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

Add A Comment

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,

John Sound Level Meter Sunday, December 28, 2008 10:28:02 AM

Hi, i'm new in this kind of things, but have always loved electronics, actually thats what im going to study when i graduate, but i have two questions, all of those triangles pointing down are ground conections, right? And the second is what kind of input can i use and how do i connect it? Can i use a normal rca jack microphone, an electret microphone or what?

darkshadow6921 Sound Level Meter Tuesday, December 09, 2008 5:09:45 PM

Built several of these, Works great. Thanks!

joan_ece_student Sound Level Meter Thursday, October 09, 2008 12:03:06 PM

1N491 diode is not available here. can you give me another as a substitute because the circuit doesn't work. i just used 4148 diode. is it ok? pls response.. tnx!

anonymous Sound Level Meter Tuesday, August 26, 2008 8:23:39 PM

I have just built this project and i can't seem to get it to work. The problem is there are some many errors I could make here. I have a feeling it is either the PnP transistor, or the Silicon Diode. If u can help me out, plus email me.

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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,
John	Sound Level Meter	Sunday, December 28, 2008 10:28:02 AM
Hi, i'm new in this kind of things, but have always loved electronics, actually thats what im going to study when i graduate, but i have two questions, all of those triangles pointing down are ground conections, right? And the second is what kind of input can i use and how do i connect it? Can i use a normal rca jack microphone, an electret microphone or what?
darkshadow6921	Sound Level Meter	Tuesday, December 09, 2008 5:09:45 PM
Built several of these, Works great. Thanks!
joan_ece_student	Sound Level Meter	Thursday, October 09, 2008 12:03:06 PM
1N491 diode is not available here. can you give me another as a substitute because the circuit doesn't work. i just used 4148 diode. is it ok? pls response.. tnx!
anonymous	Sound Level Meter	Tuesday, August 26, 2008 8:23:39 PM
I have just built this project and i can't seem to get it to work. The problem is there are some many errors I could make here. I have a feeling it is either the PnP transistor, or the Silicon Diode. If u can help me out, plus email me.