Designing a VU meter for high db ranges.

Discussion in 'The Projects Forum' started by civil, Sep 11, 2011.

  1. civil

    Thread Starter New Member

    Sep 11, 2011
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    Hey, I was wondering if I anyone could give me some help in designing a VU meter.

    Essentially what I am looking to build is a circuit that has 4-5 LEDs that light up at different levels. It would sit in loud environments to warn workers when ear protection must be warn. 1 green LED means safe, 1 yellow means getting loud, 2 yellows means protection should be worn for safety, 1 red means definitely need ear protection. That sort of thing.

    I've read into a lot of circuit diagrams and it is my understanding I will need the following:
    a microphone connected into an amp (not sure?) which then leads through various resistors/capacitors and results at an LM3915 (or LM3914 if I want linear)

    I guess what I am asking for is any direction here. Whether it being formulating equations or designing the circuit. I'm not sure where to begin.

    Many thanks
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I'd think you would want the Log version as apposed to the linear version. Have you thought of fluctuations on the noise level? The visual meter you propose would not be very effective in offering "good advice" to workers when intermittent noise is occurring.

    If using in a work environment you want to be sure the device complies with OSHA, ISO standards or liability issues may be a concern. Sometimes exposure to loud noises for a sort time is OK, but prolonged exposure would not be desirable. A device such as a micro-controller could log the exposure to certain levels and output a warning based on projected exposure. Food for thought!
     
  3. wayneh

    Expert

    Sep 9, 2010
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    One approach would be to buy one of those cheap SPL meters and route the meter voltage to the LM3914. The store-bought meter would solve a lot of the upfront issue and I imagine gives a linear voltage response to log sound level changes. If you want to start from scratch, I'd look for a good DIY SPL meter circuit and make the same modification. You may need an op-amp in between, to convert whatever the meter is seeing into the right range for the LM3914.

    Wait a second, aren't there a bunch of sound-reactive LED circuits posted here somewhere?
     
  4. #12

    Expert

    Nov 30, 2010
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    The first direction I would give is: read the datasheet. That is what tells you how to connect the LM39xx chip, set up the voltage levels on the input, and adjust the LED current on the output.
     
  5. civil

    Thread Starter New Member

    Sep 11, 2011
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    Thanks for the replies.

    I did a bit of reading through the datasheets and it appears that to configure the lm3915, I need to have a larger reference voltage than my input. This will mean that the steps taken to turn on each subsequent LED will be larger I believe.

    I'll also be running the microphone through an amp just due the the low voltage it returns.

    This is my first electronic project for class so I believe I may be able to do a proof of concept design. Mainly so I don't display it in class and have to play 90dB sound to prove it works.

    I found AudioGuru Schematics and that gives me a basic idea of what I should be doing. We haven't covered op-amps yet so I'm not sure I should be using them however it's my understanding a comparator will do roughly the same job?

    [​IMG]
     
  6. civil

    Thread Starter New Member

    Sep 11, 2011
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    I found this schematic which looks pretty similar to what I am attempting. Also cuts down on the elements I'd require

    The legend is
    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

    [​IMG]
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    AudioGuru's circuit you posted in Reply #5 works as shown, and includes all of the components necessary for proper operation. You could reduce the number of LEDs if you wished; there are two shown per output.

    The circuit you posted in Reply #6 is missing a lot of elements that you would have to fill in, in order to get the project working.
     
  8. civil

    Thread Starter New Member

    Sep 11, 2011
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    Thanks SgtWookie.

    I purchased all the components for Audioguru's circuit today apart from the MC33172 Dual Op-Amp.

    I'm not sure if it's not sold in Australia or what, but Jaycar and ebay don't seem to stock it (apart from a $7 one from Honk Kong on ebay).

    Is there an equivalent or substitute I could be using? I think an LM358 should do the job just by looking at the datasheets.

    Hopefully will be building this circuit within the next few days so I should be able to upload completed images.
     
  9. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    The LM3915 has 3 dB steps.
    If you want less leds with 6 dB steps you could alter the schematic as followed:

    [​IMG]

    This will work in Dot mode only, otherwise the current of two sources will go through the led in Bar mode.

    Bertus
     
  10. civil

    Thread Starter New Member

    Sep 11, 2011
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    Oh thanks! I did wonder how I was going to negate that.

    You don't happen to know if there is anyway to increase the dB set that the lm3915 covers do you?

    So the first yellow (LED2) will be on at roughly 80dB and so on from there. Is this a matter of increasing the resistance?
    The range quoted in the datasheet is considerably lower (I think it peaked out at about 18dB)

    Sorry for all the questions as well guys, this is my first time building a circuit and I've only been studying electronics for about 8 weeks now.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    My Sound Level Indicator project has automatic gain control that you do not want. It is very sensitive to very low level sounds like a pin dropped on the floor in the next room or a whisper. When sounds get louder than half the LEDs lighted then the AGC reduces its sensitivity so the upper LEDs show loud sounds.
    The LM3915 has a range of 30dB (10 outputs with 3dB between them). My AGC circuit increases its range to 50dB.
    The difference in sensitivity between activated outputs on an LM3915 is 3dB that cannot easily be changed. You can connect two adjacent outputs together (as was shown) for 6dB between activated outputs.

    Farnell.com is an international electronic parts distributor. Go on the web to see that their Australian buisiness is now called Element 14 and they have a warehouse in your country with the MC33172 opamps in it. An LM358 opamp will work but it has very poor high frequency response above 2kHz.
     
  12. yourownfree

    Active Member

    Jul 16, 2008
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    is this suppose to be an invention? If so I can tell you how it should be built if you really want to make some bucks, just don't forget me.
     
  13. civil

    Thread Starter New Member

    Sep 11, 2011
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    0
    Thanks Audioguru! I ended up going with a LM386 since I decided it appeared to perform the necessary job.


    Not an invention, it's a project for uni. We were tasked to look at an existing simple circuit and then chose some way to be innovative with it. The use this circuit would theoretically perform is where I see it would get marks for innovation.

    That being said, I would not say no to the help haha
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    It will not work in my circuit because the LM386 is not an opamp, instead it is a power amplifier that is designed to drive a speaker.

    Why don't you buy the proper dual opamp from where I said they are available in your country??
     
  15. civil

    Thread Starter New Member

    Sep 11, 2011
    7
    0
    We have to give a presentation and explain each element in the circuit. Since I have no idea how an op-amp works, I might not be able to fully respond to questions from the class.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    Look at Opamp in Google.
     
  17. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I found this in my email:

    As I do not answer in emails or PM, I will do it over here.
    The diodes used can be small signal diodes like the 1N914 or 1N4148, as the current will be as high as 20 mA.
    If you do not have these the 1N4001 - 1N4007 will also work.

    Bertus
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    The diodes are not needed saince the outputs of the LM3915 are the open collector of NPN transistors that when shorted together make an OR gate.
     
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