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Simple Project

Discussion in 'Electronic Basics' started by electricked, Mar 3, 2004.

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  1. electricked

    electricked Guest

    Hi all,

    I've been reading some electronics theory and I've finally decided to do a
    simple project. My project consists of having a microphone and 4 red LEDs.
    The 4 LEDs act as a volume meter. So when someone talks in the mic, the leds
    light up depending on the volume. If the volume is low, then no LEDs light
    up. If it's very high, then all 4 LEDs light up. If it's somewhere in
    between only 2 LEDs light up.

    I'm thinking on loading the circuit from a 9V battery. So my initial
    thoughts were to use a transistor and build the circuit as follows:

    | | | | |
    --- | | | |
    - VCC 9V | | | |
    | | | | |
    | | | | |
    | V V V V
    | - - - -
    | | | | |
    |/ | | | |
    -----------| | | | |
    | _ |> | | | |
    |-/ \ | | | | | |
    (Mic)| | | | | |
    |-\_/ | | | | | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    First of all, how does a microphone work? Do I need to apply voltage to it
    in order for it to work, or does it produce current on it's own when noise
    is detected?

    What I was thinking is having the microphone switch a transistor if someone
    is talking and then depending on the loudness of the volume then the
    transistor will let current flow from C to E accordingly. At least that's my
    understanding of transistor so far. I'm thinking, if the base current is
    lower, then the current in main circuit will be less, if current at base is
    higher, then current in main circuit will be higher. Having this figured
    out, I was thinking the transistor will drop 0.6 volts so now I have
    9-0.6=8.4V when the transistor is fully open. So next I was thinking I'd let
    the max current be 80mA since 4 LEDs times 20mA each in parallel will be
    80mA max current required for them to light up fully when the transistor is
    fully open. So I'll need a resistance value to limit the current in the main
    circuit. So I have R=V/I=8.4/0.08=105OHMs. So that's my resistance. Next
    thing, I figured say the transistor is half way open it will allow 40mA to
    pass and only two LEDs will need to be lighted up. But if I connect all 4
    LEDs in parallel then isn't the 40mA going to split into 4 10mA for each LED
    connected in parallel? So this is not the effect I want. This will light up
    all LEDs but with less brightness. What if I connect the LEDs in series?
    Then I'll have to change the resistance since only 20mA max have to be
    allowed at fully open transistor. But then if the LEDs are in series and I
    have 20mA then the brightness depends on the voltage, right? So each LED
    will drop about 1.6V is it?

    So I'm stuck here. If I put the LEDs in parallel will that get me the effect
    I want? Basically, if the transistor is half open only two LEDs will light
    up at their full brightness and the last two in series won't light up at
    all. Is that going to work? How do I figure the value for the resistor in
    that case?

    All help is welcome. Sorry for the long-winded post. I'm just trying to put
    my thoughts into writing so it makes sense.


  2. I think this is commonly called a VU Meter.
    I think in this circuit, all of the LEDs will glow at the same time,
    regardless of the input. I don't actually know of a circuit but perhaps
    doing a google search on "VU Meter circuit" (or something) might turn up

    It depends on the type of mic, electrets use a voltage, but I don't think
    dynamic ones do.

    Andrew Howard
  3. You need considerably more electronics than you've shown.

    First, you will have to amplify the signal from the microphone to get
    a more useful level, then use a comparator to drive each LED, with the
    comparators comparing the audio level to different preset values to
    make each LED come on at the desired level.

    National makes a few nice parts that will do the level comparison/led
    drive part of this - see for details.

    The 3915 is logarithmic, suitable for audio level meters. The 3914 is
    linear, and suitable for use as a voltmeter.
  4. grahamk

    grahamk Guest

    Have a look at
    You will need an amplifier between the mic. and the input tho'
  5. electricked

    electricked Guest

    I understand. That's why I'm doing this project so I can gain understanding
    of electronics and specific devices such as the microphone and so on.

    How would I go about building this circuit without using an IC? If I'd like
    to build my own comparator and so on...

    What's the basic idea behind this project? How can I make it so depending on
    the voltage coming in, I light only the lights that measure the voltage


  6. electricked

    electricked Guest

    Thanks graham!

    That's exactly what I need I just need less LEDs (4 to be precise).

    What I want to do though is instead of using the LM3914, I want to build it
    myself. Any resources where I could learn more about electronics and maybe
    LEDs, amplifiers, etc. that you think are worth while?



    Have a look at
    You will need an amplifier between the mic. and the input tho'
  7. Bill Bowden

    Bill Bowden Guest

    You might try something like this:

    It uses a LM324 quad op-amp (4 amplifiers in one cheap IC)
    and covers 60 to 70 dB with 3 lights, which is close to normal
    conversation. A small speaker is used as the microphone.

  8. Is that a reply to Peter's post? (It would be easier to follow if you
    followed the general convention of *bottom* posting, as I am doing
    here.) If so, IMO he has already given you the the key aspects to
    pursue. To summarise, you need to study and experiment with these

    1. Microphones; there are various types.

    2. Amplifying their typically small signals, or restricted to the
    specific type of mic you are working with.

    3. Converting that amplified AC to a DC signal, smoothed sufficiently
    to allow appropriate display of the 4 LEDs.

    4. Using opamp or comparator ICs. A single chip can handle 4 of them.
    In practice I expect you'll need to set the LED indication levels you
    want by trial and error, using 4 presets.
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