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Sound Level Indicator.

Discussion in 'Electronic Basics' started by [email protected], Jan 22, 2005.

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  1. Guest

    Hi, I teach kids to read and I am always telling them to speak louder.
    I would like to make some device to measure the sound level. I want to
    be able to put the device on the desk and if the child is loud enough a
    green light goes on. Not loud enough, a red light. They are younger
    kids so I'll use it with a reward system. If they always have a green
    light they'll get some stickers or something. I would have to be able
    to change the sensitivity also. Any ideas would be appreciated.
    Thanks, Raul
     
  2. Jamie

    Jamie Guest

    use a quodd Op-amp chip, something like a LM324.
    using 3 OP-amps as the level LED's. use feed back
    diodes to kill null the Op-AMp before that if you
    wish only to have 1 LED on at a time.
    use the last Op-Amp as the Mic preamp, the output of which
    will be driving comparators.
    P.S.
    they do make LED bar graph segment display chips that give
    you a solid or dot option display.
    LM3916
    http://www.national.com/ds/LM/LM3916.pdf
    http://wiredworld.tripod.com/tronics/audio_level.html
     
  3. Part of the challenge of doing this properly will be to get the
    sensitivity of the indicator to vary with frequency in a way
    that mimics average hearing sensitivity. If you do not do
    this, kids whose voices are near the peak in that sensitivity
    function will have an easier time satisfying your device, while
    those whose voices are off the peak may not realize that
    they need to speak up, having been misled by your device.
     
  4. Have the level adjusted by a listener with average hearing to just start
    to flash when the trainee speaks properly. Much easier than trying to
    get it right for everybody.

    Here is a circuit that works for me:
    (courier, please. Otherwise, it looks like the gibberish it may well be)

    1MEG 10k
    ___ ___
    .--|___|-o--|___|--.
    | | |
    VCC | | |
    | LED 1.5k | /| | Vcc |
    --- 9V ___ | /+|-' | |
    - GND ----|<--|___|---o---< | .-. |
    | \-|--->| | |
    GND \| | | |
    10k 1MEG U1D '-' |
    ___ ___ | |
    VCC .---------o--|___|-o-|___|---. GND |
    | | | | | |
    ..-. .-. | VCC | | |
    | |10k | |330k | + | | |
    | | | | | | | | .-------o
    '-' '-' | | | | | |
    | | | | | | | U1C |
    | | U1A | | | U1B | | |\ |
    | Cf | |\ | .-. | |\ | '--|-\ |
    | o--|-\ | | |<-)---|-\ | | >-'
    | || | | >---' | | | | >--o--->|-o----o--|+/
    o----||-----o----)--|+/ '-' '---|+/ | | |/
    | || | | |/ | |/ | |
    | | | RSens | | |
    | | | | | |
    | | ' | | |
    | Rf | | | | .-.
    | _ .-. .-. | 1uF --- | | 1MEG
    '-/ \ | | | | |1k | --- | |
    (Mic)| | | | | | | '-'
    .-\_/ | '-' '-' | | |
    | | | | | |
    -o-----------o----o--------------o--------------------o----o---- GND
    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

    U1A is a gain stage. U1B is a comparator, which feeds into a trivial
    peak detector (the diode) which decays with a tau of 1s. U1C is a
    buffer, so the decay of the peak detector isn't affected by U1D, which
    is a comparator that controls how much time the light is on.

    The opamps are A-D on a single LM324 chip.

    The pot on the V- input to U1B controls level sensitivity.
    The pot on the V- input to U1D controls how long the LED stays on after
    it's triggered.

    The MIC is a cheapo electret microphone

    Cf and Rf adjust the sensitivity of the input. A 1k resistor and 1uF
    capacitor will give a reasonably OK filter for voice, I think. However,
    playing around with different values will give you a different profile.
    the function of attenuation with frequency is

    a = Rf/sqrt(Rf^2+(1/(2*PI*f*Cf))^2)

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  5. Harold

    Harold Guest

    HI Raul:

    Try using just an amplifier and a microphone. Instead of using a
    speaker, just use a 12v tail light from the auto store. These are
    typically 4 to 12 ohms equivalent to an 8ohm speaker. As the student
    talks, the light will glow. Pretty simple.
    Harold
     
  6. Guest

    I'm sorry I can't read the ANSI schematic. I am a novice. If I have a
    schematic I can use my bread board to get it to work, then build it. I
    can't do it from the ANSI drawing. I'm not that good. Is there some way
    I can convert it to a regular schematic?

    Thanks, Raul
     
  7. Lord Garth

    Lord Garth Guest

    Use the full screen and select a fixed width font such as courier.
     
  8. Bob Masta

    Bob Masta Guest

    You might want to look into the inexpensive sound level
    meters that Radio Shack sells. They are calibrated to
    read in dB SPL and they have weighting scales so you
    can use an A-weighted response that is roughly the way
    your ear weights different frequencies. They also have some sort of
    a peak-hold function, which I believe is what you want here,
    but I'm not sure of the details. They also have analog
    and digital models. Might be worth a look.


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  9. Here you go:

    schematic: http://home.comcast.net/~rcmonsen/schematic.gif
    PCB layout: http://home.comcast.net/~rcmonsen/layout.jpg

    I haven't built the layout, I just drew it. It is for a single sided PC
    board. Flip it left<->right, shrink it to 50%, and it should be OK. The
    thick lines are traces, the thin lines are components, and the circles
    are drilled holes. (Note there is a mistake in the resistor values below
    the layout, R2 should be 10k, it seems to work better for me. The
    schematic gets it right.)

    Also, I added the 'red' led, DL2, on the fly without testing it. I think
    it'll be ok.

    I don't know how well it will work for you. However, the parts will be
    less than $10 at RadioShack, I think, including an enclosure. You may
    want to build it first on a solderless breadboard just to get a feel for
    it. The pots adjust the sensitivity and the time the green LED is on
    after being activated.

    If you don't want to make a PC board, you can use stripboard, and type
    the schematic into "StripBoard Magic" (which is available on the web
    somplace). That may be easier, but you'll have to get some stripboard.
    RadioShack usually doesn't carry it.

    Bob Masta's suggestion, to buy something premade, will clearly be a
    safer/easier option, in that it's guaranteed to work.

    If you build it, let me know how it works out.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  10. Guest

    Thanks a lot. I'll will try and build it. I'd rather do that than just
    buy something. You don't learn much that way. I'll put up pics when
    I've got it working.
    Thanks, again
    Raul
     
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