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pull down resistor problem — Parallax Forums

pull down resistor problem

the_0utsider1the_0utsider1 Posts: 13
edited 2014-01-02 14:22 in General Discussion
In my circuit is as follows 5v from external source to NO switch to 220Ohm R to pin in8 between switch and the 220Ohm Resistor, i have another 10Mohm R going to ground as the pull down.
to use this, i run a motor when the gadget pushes the NO switch, the sw stops the motor. problem is is the chip is getting a high even when the NO is not pressed. I tried using 440Ohm for pull down with the same result.
Any ideas?

TIA

Comments

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-01-02 11:34
    Well, I don't usually use pull-downs; I'd use pull-ups and go low.
    But 10Mohm resistors are rather absurd at 5 volts, I rarely go any higher than 10Kohms for pullups.
    And a 440 pullup or pulldown seems extremely low resistance value.

    How do you figure out your resistor sizes?

    The BasicStamp Manual has good examples of how to wire pullups and pulldowns with a NO switch. And it is a free download. Why not try that reference. And I would toss the 10Mohm resistors in a junk pile.

    Page 143 of the Button command has wiring diagrams for you.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-01-02 11:51
    Two things come to mind:
    1. The switch is faulty and stays closed.
    2. Somewhere in your program you've set P8 to be an output set to "high."

    In any event, use a 10K or 4.7K pull-down. 10M is so high that your input may be responding to noise. 440R is too low.

    -Phil
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2014-01-02 12:20
    Hi Outsider;

    It's quite possible that the pin on the Prop, if it is a Prop running on 3.3V, has been damaged due to the excessive current sourced by the 220Ω resistor when the switch was closed.
    (5V- (3.3VCC + 0.6Vprotection diode))/220Ω=5mA.
    Clearly this is 10 times greater than the spec allows.

    This is a better and safer circuit for use with either a Prop or a Stamp processor.
    output.png


    Duane J
    output.png
    239 x 350 - 4K
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-01-02 12:27
    Duane,

    Since the thread is in General Discussion, and he's using +5V, I assumed it was not a Prop. However, if it is a Prop, the pin is almost certainly damaged by the excessive current.

    -Phil
  • the_0utsider1the_0utsider1 Posts: 13
    edited 2014-01-02 14:22
    Phil - my bad, I originally tried 10K resistors not M.
    Duane, while i dont have a prop chip, your circuit is exactly what i have wired - except for the 3.3 k R....

    I put a voltmeter in the circuit with the NO open and i get nearly 0V... I guess i will try another pin before looking at the resistors then...
    Thx all.
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