TL072, pin 7 is the output of opamp #2 and pin 2 is an input of opamp #1.

I checked the datasheet and I told you which pins I would connect to each other I do that as a was to confirm what I plan to do, so I was expecting a confirmation from you.

you said connect the output to the negative input so that why I said I would connect pin 7(output) to pin 2 negative input and then you said connect the positive input to 0v so I said to you that I will connect pin 3( positive input) to pin4(GND or 0v).

I keep confirming the schematic I guess that's annoying now, I have a feeling that the 10k resistor in the schematic should have been placed closer to the input or I am not sure if it makes a difference where its placed once its connected from pin5 to GND.