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Sound Level Indicator

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Hi Giroup,
Why don't you make my Sound Level Indicator project?
http://www.electronics-lab.com/projects/motor_light/009/index.html

I used 20 very bright red LEDs but it would also look good with ultra-bright blue or green only 10 LEDs.

post-1706-1427914271678_thumb.jpg

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Hello!

I have some questions about this circuit:

1)Is IC1 for amplifying the electret mic, because the signal without the IC would be too weak? Can i have a such mic which does not have to have a preamp?

2)Is IC1 supplied with +9(+7,5)V or with +5 V? MC33172P is single supplied, this means that it is amplifying only positive signal (AC can't come out from there)? The AC is not needed because when we want the LED-s to shine, then it is enough from half-sine (pulsating DC)? Is it so?

3)Q1 and Q2 have supply also +9V not +5V?

napos

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Hi Napos,
Welcome to our forum. ;D


Hello!

I have some questions about this circuit:

1)Is IC1 for amplifying the electret mic, because the signal without the IC would be too weak? Can i have a such mic which does not have to have a preamp?

The electret microphone has a low output level. IC1 amplifies it, rectifies it and drives the transistors.

2)Is IC1 supplied with +9(+7,5)V or with +5 V? MC33172P is single supplied, this means that it is amplifying only positive signal (AC can't come out from there)? The AC is not needed because when we want the LED-s to shine, then it is enough from half-sine (pulsating DC)? Is it so?

IC1 has a 7.2V to 9V unregulated supply.
Any opamp will work with a single supply if its input voltages are near half the supply voltage, and then amplify AC voltages. I used an MC33172 because its inputs work at the same voltage as the negative supply, and the output voltage can go nearly down to the negative supply. The negative supply is ground in my circuit.

3)Q1 and Q2 have supply also +9V not +5V?

Yes.

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Thanks for reply

I have one more question, resistor R16 has value 10/1W, this means that the resistor is 10 Ohm and 1W?


Update:

This was already mentioned in the text, my bad

"...LM3915 gets hot so the 10 ohm/1W resistor R16 shares the heat..."

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I have one more question, resistor R16 has value 10/1W, this means that the resistor is 10 Ohm and 1W?

Correct.

This was already mentioned in the text, my bad

"...LM3915 gets hot so the 10 ohm/1W resistor R16 shares the heat..."

Yes. With a 9V supply, 10 LEDs drawing 27mA each through the LM3915 and 2V LEDs, the LED driving circuit must dissipate 7V x 270mA= 1.89W. The LM3915 will overheat.
The 10 ohm resistor will dissipate 270mA squared x 10 ohms= 0.73W, so the LM3915 dissipates the difference of the total which is 1.16W.

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I tried. Not bad. I also used each input to power a transistor to preform other
task a deferent adio levels. For exaple I used a older reapet toy to say be
quit when the noise level got to loud. A transistor and a relay did the trick.
Alot of fun guru.

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Hi Helio,
Yours talks and says, "Be Quiet!" when the noise is too loud? Great! ;D ;D
Mine just blinks its very bright LEDs with the sounds.
Mine has a rechargable 9V battery (actually 7.2V) so it is also portable.

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Hi Slackjack,
An LM3914 has equal 10% steps between its LEDs in a linear sequence. Its range is only 10 times.
An LM3915 has 3dB between each step in a logarithmic sequence. Its range is 31.6 times.
Your hearing sensitivity is logarithmic in order for you to hear very low level sound and very loud sound.

You wouldn't hear a 10% change in level, but 3dB (1.414 times) is noticable.

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Hello!

I have some questions about this circuit:

1) what are the transistor using for ???

2) how to evaluate the value when the light 10th is on ???

3) the range for measurement is from 0db to 30 db ????

thank you for ur help............

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Hello!

I have some questions about this circuit:

1) what are the transistor using for ???

2) how to evaluate the value when the light 10th is on ???

3) the range for measurement is from 0db to 30 db ????

thank you for ur help............


Hello!

I have some questions about this circuit:

1) what are the transistor using for ???

2) how to evaluate the value when the light 10th is on ???

3) the range for measurement is from 0db to 30 db ????

thank you for ur help............

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I have some questions about this circuit:

1) what are the transistor used for ???

