how to take output from this timer?

Discussion in 'The Projects Forum' started by ram das, Jan 15, 2010.

  1. ram das

    Thread Starter New Member

    Jan 15, 2010
    2
    0
    hi friends,
    i have this timer using (cd4001) nor gates and counter(4020),
    on connecting supply D2 shoud b low and after 2 minuts(selected by pin 13 of 4020) it must b high............but alaaaaaaas it is not so on breadboard...........
    can u tell me this idea is crect or
    output will appear at any other place??
    thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    One guess is your LED's. They have no resistance after being biased into conduction. CMOS logic outputs are very current limited, so the LEDs' effectively clamp the outputs to ground.

    Try adding 2000 ohm resistors between the LED cathodes and ground. That may limit current enough for the logic to operate. It is possible you have destroyed the IC's because of the excess current, though.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Another good technique for driving LEDs from a CMOS output is a simple NPN transistor, something like this...

    [​IMG]
     
  4. ram das

    Thread Starter New Member

    Jan 15, 2010
    2
    0
    thanx vry mch.....but now it gives blinking and nt a constant output at 13.
     
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