PROJECT HELP?! What are the replacements of CMOS ICs 4001 and 4070 and how can I use them?

Discussion in 'The Projects Forum' started by Mishaalijaz, May 22, 2016.

  1. Mishaalijaz

    Thread Starter New Member

    May 22, 2016
    I made a circuit on proteus with CMOS ICs 4001 and 4070..But for our project design, we are only supposed to use TTL..
    so I have to switch my ICs. I was informed to replace 4001 NOR gate with TTL 7402 NOR gate and 4070 with TTL 7486 XOR..
    But my circuit is not working properly..A picture of my circuit is attached here with CMOS ICs (which works perfectly fine) but when I switched, it didn't work properly. I need help as I have to submit my final design report in 2 days and implement it on hardware as well! By doing some research, the only thing I found on the web was that although the substitutes of 4001 and 4070 are 7402 and 7486 respectively in TTL but their wiring is implemented differently.. but at least it should work on proteus? I'm totally stuck..
    any suggestions on what to do, how to modify my design would be highly appreciated

    P.S. The circuit basically gives green light (opens the door on correct password input) when the bits of the switches are equal and gives a red light otherwise
  2. JohnInTX


    Jun 26, 2012
    You will need way smaller resistors for pull downs on the TTL inputs. To pull a TTL input low requires that you sink current out of it - how much current depends on the type of TTL (-1.6ma for standard, -.4ma for LSTTL etc.) Look up your family in the data sheet and use ohm's law to pull the required amount of current on the input port without raising the voltage above Vinput-low max.

    Example: for the standard 7400:
    Input low current is -1.6ma (must sink this much current from the input for a logic 0).
    Input low voltage (max) is .8V (the highest voltage allowed on the input to still be a logic 0)
    These two parameters must be satisfied at the same time so:

    .8V/1.6ma = 500 ohms maximum value for the pull down resistor. It should be a bit smaller for reliable operation. The same calculation (with different parameters) is done for LSTTL and the other TTL variants. As always, the datasheet tells the story.

    Note that CMOS works because the input current is essentially zero when at a logic 1 or 0 so your bigger resistors will pull the input down reliably. Note also that the TTL '1' input current is much less than the '0' current so larger pull UPs will be OK.
    Last edited: May 22, 2016