Switches as inputs

Discussion in 'The Projects Forum' started by Gadersd, Apr 2, 2013.

  1. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    I am going to use switches as inputs to my registers. The problem is that I am using ttl ics and cannot use pull down resistors to prevent floating inputs. I know I can use pull up resistors, but then the default value will be 1 instead of 0 with a pull down resister and I need the default to be 0. How can I use a pull up resister without having to invert the outputs?
     
  2. FrozenMel

    Member

    Aug 23, 2011
    34
    0
    So just to be sure, you want to use a register that has a TTL input and want your normal state to be 0 and your switched state to be 1?

    If that is the case, and you aren't worried about power loss too much, you could use a high value pull up resistor and have the switch normally closed, connected to ground. Then when you flip the switch, the ground short is disconnected and the pull up resistor takes over changing the logic level from 0 to 1.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    What's the fuss about the default being 1? Use a pull up resistor and reverse the logic in the code.
     
    Last edited: Apr 2, 2013
  4. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Inverters and inverting registers are always an option.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Are you really married to the standard TTL series?

    Any mos type can use a pull down resistor.
     
  6. absf

    Senior Member

    Dec 29, 2010
    1,490
    371
    If you like, you can use the more popular 74HCxx or the 74Cxx series of TTL. Pull down is allowed on these devices.

    Allen
     
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