bcd logic

Discussion in 'General Electronics Chat' started by duxbuz, Apr 8, 2014.

  1. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Hi
    This is continuing on from a post I created. I didnt really know whether to start a new thread but felt the old one was finished and this is slightly different.

    Anyway....

    I have a 74HC42N BCD to decimal decoder
    http://www.nxp.com/products/logic/de...s/74HC42N.html

    I am basically probing its logic just to watch it working.

    I created the circuit and I get strange results.

    Sometimes I get pins seemingly stuck in a state. Hard to explain as not in front of circuit at moment.

    I press a switch to set an input high, then the state of the output on the IC may change but then the IC output stays in new state after button released.

    The other issue is that I am not really getting the expected logic outcomes from pressing the switches.

    I have only had one time that the logic seemed to work as the truth table outlines.

    I have posted my circuit on here just to see if there are any glaring errors.

    I will continue to debug it, but I just wondered if anyone can cast an eye over it to see if it generally looks correct.

    I have tried the inputs with resistor pullups as in picture and with pulldowns. I have tried 1.5k as in picture resistors and 10k resistors.

    I have also had some outputs with leds, and resistors to ground, like a previous circuit i copied for Nand gates.

    Anyway I am a bit stuck. Could anyone help please?

    My circuit:

    http://i1025.photobucket.com/albums/y319/duxbuz/MV900F/SAM_0558_zps15b559a9.jpg

    Thanks
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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  3. crutschow

    Expert

    Mar 14, 2008
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    It sounds like your pulldown resistors are not connected to circuit ground/common.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Check that the push-buttons are connected correctly.
    Check that you are using the power and ground rails on the breadboard correctly.

    Is pin-8 GND of the 74HC42 truly at ground?
     
    Last edited: Apr 8, 2014
  5. duxbuz

    Thread Starter Member

    Feb 23, 2014
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  6. crutschow

    Expert

    Mar 14, 2008
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    You schematic looks fine so I suspect an incorrect or flaky connection on your breadboard. Either that or you have a faulty IC.
     
  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Your circuit shows the 74HC42 pins as High on Select, is this just a drawing error, because the Selected output pin is pulled low.

    E
     
  8. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Many thanks.

    I had missed off bridging over the ground from one breadboard rail. That didn't help!


    Then it seems my switches are a bit poor at seating
     
  9. ScottWang

    Moderator

    Aug 23, 2012
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    Using multimeter to make sure pin 8 at the GROUND, and the pin 16 has +5V, and measuring the voltage of output 0~9 when you press any input key.
     
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