74LS08 trouble

Discussion in 'Homework Help' started by krzysiudobry, Sep 1, 2010.

  1. krzysiudobry

    Thread Starter New Member

    Sep 1, 2010
    2
    0
    Hello, I'm new

    I'm having trouble with the 74ls08:

    [​IMG]



    So here's what I get:
    When I connect the power to pin 14 (using resistor voltage divider because I use a 9V battery and I need 5V for this one) and ground pin number 7 I get 4V from pins number 11, 8, 3 and 6 without any input to all 4 gates!.

    when I connect the power to lets say pin number 13 and 12 I still get 4V from pin 11, when I ground any of the input pins i get 0V from output (for example pin 13-grounded, pin 12 - 5V, pin 11 - 0V) when i ground both inputs i still get 0V as output. Please help.

    here's what says on the ic: HD74LS08P OC16
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If you need to use a 9 volt source, CMOS logic is better. 74LSxx logic will fail if the Vcc goes above 5.5 volts. TTL logic tends to fail with the input shorted, so connecting an input directly to Vcc may cause problems. It's best to use a pullup resistor of around 10K ohms.

    A CD4081 has the same function, but will work from about 3 volts up to 15 volts.
     
  3. DonQ

    Active Member

    May 6, 2009
    320
    11
    Or at least use a 3-pin regulator like a 7805 or a lm317 type of regulator (or any of a wide variety of 3-pin options). This will give you a 'real' 5V supply with minimum hassle.

    Voltage dividers go wacko when the load changes, like when you connect different inputs, or even if the output changes, or depending on the values (it can even change when you just attach a meter to read the voltage).
     
  4. krzysiudobry

    Thread Starter New Member

    Sep 1, 2010
    2
    0
    thank you for info i'm gonna try that CD4081 and a 7805.
     
  5. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    Other than the voltage, I'm not sure what the issue is? The 74LS08 is a quad AND gate; the output is a "1" only when both inputs are "1", and "0" otherwise. On TTL, unconnected inputs tend to float hi.
    /mike
     
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