Pull Up resistors

Discussion in 'The Projects Forum' started by jpitz31, Dec 3, 2010.

  1. jpitz31

    Thread Starter Active Member

    Oct 24, 2007
    Hello All,

    I am working my way through the Book "Make Electronics" by O'Reilly.

    In Chapter 4, page 156. The text indicates:

    "R5 and R6 are known as "pull-up resistors" because they pull the voltage up.
    You can easily overwhelm them by adding a direct connection to the negative side of the power supply"

    Enclosed is the schematic being referred to.

    My question is, As long as S1 and S2 are not pressed the voltage is up, almost to the power supply voltage, so why would you need the pull up resistors?

    How do pull up resistors pull the voltage up?


  2. tyblu


    Nov 29, 2010
    Without those resistors, the inputs (1 and 4) are left "floating". Their value is not guaranteed to be anything, despite what you measure. It means that it can easily change if the environment changes, such as being connected to a trace on a PCB and subjected to a little bit of EM interference. A resistive tie to the positive terminal, with very little or no current flowing through the resistor, 'pulls' the voltage up to very near the positive rail voltage. Pull-up and pull-down resistors are often used in digital systems, such as IO pins on microcontrollers, glue logic, or transistors, to guarantee their state when they are not 'activated'.
  3. radiohead

    Senior Member

    May 28, 2009
    Resistors oppose change in circuit current. In doing so, they generate heat, and develop a voltage drop across them. Pull up resistors are typically used in logic circuits to ensure the inputs don't have excessive current running through them.

    Scroll down to the very bottom of this page, and you will see other posts with questions about pull up resistors.
    Last edited: Dec 3, 2010
  4. jpitz31

    Thread Starter Active Member

    Oct 24, 2007
    Thanks tyblu and radiohead for the great answers.