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Question regarding 7402 ic

  1. Feb 6, 2013 #1
    Hello everyone...!!!!
    i have made different logic gates using 7402 ic (quad 2-input nor gate ic). what actually occurs is that after joining different gates for making "and" gate & "or" gate until i give the input the LED glows( i have connected the negative terminal of LED to negative terminal of battery and the output of final nor gate to the positive terminal of battery).
    But before giving high or low input on any of these gate (i.e. 0 volts in the input terminals), the bulb glows, but as soon as i connect low voltage source (i.e. 0 input) the bulb doesn't glows(which should happen)..
    so i didn't found it why the LED was glowing when i didn't gave any input. Can anyone please help?????
  2. jcsd
  3. Feb 6, 2013 #2


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    If you leave any of the inputs disconnected the voltage on that input is undefined.

    CMOS logic gates have a very high input impedance. The pins on an unconnected input can act like an aerial and pick up noise (static electricity, induced voltage from the 50/60Hz mains or even radio waves). The results are unpredictable, the logic gate may well amplify this noise or treat it as a logic 1 or 0 causing the LED on the output to light up.

    It's quite possible the LED is flashing at 50 or 60Hz or trying to flash at radio frequencies!

    When you connect the inputs to 0V (or VCC) with a wire you not only force the input to be a logic 0 (or 1) but you also change the impedance of the input node and this stops the input picking up noise.

    When you design a circuit all unused/spare inputs should be connected to 0V or VCC.
  4. Feb 6, 2013 #3
    thanx CWatters
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