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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Alan Sound Level Meter Sunday, October 12, 2014 4:58:58 AM

I want to make a sound level circuit for detecting noise source under a car bonnet (or wherever it might be). Can I use electret microphones with a switched input from mic to mic and thus see which is nearest the noise source?

anonymous Sound Level Meter Sunday, February 24, 2013 3:33:30 PM

I am having the problem "of about 0.6V to pin 5". Regarding "The solution is quite simple though.", please share. I am using a microphone into an LM386. Without the Peak Detector it works as expected. If at all possible I would like to stick the the LM386. The only purpose for my circuit is to make the bar graph respond when someone speaks into the mic. I am not using the sound in any way (not even playing it into a speaker). Thanks!

peter west Sound Level Meter Monday, July 09, 2012 3:30:55 AM

I agree that the peak circuit is shown in the data sheet. I also agree it does not work. There is a good reason why not - the Vbe for the transistor is about 680mv, while the voltage drop across the diode is only 120mv [because there isn't any current with no input] The resulting 500mv+ turns all the leds on, as a number of people has remarked. The solution is quite simple though.

anonymous Sound Level Meter Friday, July 06, 2012 3:20:18 PM

This circuit WILL NOT WORK for line level signals; there will be a constant voltage of about 0.6V to pin 5, which will keep the lowest LEDs on constantly. Don't bother building this - use an opamp detector instead.

Pio Sound Level Meter Tuesday, April 03, 2012 8:16:44 AM

could any1 help me make a sound level meter that goes up to eleven? 11 LEDs that is. can this b altered to do so? also is it possible to add a Pezio elctric microphone to this circuit diagram??

anonymous Sound Level Meter Thursday, December 01, 2011 2:46:05 PM

Does it matter if you use Carbon or Metal film resistors?
(Editor's notes: Nope.)

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!!!

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Alan	Sound Level Meter	Sunday, October 12, 2014 4:58:58 AM
I want to make a sound level circuit for detecting noise source under a car bonnet (or wherever it might be). Can I use electret microphones with a switched input from mic to mic and thus see which is nearest the noise source?
anonymous	Sound Level Meter	Sunday, February 24, 2013 3:33:30 PM
I am having the problem "of about 0.6V to pin 5". Regarding "The solution is quite simple though.", please share. I am using a microphone into an LM386. Without the Peak Detector it works as expected. If at all possible I would like to stick the the LM386. The only purpose for my circuit is to make the bar graph respond when someone speaks into the mic. I am not using the sound in any way (not even playing it into a speaker). Thanks!
peter west	Sound Level Meter	Monday, July 09, 2012 3:30:55 AM
I agree that the peak circuit is shown in the data sheet. I also agree it does not work. There is a good reason why not - the Vbe for the transistor is about 680mv, while the voltage drop across the diode is only 120mv [because there isn't any current with no input] The resulting 500mv+ turns all the leds on, as a number of people has remarked. The solution is quite simple though.
anonymous	Sound Level Meter	Friday, July 06, 2012 3:20:18 PM
This circuit WILL NOT WORK for line level signals; there will be a constant voltage of about 0.6V to pin 5, which will keep the lowest LEDs on constantly. Don't bother building this - use an opamp detector instead.
Pio	Sound Level Meter	Tuesday, April 03, 2012 8:16:44 AM
could any1 help me make a sound level meter that goes up to eleven? 11 LEDs that is. can this b altered to do so? also is it possible to add a Pezio elctric microphone to this circuit diagram??
anonymous	Sound Level Meter	Thursday, December 01, 2011 2:46:05 PM
Does it matter if you use Carbon or Metal film resistors? (Editor's notes: Nope.)
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!!!