2) how to evaluate the value when the light 10th is on ???

3) the range for measurement is from 0db to 30 db ????


The text of my project answers your questions:
"4) Three 2N3904 transistors are used as emitter-followers:
a) Q1 is inside the negative feedback loop of the 2nd opamp as a voltage reference for the other two transistors. Hopefully the transistors match each other.
b) Q2 emitter-follower transistor quickly charges C8 which discharges slower into R13 and is used as a peak detector.
c) Q3 transistor is the automatic gain control. It is also a peak detector but has slower charge and discharge times. It drives the comparators’ resistor ladder in the LM3915 to determine how sensitive it is. R15 from +5V is in a voltage divider with the ladder’s total resistance of about 25k and provides the top of the ladder with about +0.51V when there is a very low sound level detected. Loud sounds cause Q3 to drive the top of the ladder to 5.1V for reduced sensitivity."

The 3rd transistor provides automatic gain control so the max indication of the LM3915 is increased from 30dB to 50dB. Because of this AGC you don't know what is the actual sound pressure, but the LEDs always indicate without staying at the end of the scale during loud sounds.

The actual sound pressure level for quiet sounds depends on the sensitivity of your mic. My mic is very small from a cell phone so maybe is not very sensitive. I designed the gain of the circuit to be high enough so it indicates on a few LEDs a pin dropped on the floor a few meters away and people whispering about 3m away. I didn't measure the actual sound pressure levels.

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May I know what is c2 for in the first op-amp?

It blocks DC so that IC1a has a DC gain of 1 like a follower with its DC output voltage averaging +2.5V, but allows an AC gain of 101.

If R6 is connected to ground then the DC gain of the opamp would be 101 and its output would be saturated at near +5V, because R2 and R3 bias its input at +2.5V. With its gain of 101 its output would try to be +2.5V x 101= +252.5V!

post-1706-1427914324211_thumb.png

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Hi AudioGuru,

I would like to know what to do if I want the range to be from 30 dB to maybe 120 dB?
And can you provide the image of the back of you circuit board, to see how the connections are done?

J.

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Hi Jon,
My circuit is very sensitive since it indicates a pin dropping on the floor fairly far away. It indicates that my quiet refrigerator is running. It indicates that I am breathing quietly. Its range is probably from 45dB to 95dB.

If you have it as sensitive as 30dB then it would show noises from way down the street and show its own internal noise. It would need to have the input offset voltage nulled on its first and second opamps.

120dB is extremely loud and hardly ever occurs. An ordinary electret microphone would be overloaded by such a loud sound.

I used stripboard to make the circuit board. Its component side is shown in the project with some track cuts and a few jumpers are shown.

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Hi AudioGuru,

It's me again. I want to know how many LEDs will light for how much dB if the range of the Sound Level Indicator is 45 dB to 95 dB? And another thing, who can it be 45 dB to 95 dB, the LM3195 only has 10 outputs in 3 dB steps for a total of 30 dB?

J.

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Hi Jon,
Yes the LM3915 has a range of only 30dB. But I designed the circuit to have an additional 20dB of automatic gain control.

For low level sounds, the input is indicated directly on the LEDs of the LM3915.
For low level sounds, the Rhi voltage of the LM3915 is set by the voltage divider of R15 (220k) and the LM3915 divider's resistance of 28k. So the voltage is only 0.56VDC.

For loud sounds, Transistor Q3 increases the voltage of Rhi to about 5.6V which is 20dB higher.

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Hi Audioguru,

It's me again after a long time, but I want to know what do you use to supply the circuit with current in the +9 adapter? Can't I use a 9 volt battery? Because I don't have any 7.2 volt battery.

Jon

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Hi Jon,
My Sound Level Indicator uses a "9V" rechargable Ni-Cad battery that is really only 7.2V because it has six 1.2V cells. It is almost always fully-charged at 8.5V.
The Ni-Cad battery powers it for about 20 minutes with the brightness control at max (26mA for each LED) and a loud signal so most LEDs are turned on.

My AC-DC adapter is 9VDC/350mA. A 500mA adapter would also work fine.

An ordinary 9V alkaline battery will power it for about 1.5 hours at max brightness with most of the LEDs turned on. The 9V alkaline battery will last for about 62 hours with no sound so that all the LEDs are off.

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