very annoying java breadboard problem

Discussion in 'General Electronics Chat' started by circuitnuts87, Jan 25, 2007.

  1. circuitnuts87

    Thread Starter New Member

    Jan 25, 2007
    1
    0
    hey guys, i ran this applet off of

    http://www.cs.york.ac.uk/netpro/bboard/jbreadboard.html

    anyhow this is my problem

    I wanted to create a simple and gate using the SN7408J chip.

    I initially set up my circuit as shown below,
    [​IMG]

    (you might have to click on the image again, when the new tab opens up to see it in better quality)

    this was supposed to be my 00-LEDoff circuit because the two switches are set to off and 0 represents off. however the LED lights up! Its an AND gate so its supposed to light up when both switches are turned on. I hope u guys got my problem.

    This is what my 01-LEDoff circuit looked like (it also shows what pin does what)

    [​IMG]

    This is what my 10-LEDoff circuit looked like

    [​IMG]

    and this is supposed to the 11-LEDon circuit, but it doesn't light up!

    [​IMG]

    thnx 4 ur time
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Your problems are due to not having your inputs properly pulled up or tied to ground. A floating input will rise to the operating point of the logic and cause oscillations. It's even possible for these oscillations to overheat and destroy the chip. For TTL, the pull-up resistors should be 4.7K. Just about any other logic family can use larger resistances of 20K.

    To work right, your switches should switch to ground. The inputs should be pulled up to Vcc as above.

    Your LED needs a current limiting resistor, too. See http://www.allaboutcircuits.com/vol_3/chpt_3/12.html for how to calculate its value.

    Things should work better after the changes.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Try connecting your gate inputs to +5 instead of ground. It should work then.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Not to be disagreeable, but straight TTL inputs should never be tied to Vcc. Always use a resistor. It's safe to go direct with any of the CMOS families.

    To expand on this, pins 4 & 5, 10 & 11 and 12 & 13 need to be tied to ground or pull up high. These are the unused inputs.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Quite right. Mea culpa.
     
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