Sound Level Indicator

[audioguru2]

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.

 

[jon1]

Ok.
What's shielded audio cable? And what's the use of it?

J.

 

[audioguru2]

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.

 

[jon1]

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...)

 

[jon1]

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.

 

[audioguru2]

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.

 

[jon1]

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.

 

[audioguru2]

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.

 

[jon1]

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.

 

[audioguru2]

I am glad that you got it to work properly.

 

[jon1]

Where is R18 on the Veroboard circuit?

J.

 

[audioguru2]

Where is R18 on the Veroboard circuit?

It is on the left side of the LM3915 and it connects to pin 1. It should be 22k for two LEDs in series but 10k also works to keep the first LED from lighting dimly when it should be off.

 

[jon1]

Ah ok! Now I understand why LEDs connected to pin 1 were always lighting...

Thanks,
J.

 

[jon1]

Why is there an arrow-like thing on the (9,17) point? And what does it mean?

J.

 

[audioguru2]

R17 charges my 7.2V rechargable battery when the AC adapter is plugged into the adapter jack. The arrow on the adapter jack shows which way its built-in switch moves when the adapter's plug pushes it.
Then the battery is charged through R17 and the circuit gets +9V from the AC adapter.
When the adapter's plug is out then the switch in the jack moves to short R17 so the circuit gets the full power from the battery.

 

[jon1]

Do you think there might be any problem on your Veroboard schematic? Because it doesn't work...

J.

 

[audioguru2]

I built my project according to my Veroboard layout and it works perfectly.
What is the problem with yours?

 

[jon1]

I don't know, that's the problem... There's too many weldings...

J.

 

[audioguru2]

Hi Jon,
You said that it was too sensitive. Did you try changing R5 to 10k to reduce the sensitivity 20dB?

You said the voltages at the pins of the LM3915 without a signal were 0.04V for pin 5 and 0.5V for pin 6. Pin 6 is correct but pin 5 is a little high that would make the LEDs #1 and #2 light.

You said that your battery was weak. Did you replace it with a new one?

 

[jon1]

Yes I did all those things, but I think the problem is on my weldings, maybe I connected some wires not correct or I connected some wires that should not be connected together, I don't know. I'll check them...

J.