New and Need some help

Discussion in 'The Projects Forum' started by WVU Engineer, Jan 18, 2008.

  1. WVU Engineer

    Thread Starter Member

    Jan 18, 2008
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    Hi i am a new member to this forum and would like some suggestions/help from some people who know better.

    My project is as follows:

    I am trying to build a VU Meter, a very large one. I was planing on using a couple of LM 3915 to drive all of the LEDS, about 60 in total. I was thinking about using two PC power supplies because they would convert AC to DC. One of the power supplies would power the LED's and one of them would supply power to the LM3915 chips. I am not sure of a circuit for which to do this. i have read through the LM3915 datasheet, and i am unsure of how exactly it would interface with an Electret condenser microphones. In the end i want this meter to read how loud it is in a room... i plan on building it right into a table. Any suggestions/advice anyone can give would be a huge help, i dont want to blow anything up. Thanks again.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I believe this is along the lines of what you had in mind.

    vu meter circuit

    An LM3915 will only support 10 LEDs so unless you plan to double up on some of the LEDs you will need six LM3915's to handle 60 LEDs.

    I don't believe you will need more than one PC supply.

    hgmjr
     
  3. WVU Engineer

    Thread Starter Member

    Jan 18, 2008
    15
    0
    yes it is.. however how would i interface the mic with that.. and also if i use the 12V supply to power all 6 LM3915 would that hurt the power supply...
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The VU meter circuit is incomplete. It needs a peak detector circuit at its input as shown on the datasheet for the LM3915 so that the peak sound levels will be bright and clear. The circuit that is posted will be just a dim blur.

    Use only a single LM3915 for each 30dB range. Then use a transistor for each output to drive each group of LEDs.

    My Sound Level Indicator project has a peak detector circuit and an Automatic-Gain-Control circuit. Its AGC allows it to show a range of 50dB. It is mostly powered from an AC-DC adapter but also has a 9V rechargeable battery inside. It has an electret microphone and can indicate a pin dropping on the floor in the next room and loud music nearby.
    It has a switch for either a bar of LEDs or a dot.
    As a bar, its 20 LEDs are pretty bright.
    http://www.electronics-lab.com/projects/motor_light/009/index.html
     
  5. WVU Engineer

    Thread Starter Member

    Jan 18, 2008
    15
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    i am trying to build this to put in a table for parties and such... i am unsure of what you mean by placing in a transistor... i dont understand why i would need one for each LED... also would i need an AGC for this... cause i only want it to detect louder sounds like the beat of music... and would the Sound Level Indicator circuit that you have shown me work for my type of application, and do you think that my power supply from a computer idea would work effieciently? thank you alot of your input i see how the mic has to interface
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    You want 60 LEDs. The LM3915 has 10 outputs. Each output has a max allowed current of 30mA and each ordinary LED has a max allowed current of 30mA.

    Six 2V red LEDs in series need 12V plus the output of an LM3915 needs at least 1.5V.
    Six 3.5V blue LEDs in series need 21V plus the output of an LM3915 needs at least 1.5V. So you need a power supply from 13.5V to 22.5V depending on the colour of the LEDs. A computer power supply has only 12V. But a little wall-wart power supply can be used instead.

    You could connect three LEDs in series and in series with a current-limiting resistor. This is one string. Two strings can be driven with a transistor and then the voltage requirement is half. With transistors driving the LEDs then you can use brighter LEDs that use more current than the LM3915 can provide.

    You don't need my AGC circuit just for a loud beat of the music.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Here's an alternative to using discrete transistors on the outputs of the LM3915.

    An ATX form factor PC power supply has +12V and -12V, along with the other voltages.

    From -12V to +12V is a 24V potential at around 800mA. If you were using standard 15mA LED's rated in the vicinity of 1.7V to 2.2V, you could run quite a string of them across 24V. It all depends upon what the rating is of the LEDs you're planning on using. All of the LEDs on any one LM3915 should all have the same current rating if you wish to use the internal regulator, which will save a lot of wiring.

    You could tie the LM3915s' Vcc pin to the power supplys' GND and the LM3915s' GND pin to -12V as a "virtual ground". Everything's relative.

