Help With Clock Signal

Discussion in 'The Projects Forum' started by Adrastos, Apr 4, 2009.

  1. Adrastos

    Thread Starter Member

    Jan 5, 2009
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    Hello all,

    I came across this circuit for generating a somewhat precise clock frequency (the LED in the included schematic should be flashing at 2 Hz, by my calculations). However, when I built it on a breadboard, the LED stayed constantly on (no flashing!!) :S

    So, I need some help.. what's going on here? Should this not work? Thanks!

    (that's a CD4060BCN chip in there)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    U10 is a CMOS 4060 IC, right?

    I don't know why they grounded Q4 through Q13. Those are outputs, not inputs. You don't have to worry about unused CMOS outputs; only the unused inputs. Grounding those outputs may have damaged the IC.

    Pin 9 should not be grounded (if the IC in question is indeed a 4060) - that will kill the crystal oscillator circuit.

    At this point, you'd be better off starting over by looking at the datasheet for a 4060 IC. Look at Motorola/ONsemi's datasheet for an MC14060B.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    There's also the possibility that your 32.768kHz crystal is dead. They're very fragile. If it's ever been dropped on a hard surface from more than a foot height, it's probably toast.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Oh, and trying to breadboard a crystal clocked circuit isn't a good idea.

    Breadboards have a great deal of capacitance. That by itself could interfere with the operation of the oscillator.

    Try building the circuit using the "dead bug" technique. Google is your friend here.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Are you talking about this Wook?

    Never seen this technique myself, though I've used some odd ones, including point to point without a PC board.
     
  6. Adrastos

    Thread Starter Member

    Jan 5, 2009
    17
    0
    Thanks very much! For this project, I only need a 1 Hz clock signal for 120 seconds, so I suppose any error introduced by using an RC-oscillator wouldn't be that bad. However, I've looked at three different datasheets for the 4060 (and yes, that is a 4060 in there), and each give slightly different coefficients in the RC-frequency formula. Which should I use - 2.3, 2.4, or 2.5, in:

    f=1/(2.3*R*C)

    ??

    Thanks!!
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    The "dead bug" construction technique is tried & true by amateurs and professionals alike, particularly in RF circuits. We have a number of HAM radio folks on the boards who will confirm this, and I've seen it used numerous times on avionics development programs.

    When you get into higher frequencies, a breadboard just doesn't cut the mustard; there are simply far too many stray coupling paths.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Errors, like enemies, accumulate. :eek: ;)

    If you're hell-bent on using RC, your best bet would be to get a mylar, metal-film or poly capacitor and use a potentiometer to "dial in" the frequency.

    Even then, you won't have any kind of decent accuracy. Variations over temperature will throw things wildly out of whack.

    A basic crystal oscillator is orders of magnitude more stable and accurate than an RC or LC network. It's rather difficult to describe without showing you displays from a spectrum analyzer or network analyzer, but I don't happen to have one handy at the moment.
     
  9. Adrastos

    Thread Starter Member

    Jan 5, 2009
    17
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    I'd actually really rather not use RC, I'm in engineering and know all about the propagation of errors :p

    The real problem at the moment is cost - the only electronics shop I've been able to find here is terrible, and I have to order most things online, which gets expensive with shipping when you only need a few parts here and there.

    However, now I'll try to "deadbug" it, when I have some time (after finals perhaps..)

    Thanks!
     
  10. Adrastos

    Thread Starter Member

    Jan 5, 2009
    17
    0
    In case anyone was wondering, I finally grabbed the time and money (being in school, I haven't gotten a paycheck since last August..), and created a slight variation of your `dead bug' technique.

    And it worked PERFECTLY!

    Thanks guys!!
     
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