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Discussion in 'General Electronics Discussion' started by screwball, Apr 6, 2012.

  1. screwball

    screwball

    89
    1
    Jan 9, 2012
    Hello,

    I am testing components before making my final alarm circuit but i have a problem,
    Im testing logic gates atm (an 4input OR gate) and i am wiring pin 7 to 0v and pin 8 to 9v as supposed to, then obviously pin 1 to the led and then the return side of the led to 0v, but the led lights up even though none of the inputs for the OR gate have an input yet, why is this?

    when i put 0v to all the pins then leave it then the led will be off and when i remove the 0v from the pins on the chip led will still be off but the second i unplug 9v and plug it back in the led comes on again, is this because of static? if so how can i fix the problem? the easiest way preferably as i dont have 60million capacitors for all the logic gate chips im using in the circuit

    thanks in advance
     
  2. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    pull down resistor on the input pins. You can not leave pins floating. I am sure if you read the datahseet for the IC it will state something very similar.
     
  3. screwball

    screwball

    89
    1
    Jan 9, 2012
    pull down to 0v yeah?

    would 1000ohm 1/4watt be fine? as ive got hundreds i need to get rid of

    cheers
     
  4. screwball

    screwball

    89
    1
    Jan 9, 2012
    ive just tried it on the OR gate and it works fine, would this work the same on all ICs on the input pins such as 4063 and 4017?

    Thanks
     
  5. shrtrnd

    shrtrnd

    3,666
    453
    Jan 15, 2010
    Read the data sheets, they're readily available on-line.
     
  6. screwball

    screwball

    89
    1
    Jan 9, 2012
    i looked but didnt have a clue what meant what :(
     
  7. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    If you use a standard 4xxx series cmos chip at 9V, pullup, and pulldown resistors should be a bit higher than 1k. 22k should do nicely, this will load any output connected with 0.4mA. If you use 1k you'll 'over'load each output with 9mA, way to much.

    For external signals that can supply 10mA and higher 1k can be used, but you'll draw much more current than necessary when activating the signals.

    TOK ;)
     
  8. davenn

    davenn Moderator

    13,260
    1,748
    Sep 5, 2009
    That's ok :)

    I looked at the data sheets for both the chips you mentioned. There is NO reference to using pull down(up) resistors. You didnt miss anything.

    Just be aware thats its common knowledge/practice to do that to ALL unused inputs on CMOS chips in particular, else you get all sorts of weird results in circuit operations.

    I have not much experience with small micro's like Audrinos and PIC's maybe some here can state if its common practice with them as well

    cheers
    Dave
     
  9. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    4xxx cmos series ic needs pull-down or pull-up resistor to protect from static discharge sensitive unused input pins only. No need to place pull-up or pull-down resistor to protect output pins.
    If its really a "unused" input pin. You can directly connect to vcc or ground to protect it without resistors.
     
  10. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    Sometimes you use pullup on signals controlled by tri-state outputs, to establish a default state, or in a wired-and/ -or situation to save gates. If the resistor is too low you'll have problems driving it. Fan out and in is a little discussed theme today, with CMOS all over the place, but still a possible problem.

    I agree that a normal output to input connection does not need additional pullup/ down resistors.
    I find that when I do prototypeing, it is always wise to use pullup/ down resistors on unused inputs. If you suddenly need to use this input, you can use it directly without disconnecting a fixed Vcc or 0V.

    This is a technique I use when laying out PCBs too, for unused signals on a microcontroller. With high density SMD components it is much easier to solder a wire to a resistor pad or remove a resistor to add a signal than add a wire to fine pitch chips.

    TOK ;)
     
  11. screwball

    screwball

    89
    1
    Jan 9, 2012
    Hey, thanks for all the replies, its been so much help, ive got a few 27k resistors about so ill use them on unused input pins, so i dont need to put pull down resistors on the inputs that i use atall?

    ill probably put all the unused input pins straight down to 0v and if it works then it works, if not ill use pull down resistors on all inputs, im using stripboard for this circuit and my soldering iron just broke as one of the connections came out of the tip and i cant get to it because of the way it was manufactured so ive thrown it and ordered a new one so youve got a few days of peace from my lame questions lol

    Cheers
     
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