Help with basic logic gates (practical)

Discussion in 'General Electronics Chat' started by Shagas, May 31, 2013.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Hello ,

    I just started to play around with the digital realm and I got myself some basic gates .

    I'm trying to connect this NAND gate I have ( SN7438AJ ) and play around with it but it's not working as it's supposed to be .

    ***
    First of all in the datasheet it says that it's a quad 2-input NAND buffer . I know what a buffer is , but does that mean that I can use this IC as a normal NAND and that it's simply buffered for more current sinking/sourcing capability? Or does this IC need a normal NAND IC to function?

    ***

    It also says that it's 'open collector' . I checked out some schematics to see what they mean but I cannot get a satisfying explanation in terms of how I should connect the gate (any advice would be appreciated) .

    Also do I need pull-up/down resistors (why ? ) on the inputs/outputs for the gate to function properly?

    The reason that I'm asking is because I'm trying to connect the gate and make it work but the gate is behaving like an 'AND' gate instead of 'NAND'
    I've tried connecting an LED through a resistor with the positive terminal hooked to + voltage and negative to the output of the first gate and the LED lights up when both inputs are high and off otherwise (that's an 'AND' function right? )

    I can't find any sources on the net that would explain practical wiring of Logic gates , I'd be very gratefull if someone pointed me in the right direction :)

    Datasheet of the IC i'm using :
    http://www.home.hs-karlsruhe.de/~bebe0001/74LSXX/7438.pdf
     
    Last edited: May 31, 2013
  2. MrChips

    Moderator

    Oct 2, 2009
    12,420
    3,355
    Open collector means that the output stage consists of a single NPN transistor with an uncommitted collector. That is, the collector is not connected - it is open.

    The purpose of this is the open-collector can be used to output voltages that are different from the standard TTL supply voltage of 5V.

    A second reason is that multiple open-collector outputs can be directly wired together to create a wired-AND logic function.

    To use an open-collector output you need to provide a load resistor to Vcc.

    Try using a resistor from 2.2k to 4.7kΩ from the output pin to Vcc.

    Next time, get a 7400 NAND gate instead, preferably a 74LS00.
     
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  3. Shagas

    Thread Starter Active Member

    May 13, 2013
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    eyy thanks for the info ;)
     
  4. sheldons

    Active Member

    Oct 26, 2011
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    heres an example of what you can do with these devices plus some info....
     
    Last edited: May 31, 2013
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  5. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Thanks sheldons , i'll check it out :)
    I know some basic theory about them but I'm trying them out on a breadboard for the first time
     
  6. sheldons

    Active Member

    Oct 26, 2011
    616
    101
    if you need any more info ill upload some more.....
     
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  7. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
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    If you've got some interesting things that I can make out of Basic NAND , NOR , OR gates then sure !
     
  8. crutschow

    Expert

    Mar 14, 2008
    12,984
    3,223
    The reason the NAND looks like an AND gate with the LEDs connected as you have them is, for a NAND gate, the output is logic low (conducts current to ground) when all inputs are logic high. Thus an output low, turns on the LEDs.
     
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