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Old homebuilt capacitance meter

Discussion in 'Electronic Design' started by Norm Dresner, Oct 30, 2003.

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  1. Norm Dresner

    Norm Dresner Guest

    I just found in a pile of boxes in the basement a capacitance meter I wire
    wrapped in the early 80's and it no longer works. The worst part is that I
    can't find the schematic from which I built it. I'm hoping to jog someone's
    memory who might be able to find a copy (I have absolutely no memory of the
    original source, but it might have been an Radio Electronics article).

    From what I can see of the circuit, it uses a bunch of 74C90 counters and a
    4 MHz crystal-based oscillator to time something. Since there's a 556 and a
    555 in there, I'm guessing that it times the RC time constant for one of
    them. The only part I don't immediately recognize is an MC14553B. Other
    parts in the circuit are the obvious glue.

    Thanks in advance
  2. Robert Baer

    Robert Baer Guest

    A scheme sometimes used that might use parts like that comes to mind.
    Basically, a transition of a square wave (or pulse) is used to run two
    drivers, and one driver gpes to an RC network, where the user supplies
    the C, and the other driver goes to a NAND gate - the output of the RC
    goes to the other input.
    Net output of the gate is a pulse whose width is porportional to the
    Filter that pulse for a DC voltage which goes to your DVM for a
  3. Bob Stephens

    Bob Stephens Guest

    Or spend $29.95 for a cheap DVM with a capacitance bridge built in.
  4. Norm Dresner

    Norm Dresner Guest

    Well, at least I found the folder with the original schematic and some
    pencil-notes on my revisions to it, like substituting the 14553 3-decade
    counter for three '90s, a '76 for a '73, and stuff like that.

    The original circuit was adapted from an article in the September 1978 issue
    of Radio-Electronics and judging by the date codes on ICs, the meter wasn't
    built much after that.

    Now I can go debug the thing.

    Thanks to all who tried to help.
  5. Robert Baer

    Robert Baer Guest

    THe article can be newer that the date lot codes; one does not
    necessarily get "recent" ICs from vendors.....
  6. Baphomet

    Baphomet Guest

    It looks like a Radio Electronics Magazine webpage is coming soon at: I hope they have
    archived articles. It was, along with Popular Electronics and Electronics
    World, a great hobbyist magazine.
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