    Using discrete output transistors would defeat the built-in current regulation circuit of the LM3915.
     
  8. WVU Engineer

    Thread Starter Member

    Jan 18, 2008
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    i wish to use a Electret microphone for this project. I am just now beginning to realize that i am more confused about this than i had originally thought... In the data sheet for the LM3915 it says that if you cascade them together, the lower sets would not be that accurate. Is there any way to fix this situation. Also could Audioguru please explain to me how your Sound Level Indicator works i would really like to know. I have a hard time figuring out how to build and analyze circuits such as that. An extra added input to this project would be a large help.
     
  9. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    The LM3914 is a standard equal ladder network, the LM3915 is in 3dB steps (power), and the LM3916 was designed for VU meters.

    Let's say you want 60 LED's, and you were going to run it from a 12V supply. 12/60 = 0.2V per step (LM3914). So, you need a microphone output that will go from 0-12V, and each LED will light for every 0.2V (200mV) of audio signal. Obviously, the mic isn't going to supply this large 12V output on its own, so amplification is required. Also, if you don't want the LED's to zip back and forth so fast you can hardly see them, then the audio needs to rectified (or peak detected) before it goes into the LM391x IC.

    The Analog Devices SSM2166 is a relatively simple IC for an electret microphone that will provide amplification along with AGC (the output level will not vary much with the audio frequency). However, given your application, it may be more interesting without AGC.

    Therefore, using just an amplifier and rectifier circuits to drive the LM391x may do you well.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Summary:
    The electret mic feeds a mic preamp designed for an electret mic.
    The preamp feeds a peak detector circuit that feeds an LM3915.
    Each output of the LM3915 can feed 6 LEDs in series if the supply voltage is high enough.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    WVU Engineer,

    Go to National Semiconductor's site and download their datasheet for the LM3915, or just click here:
    http://www.national.com/ds.cgi/LM/LM3915.pdf

    Application hints begin on page 9, but information specific to cascading the device begins on page 11.

    Audioguru's project is certainly worth a good looking over. Right there he has an amplifier for an electret mic, just begging for you to examine it. ;)
     
  12. WVU Engineer

    Thread Starter Member

    Jan 18, 2008
    15
    0
    Audioguru, i have bought the parts described in your audio level indicator circuit, i am off by a few pF in total... when i hook it to one LM3915 all of the LED's light except the 10th one... which DOES respond to a finger snap or a loud yell..... any ideas as to why all the first nine LED's are lighting.. or as to how i can fix my problem....i have checked the voltages at the indicated points, and all match up exactly. Also, i have to cascade three LM3915 if i can run 6 off of each output.... do you have any ideas? thanks for any help. A
     
  13. WVU Engineer

    Thread Starter Member

    Jan 18, 2008
    15
    0
    SgtWookie,
    I Have a hard time looking at a circuit and seeing how it works (i am working on it). I have actually printed off and read through (several times) the data sheet for the LM, i have used the sound level indicator from Audioguru, however i am still having trouble.... any hints?
     
  14. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    My Audio Indicator project has an electret mic which is powered by a regulated 5V supply and it feeds preamp IC1a. The second opamp IC1b is biased at ground (it is a type of opamp that works when its input voltages are at ground) and is used as a rectifier with gain. Its output transistors supply matched positive high currents but are not a load on the peak-hold capacitor.
    Did you use a type of opamp that works when its input voltages are at ground? An old MC358 will work but it has poor high frequency response. The Motorola (ON Semi) dual opamp I used is much better.

    My project has a switch for a DOT of light or a BAR of light. Which mode give you trouble with the 10th LED?

    I don't think you want to cascade three LM3915 ICs. Then the range will be 90dB which is far too high.
    I think you just want to use a transistor at each output of one IC to boost the current high enough to drive 6 LEDs. Or if the LEDs have a low voltage (red ones) they can be wired in series and driven from an output of the LM3915 directly.
     
  15. WVU Engineer

    Thread Starter Member

    Jan 18, 2008
    15
    0
    I figured it out. Will post the final schematic as well as pictures when i have finished. (it will be a while) Thanks to all who helped.
     
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