AND gates always on?

Discussion in 'Digital Circuit Design' started by Noisettes, Oct 7, 2017.

  1. Noisettes

    Thread Starter New Member

    Oct 7, 2017
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    Hi, so im trying to build this circuit

    [​IMG]
    But im stuck at the AND gates. Theyre supposed to be a keypad password that when the buttons 1,4, and 7 are pressed in order...theyll unlock or output high at the last AND gate.
    Problem is even without pressing the buttons the last gate already outputs high. Is it because im using 74LS08 instead of the recommended 74HC08 in the book?
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    If you press any of the numbered buttons other than 1, 4, or 7 does that last gate output go low?
    If not measure the voltage on all the AND gate pins with no buttons pressed and report here.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Could be.
    Substitution of parts without noting what problem the differences may cause is not good design practice.

    What are the value of the resistors at each gate input going to ground?
    Look at that resistor value and then the input current required to pull the AND gates to a logic zero (which is the purpose of those resistors).
     
  4. AlbertHall

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    Good point. They could be too high for 'LS chips.
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What are the component values?
    There are none given.
    It could be that the pull-down resistors will not work with the LS08.

    Bertus
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    If the circuit calls for 74HC08 then install 74HC08. It is not interchangeable with 74LS08. They are two completely different families and semiconductor technology.
     
  7. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Welcome to AAC!
    Ignore any publication promotes schematics drawn in that fashion. No component designators or values??

    Do yourself a favor and don't waste your time, or ours, on such junk.
    Newbies should not make substitutions when they don't understand enough to know what problems they'll be creating or know how to troubleshoot them. 74LS and 74HC are not interchangeable.

    What values are you using for the pull down resistors?
     
    Last edited: Oct 7, 2017
  8. Noisettes

    Thread Starter New Member

    Oct 7, 2017
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    Yep, if its any other button, the gate goes low.
    The resistors are 10k...too high? What would happen if they are too high?
    I dont think HC are common around here -_-
     
  9. Noisettes

    Thread Starter New Member

    Oct 7, 2017
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    Also, since im pretty much a noob, how can i measure the voltage of the AND gates? Im comfused where to put the voltmeter probes
     
  10. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    The power and ground pins (or any other pins) are not shown on the gates. I hope you connected them?

    One probe goes to the negative of the power supply, the other to the gate your trying to measure.

    Ones and zeros are defined by the logic family. HC, LS, C etc.
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    What? 74HCxx not available where you are?

    If you insist on using a 74LS08 then the resistors connected to GND (-) are pull-down resistors and will have to be between 100 - 220Ω.
     
  12. Noisettes

    Thread Starter New Member

    Oct 7, 2017
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    Sorry about that, heres the more conc[​IMG][/URL][/IMG]rete version
    [​IMG][/URL][/IMG] [​IMG]
     
    Last edited by a moderator: Oct 7, 2017
  13. Noisettes

    Thread Starter New Member

    Oct 7, 2017
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    Sorry about that^
     
  14. AlbertHall

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    Replace all the resistors connected to gate inputs with 1k resistors - 4 resistors.
     
  15. Noisettes

    Thread Starter New Member

    Oct 7, 2017
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    Wow...that is small...
    Thanks for the info :)
     
  16. AlbertHall

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    They don't need to be that low for 74LS - input current 0.36mA.
     
  17. dl324

    AAC Fanatic!

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    1k will do, they could be larger, but 1k is a pretty typical value for TTL.
    upload_2017-10-7_10-47-6.png
     
  18. WBahn

    Moderator

    Mar 31, 2012
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    Also, what are you doing with the unused gates in the package. The inputs should all be tied to one of the power rails. This is not as important on TTL families, as they tend to pull high internally, but is critical on CMOS families.

    The original schematic looks like buttons 1-4-7 have to be held down together, no pressed in any particular order. There is no memory element involved to remember that the 1 was pressed before the 4, for instance.
     
  19. crutschow

    Expert

    Mar 14, 2008
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    That's okay for LSTTL but not the old standard TTL.
    Standard TTL has a input current of -1.6mA for a logic zero so the resistor must be less than 500Ω to keep the input voltage below the 0.8V maximum zero value.
     
  20. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I retrieved the intended image from post#12 with the component values:

    and_gate_Screenshot_2017-10-08-01-22-36_zpsgn69cx5c.png

    Bertus
     
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