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VU meter

Discussion in 'Electronic Basics' started by [email protected], Apr 14, 2007.

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  1. Guest

    I have found an audio level meter in this website as follows

    I need some help to understand. What does it mean 5 stage
    feedback circuit. What are the A and B markings on the
    diagram, is this some kind of switch?

    Any ideas?
  2. Eeyore

    Eeyore Guest

    Can I suggest that if you want to do something like that there are much much
    better and easier ways to do it ?

  3. BobG

    BobG Guest

    The text explains it... its a log amp... piecewise linear approx of
    log... dBs are 20*log(vout/vin). Use a $1 tiny avr
  4. Eeyore

    Eeyore Guest

    When that method originated there were no AVRs of course.

  5. Lord Garth

    Lord Garth Guest

    Points A & B connect to points A & B in the LED chain.
  6. Guest

    Is it switch though? what is so special about the 6th and 7th leds,
    if it is a switch, is A always one led under B or can they move up and
    down independently? Where are the five stages? we have 10 leds so
    does this mean every stage has two leds.
  7. Phil Allison

    Phil Allison Guest

    ** You are utterly clueless about circuit analysis.

    Build the damn thing and maybe you will SEE how it works.

    Somehow, I doubt even seeing it will make you one bit wiser.

    ........ Phil
  8. You're in SEB Phil, please try to be nice.
    Is that your professional analysis of the circuit?
  9. Wim Lewis

    Wim Lewis Guest

    No, the A and B are just indications that the arrows marked A and B
    should be connected to the points A and B on the right --- this makes
    the schematic diagram a little less cluttered.

    As for the "5-stage feedback": I think BobG has the right interpretation.
    A VU meter is supposed to have a logarithmic response, but all the
    amplifiers in this circuit have constant gain (linear response).
    As the diodes in the diode string light up, they switch some extra
    resistors into the third opamp's input, which changes the circuit's
    overall gain, and presumably this ends up approximating a logarithmic
    response. I don't see why it's described as "5-stage" though. It
    looks like it only has 3 or 4 different gain settings.

    These days, you can buy VU-meter chips with a logarithmic amplifier and
    a bunch of diode drivers. Or you can do it in firmware in a cheap
  10. circuithelp

    circuithelp Guest

    I am not consered with the logaritmic scale. I wanted to ask about
    the leds and th driver transistors, how this works, and if A and B
    signal switch, as otherwise, I see no reason why someone would
    start using A B C etc to denote nets.

    Ok, hope that's clear
  11. Believe me, it's far better to do that than have traces going from
    one side of the schematic to the other merely to connect those
    points (as opposed to a line that is being used throughout the circuit,
    such as the power supply line). You either have to have those long
    connections riding over all kinds of things, or have them go far out
    of the way just to keep it clear of the rest of the schematic.

    It's really clear, once you know this is happening (and I admit that
    in this case, it wasn't so clear because of the way the schematic
    wsa drawn). The way I saw it when I was a kid was to have a triangle
    on it's side (so the point faces left or right) and then the other point
    has a similar triangle pointing towards the first one. It was then pretty
    obvious that they were two points to connect together, because they
    were special symbols for the purpose.

    You can even do it so there is one source, and multiple "receivers",
    which can be useful for showing where the power supply line goes, again
    so you don't have long traces all over the schematic. In essence, that's
    just a variant on the ground symbol. You know those all connect together.

    The thing about schematics is that they should be drawn so you can
    easily interpret them by looking. THis is not the same thing as circuit
    layout. If you drew them the way you'd wire a circuit, it would usually
    be a real mess to follow, yet the schematic properly drawn spreads
    the circuit out so two points that might on a circuit board be next
    to each other are at opposite ends of the schematic.

  12. Eeyore

    Eeyore Guest

    I find Phil's restraint admirable.

  13. circuithelp

    circuithelp Guest

    All BEWARE

    Phil Allison and Eeyore are well known intelectual property
    theives actively targetting this newsgroup.

    The way they operate is this:

    first they would give you some sort of irrelevant answer
    and say that you should do it their way (but not
    revealing their way)

    The next step they will start becoming dismissive of you
    and try to lead you to believe that you are an idiot and
    that they are the oracle of knowledge

    After that, they will slide into insults and mud throwing
  14. Don Bowey

    Don Bowey Guest

    Just what the board needs........ Another idiot troll.
  15. Guest

    When they do slip up and post something with apparent technical
    content, they are often wrong, thereby displaying their profound
  16. Don Bowey

    Don Bowey Guest

    It is more likely that the basic/beginner poster does not understand the
    answers they are given, and will not ask for clarification. I noted several
    recent posters who received valid replies, but became belligerent over them.
  17. Lord Garth

    Lord Garth Guest

    I don't think you can trust the descriptive text. He begins with the
    statement that the audio amp runs on 0v-18v power rails but the
    schematic clearly shows +18v and -18v from the amp.

    You should probably look for a better circuit such as one based
    on this chip:
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