Sound Level Meter

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This nifty sound level meter is a perfect one chip replacement for the standard analog meters. It is completely solid state and will never wear out. The whole circuit is based on the LM3915 audio level IC and uses only a few external components. This circuit can also be integrated into audio amp projects.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
C1	1	2.2uF 25V Electrolytic Capacitor
C2, C3	1	0.1uF Ceramic Disc Capacitor
R1, R3	2	1K 1/4W Resistor
R2	1	10K 1/4W Resistor
R4	1	100K 1/4W Resistor
R5	1	1M 1/4W Resistor
D1	1	1N914 Silicon Diode
Q1	1	2N3906 PNP Transistor
LED1-LED10	10	Standard LED or LED Array
U1	1	LM3915 Audio Level IC
MISC	1	Board, Wire, Socket For U1

Notes

V+ can be anywhere from 3V to 20V.

The input is designed for standard audio line voltage (1V P-P) and has a maximum input voltage of 1.3V.

Pin 9 can be disconnected from +V to make the circuit use a moving dot display instead of a bar graph display.

Thanks to help from the forum, this circuit has been improved from the original version to include a peak detector for a more stable and viewable output. This page has been updated with the new circuit.

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Comments

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Dimitris Sound Level Meter Sunday, August 08, 2010 8:46:35 AM

I think that u connect 5 with 3. There is no connection between them. 9 is directly connected to 3. (" All LED switches on when power is connected")

anonymous Sound Level Meter Thursday, June 03, 2010 10:48:17 AM

Built this circuit but it does not work properly. All LED switches on when power is connected. Any workaround for this problem?

Eric Sound Level Meter Sunday, April 11, 2010 9:12:47 PM

I built this exact circuit using a 15V power supply and my LM3915 is overheating very badly. I am using a 1N4148 instead of the 1N914 but otherwise it is all the same. Does anyone have any ideas as to why this is happening? Also, before it gets too hot all the LEDs are lit when no audio signal is present. This would seem to indicate incorrect wiring but both I and another individual have checked it thoroughly. A photo of circuit with all wiring can be provided if needed.

jcs Sound Level Meter Wednesday, January 27, 2010 12:16:39 PM

Just received some RGB LED's from someone made by this company: http://www.lumiron.com/SPECTRO%20300RGB%20-%20T.html I would like to try to drive them with this Sound Level Meter circuit. The supply voltage for the LED's is 12 volts, so it should work fine. Any ideas or suggestions would be much appreciated. I figure 16.7 million colors is enough color to experiment with!!! LOL!!!

J & R Sound Level Meter Tuesday, November 17, 2009 8:41:26 PM

This circuit is basically the reference circuit and the reference peak detector from the LM3915 Datasheet. For those who claim it will not work, I was wondering, why then would the same circuit be contained in the LM3915 datasheet, from the manufacurer of the I.C.? The answer has to be that the circuit works. Actually, I've been building these for awhile (blame Mr.MIMs III) and they work fine. For the person who asked, the 1N4148 is a fine replacement for the 1N914 in this circuit (even the datasheet says so). Have a good day :-)

anonymous Sound Level Meter Sunday, August 23, 2009 4:02:39 AM

Yet another circuit on this site which doesn't work. Nice one..
(Editor's notes: Works fine. See multiple posts in the forum. )

anonymous Sound Level Meter Thursday, July 23, 2009 7:10:17 AM

can i connect it to a mike to check the audio level of a person singing into it(mike)?

Bradley Glen Sound Level Meter Saturday, July 04, 2009 11:49:05 AM

I tried this and the problem is that the diode is forward biased through the Diode from the supply . If any DC leaks onto pin 5 of the IC then you are not going to get the proper reading .

simone Sound Level Meter Friday, June 19, 2009 9:28:18 AM

The Q1's bias will not work if the Vin is less than about 0,6V... try an auto bias circuit using the V+ insead of Vin... Then remember that the D1 introduce a voltage drop of about 0.7 V

anonymous Sound Level Meter Saturday, March 21, 2009 5:00:25 PM

Putting an audio signal in makes no difference. Also when I connect the power all LEDs turn bright on too how can I fix this? Any help would be appriciated,

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Dimitris	Sound Level Meter	Sunday, August 08, 2010 8:46:35 AM
I think that u connect 5 with 3. There is no connection between them. 9 is directly connected to 3. (" All LED switches on when power is connected")
anonymous	Sound Level Meter	Thursday, June 03, 2010 10:48:17 AM
Built this circuit but it does not work properly. All LED switches on when power is connected. Any workaround for this problem?
Eric	Sound Level Meter	Sunday, April 11, 2010 9:12:47 PM
I built this exact circuit using a 15V power supply and my LM3915 is overheating very badly. I am using a 1N4148 instead of the 1N914 but otherwise it is all the same. Does anyone have any ideas as to why this is happening? Also, before it gets too hot all the LEDs are lit when no audio signal is present. This would seem to indicate incorrect wiring but both I and another individual have checked it thoroughly. A photo of circuit with all wiring can be provided if needed.
jcs	Sound Level Meter	Wednesday, January 27, 2010 12:16:39 PM
Just received some RGB LED's from someone made by this company: http://www.lumiron.com/SPECTRO%20300RGB%20-%20T.html I would like to try to drive them with this Sound Level Meter circuit. The supply voltage for the LED's is 12 volts, so it should work fine. Any ideas or suggestions would be much appreciated. I figure 16.7 million colors is enough color to experiment with!!! LOL!!!
J & R	Sound Level Meter	Tuesday, November 17, 2009 8:41:26 PM
This circuit is basically the reference circuit and the reference peak detector from the LM3915 Datasheet. For those who claim it will not work, I was wondering, why then would the same circuit be contained in the LM3915 datasheet, from the manufacurer of the I.C.? The answer has to be that the circuit works. Actually, I've been building these for awhile (blame Mr.MIMs III) and they work fine. For the person who asked, the 1N4148 is a fine replacement for the 1N914 in this circuit (even the datasheet says so). Have a good day :-)
anonymous	Sound Level Meter	Sunday, August 23, 2009 4:02:39 AM
Yet another circuit on this site which doesn't work. Nice one.. (Editor's notes: Works fine. See multiple posts in the forum. )
anonymous	Sound Level Meter	Thursday, July 23, 2009 7:10:17 AM
can i connect it to a mike to check the audio level of a person singing into it(mike)?
Bradley Glen	Sound Level Meter	Saturday, July 04, 2009 11:49:05 AM
I tried this and the problem is that the diode is forward biased through the Diode from the supply . If any DC leaks onto pin 5 of the IC then you are not going to get the proper reading .
simone	Sound Level Meter	Friday, June 19, 2009 9:28:18 AM
The Q1's bias will not work if the Vin is less than about 0,6V... try an auto bias circuit using the V+ insead of Vin... Then remember that the D1 introduce a voltage drop of about 0.7 V
anonymous	Sound Level Meter	Saturday, March 21, 2009 5:00:25 PM
Putting an audio signal in makes no difference. Also when I connect the power all LEDs turn bright on too how can I fix this? Any help would be appriciated,