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Sound Level Indicator

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Thanks for the image, it's a bit better, and now I'm done with the circuit but when I supply it with current every LED lights and there's no sound around! What might be the problem? (And what's the order for the LEDs, is it 10 to 18 on the LM2915 or is it 18 to 10?)

J.

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when I supply it with current every LED lights and there's no sound around!

Did you substitute any parts? The opamps?
Without a signal, pin 5 of the LM3915 should be very close to 0V. The violtage rises as the sound picked up is louder.
Without a signal, pin 6 of the LM3915 should be about 0.47V. Then the 10th LED should light when the voltage on pin 5 is 0.47V or more, and the 1st LED should light when the voltage at pin 5 is 0.015V.

what's the order for the LEDs, is it 10 to 18 on the LM2915 or is it 18 to 10?

The datasheet for the LM3915 shows the order of the LEDs. Pin 1 is the most sensitive LED and pin 10 is the least sensitive.

J.

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I don't think I substituted any parts. When I plug the battery to the circuit every LED works (even when there's no signal...) except the ones connected to the 10th pin on the LM3915. I looked around and I think it's maybe because of the 3rd pin of the LM3915 connected directly to the +9V, because when I take it off no LED works. The brightness pot and the mode switch works fine, but when I turn the brightness pot to the max some LEDs stop working (even when there's no signal...) I think that there's no current passing after the LEDs and I don't know why...

Thanks,
J.

(Is there any e-mail to contact you, because I have to finish this project fast and you only seems to check this forum twice a day and I really need your help)

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Hi Jon,
Pin 3 of the LM3915 is its positive power supply input. It won't work if it is disconnected.

Measure the DC voltage at pin 5 and at pin 6 of the LM3915 and see if they are what I said in my last post.
Also measure the regulated 5V supply to see if the regulator works properly.

When you turn up the brightness then the battery current becomes 270mA which is too much for some 9V batteries, I think its voltage drops too low with such a high current. Measure tha voltage of your battery with the brightness control at max.

My Ni-Cad battery can supply the high current for about 20 minutes. A brand new 9V alkaline battery should be fine for nearly 1.5 hours.

I don't carry e-mail around with me. I check replies about a few times per day. 

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Ok I'll check the voltage for the pins. And maybe the problem is in the transistors' connection. Where do I connect  the + of Q1 and Q3, on the +9V or the +5V? And another thing, I connected the LM2931AZ-5.0 on the same line as the LEDs, so do I have to supply it with current with the +9V? The same thing with the 10/1W resistor. I'm going to check it...

(Another question, what does all the red X's on your Veroboard layout means?)

J.

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Where do I connect  the + of Q1 and Q3, on the +9V or the +5V?

The "+" is +9V. The 10 ohm resistor in series with the LEDs, C6, the input of the LM2931, pin 3 of the LM3915, pin 8 of the dual opamp and the collectors of the transistors all connect to +9V.

I connected the LM2931AZ-5.0 on the same line as the LEDs

Then you have shorted the 10 ohm resistor. Connect the 10 ohm resistor in series with the LEDs as shown on the schematic.

what does all the red X's on your Veroboard layout means?

Each red X is where the track is cut with a drill bit. Here is a picture:

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Ok now, does that mean that R1, R2, R15 and C3 are connected on the line of the LM2931's output (so the +5V)? If yes then I tried it but it does not work...

J.

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Ok now, does that mean that R1, R2, R15 and C3 are connected on the line of the LM2931's output (so the +5V)? If yes then I tried it but it does not work...

Yes, but also R15 must be connected to +5V or many of the LEDs will light, because then pin 6 of the LM3915 will not have 0.47V but instead it will be 0V.

Please measure the DC voltage on pin 5 and on pin 6 of the LM3915 when there is no sound and report the measurements here.

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I replaced the dual opamp but it didn't change a thing, one the LEDs works all at once or nothing works and the battery gets hot. I'm still trying to get the voltages...

J.

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Since R15 connects to +5V then it makes a voltage divider with the resistors in the IC which makes pin 6 about +0.5V. Without a signal, pin 5 of the LM3915 is supposed to be 0V. Then none of the LEDs will be on. When a signal causes +16mVDC at pin 5 then the first LED will turn on. 22.6mVDC turns on the second LED and 32mVDC turns on the third LED, etc.

All the LEDs turn on when the voltage on pin 5 of the LM3915 is slightly higher than the voltage on pin 6. If pin 6 is 0V (shorted to ground or R15 does not connect to +5V) then all the LEDs might be on all the time.

If the microphone doesn't have shielded audio cable connecting it then its wiring will pickup mains hum which will give an indication on the LEDs. Even though the wires connecting my microphone to the circuit board are only about 3cm long, I used shielded audio cable to make the connections.

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Shielded audio cable is used to join a microphone to a preamp and to join any audio source to an amplifier. It has a center signal conductor that is surrounded by the grounded shield.
It stops mains hum and other electrical interference that would be picked up by ordinary wires.

I can buy a reel of it but frequently I cut off RCA connectors from cables that come with audio equipment or I buy the cables with RCA connectors at The Dollar Store.

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Ok, I just completed the circuit another time. Now the LEDs seems to be reacting to the electret mic, but in another hand it works even when I remove the mic... The LEDs are lighting all at once and then they gradually stop and then gradually works, and it does that all time, none stop... I just don't understand what might be the problem... I'll continue checking it.

J.

(And I'll soon have the voltages...)

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The Sound Level Meter works! The only problem is that it's too sensible, I don't think it's range is 45 dB to 95 dB, all the LEDs are reacting for all and every sound. Is there a way to reduce the amplification? And the other little problem is that LEDs connected to pin 1, 2 and sometimes 3 and 4 on the LM3915 are working all the time, even when there's no signal...

Oh and the voltage of pin 5 when there's no signal is 0.04V and pin 6 is 0.5V.

J.

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When my rechargable battery is nearly dead all the LEDs light then turn off then light then turn off over and over. Maybe your battery is weak. All 10 LEDs use a high current and a weak battery cannot supply a high current.

Without a signal, R7 holds the pin 5 of the opamp IC1b at 0V. Then the emitters of the transistors should also be 0V because they are the same and have their bases connected together. The emitter of Q2 connects to pin 5 of the LM3915.

Something is causing a voltage 0f 0.05V instead of 0V.
Maybe the microphone's wires are not shielded and are picking up mains hum or radiation from a computer or its monitor or from a TV.

My Sound Level Indicator has no LEDs on when there is no sound. An extremely low sound level (about 45dB) lights the first couple of LEDs then a little louder lights more. When nearly all the LEDs are lighted then the automatic-gain-control requires more and more sound level to light all LEDs.

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Ok, but I think the range of this Sound Level Indicator is between 15 dB and 45 dB because a pin dropped at one meter is 15 dB and the LEDs react to it. It even detects someone walking five meters away in another room, with the door of my room closed! And yes, all the LEDs light and turn off when the battery is week, I verified it. I think I can change the amplification by reducing the value of R5 or R8.

J.

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I made the circuit very sensitive. The LEDs bounce when the TV or stereo is playing.

The automatic-gain-control reduces is sensitivity 20dB when there are loud sounds.

If the R5 value of 100k is changed to 10k then it will be 20dB less sensitive.

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Thanks for everything! Everything is working fine till now! (and I hope not to have any more problems, and if I do I'll need more of your help...) But now I just have to understand your Veroboard drawing so I can reproduce it and finalize it all...

Thanks again AudioGuru!
J.

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