CD4070 vs 74LS86, what's the difference?

Discussion in 'The Projects Forum' started by bradlymathews, Feb 14, 2019.

  1. bradlymathews

    Thread Starter New Member

    Feb 14, 2019
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    They seems to be the same thing except for the pinout and the price. For building a full adder in a breadboard, does it matter if I use 74xx or CD40xx?

    Irrelevant background: I am doing a project with 4th and 5th graders where each student will eventually build his own full adder and then we wire them all together to make a nice big adder. In the run up we have lessons on binary, boolean and very basic electronics (almost done with boolean). I failed to build a calculator when I was in 6th grade back in the early 80's (ended up in software rather than hardware) so I am correcting that regret with the help of YouTube and 31 nine and ten year olds!
     
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  2. crutschow

    Expert

    Mar 14, 2008
    20,876
    5,929
    The difference is in their structure.

    The CD series are CMOS devices which used MOSFET transistors for the logic.
    These devices use essentially no current in the static state, can operate at from 3V to 15V supplies, and the output logic level goes from zero to the supply rail.
    The can readily operate from batteries with a long life-time.

    The 74LS use BJT (bipolar transistors) for the logic generation.
    These devices take a bias current even when in the static state and operate only with a 5V supply.
    The output logic goes from zero to about 3.5V.
    The steady current consumption and narrow supply voltage tolerance makes them not a good choice for battery operation
    They are however, faster than CD devices.

    I would go with the CMOS devices because of their very low power and wide power supply voltage operation.
    I see no advantage to using 74LS devices unless you really need the extra speed.
     
  3. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Welcome to AAC !

    I'd go with CMOS. It's lower power, has a wider operating voltage (you could power from a 9V battery), it's less expensive than TTL, is more readily available.

    One downside is that it's static sensitive.

    Don't try mixing logic families. It can be done, but you're students will get lost in the weeds.

    If you need faster logic, use 74HC. It has a smaller operating voltage range than CD4XXX, but not as power hungry as any TTL family.
     
  4. eetech00

    Senior Member

    Jun 8, 2013
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    I would recommend CD4000 series. They operate with a wide voltage range, are low power and all around more flexible. But they don’t output much current...about 6ma and are static sensitive. If you need to drive LEDs , you’ll only have about 5ma to drive them.

    Don’t mix 74 and CD series devices...they don’t play well together without special circuitry..

    eT
     
  5. bradlymathews

    Thread Starter New Member

    Feb 14, 2019
    2
    1
    Thank you all for the education you have provided me!

    I am thinking of going with the 74LS for two primary reasons:

    1. They are less sensitive to static. I have a bunch of kids in a classroom putting these together so robustness is a very useful feature.
    2. They output more current to drive the LEDs which are the output of our little calculators.

    I am using solid 5v power supplies so no worries there on either consumption or voltage range. Switching speed is irrelevant.

    If anyone thinks I am taking the wrong path here, please tell me before I order the parts! Again, thanks.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    That is false perception. All electronic components are susceptible to damage from ESD.
    However, if you feel comfortable using 74LSxx components that is fine. Just remember that you cannot mix 74LS family with 74HC, 74HC or CD4000 series.

    The other reasons for choosing 74LS vs CMOS are:
    1) Availability
    2) Cost
    3) Desired logic functions

    (besides supply voltage range and speed already mentioned).
     
  7. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Agree. All active devices are vulnerable to ESD, but some are way more vulnerable than others. Modern CMOS parts have ESD protection built-in, but in my experience they still are more delicate than good *old* TTL. No matter what you wrap around it, a sheet of glass that is 10 atoms thick is just a delicate little flower.

    ak
     
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