Help with and gate

Discussion in 'General Electronics Chat' started by gregcoll, Oct 25, 2009.

  1. gregcoll

    Thread Starter New Member

    Oct 11, 2009
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    0
    Hello

    Once again I am here begging for advice. I have included two rudimentary diagrams for the logic portion of my circuit.

    *Please excuse the figures, I could not find quad components in LTSpice and looked online for downloads but did not find any so I simply placed the components of one device together (i.e. A1 and A2 are part of one quad clocked d latch and A5 and A6 are part of the other) and I used the Pre input to signify the Polarity.*

    In fig 1 it is seen that I am using two LM14042B quad clocked d latches (I only use two channels from each one) to send signals out. They are wired so that the upper one clocks at Vss and the lower one clocks at Vdd. This setup works correctly by storing data from the previous output of the upper into the lower one so that this data can be sent to change the functioning of the upper one when the clock signal goes low. Anyway, it is a fairly simple setup that produces a repeating sequence out of the upper one like this:

    Q1 Q1bar Q2 Q2bar
    0 1 0 1
    1 0 0 1
    1 0 1 0
    0 1 1 0

    These signals do work to control the logic MOSFET's I am using for the power output. However, it is energizing two circuits at once and I would like to change the final output to one output at a time by using a quad AND gate (I am trying to use an NTE74LS08) in the configuration seen in fig 2 so that the final output would be:

    Q1 Q1bar Q2 Q2bar
    0 0 0 1
    1 0 0 0
    0 0 1 0
    0 1 0 0

    Again, the logic is simple and works well on paper but when I attach to the AND gate things stop working. I am using an NE555N timer which does its job well and has a 220 uF and 0.1 uF capacitor across the inputs as previously recommended to keep from causing signal corruption. I have tried using various resistors and diodes from different outputs but no significant effect was seen. In other words, I don't know what I am doing here so any advice on how to make this AND gate function is greatly appreciated.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The problem is that you are trying to use a 4000 series CMOS logic IC to drive a 74xx series TTL logic IC.

    The TTL inputs are too heavy of a load for the CMOS logic outputs.
     
  3. gregcoll

    Thread Starter New Member

    Oct 11, 2009
    21
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    Thanks very much once again. I will try to plan better in the future.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You're welcome.
    A 4081 quad AND gate is what you need.
     
  5. gregcoll

    Thread Starter New Member

    Oct 11, 2009
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    Outstanding. This was the same device I looked up at Digi-Key to use and so your verification makes me feel better. I am going to try to read a little on the differences in these systems so that I might design more suitable circuits. Thanks again for all the help.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    You're welcome.

    As long as you're ordering a few things, might as well pick up a few 4093 quad Schmitt-trigger NAND gates. These are very useful gates; they can "square up" slowly transitioning waveforms due to the Schmitt-trigger inputs, and using NAND gates you can build any other type of logic gate.
    See this Wiki entry: http://en.wikipedia.org/wiki/NAND_logic

    Also, using a single Schmitt-trigger NAND gate, a capacitor and a resistor, you can make an astable multivibrator with a 50% duty cycle.
    1) Both inputs are tied together.
    2) Connect the cap from the inputs to GND.
    3) Connect the resistor from the output to the inputs. The resistor should be >10k Ohms in value.
     
  7. gregcoll

    Thread Starter New Member

    Oct 11, 2009
    21
    0
    You recommended the Schmitt triggers before and so I did buy some to play around with but guess what... yes, they are 74 series with TTL applications. Lucky I didn't try to use those also. So now I need to come up with another project so that I can use these parts. After, that is, I do some research into low side current draw and little things like that. Cool stuff.
     
  8. roy1234

    New Member

    Oct 26, 2009
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    0
    I am going to try to read a little on the differences in these systems so that I might design more suitable circuits. Thanks again for all the help.
     
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