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Analog multiplexor help

Discussion in 'Electronic Basics' started by Jan Nielsen, Apr 17, 2007.

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  1. Jan Nielsen

    Jan Nielsen Guest

    I was recommended to use a analog multiplexor to expand the number of
    ADC ports on a picaxe "cpu" earlier.

    I have now tried to use them for a few days and cant get it working, any
    help please :)

    When I connect everything theres light in 2 of the 8 LEDs I used for a
    test, even when all 3 pins are low, if I remove the signal wire the 2
    LEDs still light.

    I connected the 8outputs to a LED each (with an resistor) and the pin3
    to a free output pin on the picaxe, and tried directly to +VDC too.

    GND to gnd ofcause, and +VDC to +VDC.
    I left the E and Vee unconnected, what are they used for anyway ?

    What am I doing wrong ?

  2. John  Larkin

    John Larkin Guest

    Not connecting E and Vee, for starters. Read the datasheet.

  3. Chris

    Chris Guest

    Hi, Jan. I'd guess you're talking about the 74HC4051 mentioned in an
    earlier post.

    As Mr. Larkin said, the datasheet usually tells all. At least, for
    those with a basic electronics background who know where to look.

    Since you're a newbie, though, you might want to check the website of
    the place where you purchase your ICs. Many have links to the IC
    datasheets available on the page where you see the price and

    One source for the HC4051 is On Semi -- here's a link to their

    The 4051 is a curious chip, with an analog section and a digital
    section. It's made so you can switch ground-referenced AC signals
    that go below GND. To do that, you have to provide a positive and
    negative analog voltage which is higher and lower than the maximum and
    minimum amplitude of your signal at Vcc and Vee (i.e. +/-5V analog

    However, and I don't believe it directly says this on this datasheet,
    if all your input signals are between the digital supply rails (like
    your LM35), you can tie Vee to GND and get away without a dual supply.

    One thing it does say pretty clearly is that the "E" is an enable, and
    it has to be tied low to get it to work. This can be neat if you want
    to parallel the single multiplexed pins and, say, make a 24-to-1
    analog multiplexer from three 4061s. But if you're just using one
    chip, and you don't want to turn the output off, just tie it to GND
    and be done with it.

    The curious thing is, all datasheets aren't the same. If you look at
    the Fairchild datasheet:

    the information about Vee is specifically mentioned on the first
    page. Go figure. In days of yore, I always kept the National Semi
    Logic databooks on the shelf for reference. I generally found them to
    be the most descriptive. You can now find that set of logic
    datasheets at the Fairchild site.

    Good luck with your project

  4. Jan Nielsen

    Jan Nielsen Guest

    Chris skrev:
    For a test the 8in/out puts are connected to a LED and the input/output
    is connected to VDC+.

    So by setting the 3 signal pins I should be able to control which LED is
    on, right ?

    The signal to the pins is from a picaxe, so its probaly around 4.7V and
    the supply is about 4.7V too.
    I have tied E and Vee to GND now, but it didnt help.
    When I power the chip, it still sends out power to 2 of the LEDS.
    I thought that 1) all outputs is off till I select one, 2) only one
    output in on at a time.

    I can disconnect the pin3 that holds the power I want to switch and they
    keep lit, so its using its own power to lit these 2 leds.

    Its Y2 and Y6, the second top pin on both sides.

  5. ehsjr

    ehsjr Guest

    Sounds like you might have a problem on the channel C
    input. What does it connect to? You cannot disconnect
    it - it has to be connected to either a high or a low.
    Same is true of the other pins.

    Can you post a schematic?

  6. Jan Nielsen

    Jan Nielsen Guest

    ehsjr skrev:
    Right, it cant be disconnected since it will fluctate ?
    The 3 pins are connected to digital output pins of the picaxe, so will
    be 0V og 4.7V when off/on.
    I have assembled it on breadboard, but I can draw up a schematic, will
    post it in an hour or 2.

  7. John E.

    John E. Guest

    I have assembled it on breadboard, but I can draw up a schematic, will
    You must post to alt.binaries.schematics.electronic , of course, not
    sci.electronics.basics. s.e.b. doesn't allow images to be posted. Just in
    case you weren't aware...

  8. Jan Nielsen

    Jan Nielsen Guest

    I have assembled it on breadboard, but I can draw up a schematic, will
    Here is my attempt in pad2pad, its a board design instead of a schmatic,
    but hopefully it gives you the information about connections.

    The layers is just there for clarity, and I have omitted the LED

  9. ehsjr

    ehsjr Guest

    It will be "floating" at an indeterminate level, making
    the result unpredictable.
    Good. Temporarily remove the connections from the channel select
    inputs to the PICAXE, and connect them to either low or high.
    You can manually test the chip that way by tieing each channel
    high or low per the datasheet to make sure you get the correct
    result. If it works properly that way, then the PICAXE is
    pulsing the C input, or putting it at an invalid level. Don't
    overlook the possibility of a flaky connection in the breadboard.

  10. Jan Nielsen

    Jan Nielsen Guest

    ehsjr skrev:
    With a new chip (multiplexor) and the signal wired to GND I get a +vdc
    on a6 and a1.

    Actually nomatter where I test the signal cables, those 2 and only those
    are active.

    Could I have the wrong datasheet?
    "4051 - 8 CHANNEL ANALOG MULTIPLEXER" is all the data I have on it, and
    no link to datasheet from the supplier.

    the chip reads hef4027bp which seems to be something different ?

  11. Of course it's something different, that's why they put part numbers on
    them. ;-) According to its datasheet that chip is a flip-flop:
    Who was your "supplier"?

    Hopefully you haven't killed any output port pins on the PICAXE. From now
    on, you should probably use 1K resistors between the PIC i/o pins and the
    logic device being driven. That way if you accidentaly try to drive a pin
    high and it is shorted to ground, it won't burn out the pin since the
    resistor would limit the current to 5mA. The resistor will have virtually
    no effect on things when an output pin is connected to a true input pin.
  12. Jan Nielsen

    Jan Nielsen Guest

    Anthony Fremont skrev:
    My supplier is just one of the electronics shops in denmark, I will try
    to get the right one.

    I dont think the pic is damaged, but even if, the small picaxe is only 3$ :)

  13. A 4027 is a dual JK flip-flop - a totally different part. The pins
    you are calling A6 and A1 are the Q output from one flipflop, and the
    /Q output from the other.

    You need a chip that is marked 4051 (or has "4051" somewhere in its
    part number).

    Do a google search for "4051 datasheet" (without the quotes) - the
    first link I see is to Fairchild's product folder on the CD4051. All
    the details on that part are available there.

    Note that the letter prefix and suffix around the number will vary
    between makers, but the number indicates the function of the part.

    Peter Bennett, VE7CEI
    peterbb4 (at)
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