comparator with Transistor NAND Gate

Discussion in 'General Electronics Chat' started by devalvyas, Jul 4, 2009.

  1. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    i am using the following schematics for a NAND Gate. The two inputs of NAND Gate comes from output of two voltage comparators working in monoshot mode. on being triggered both the comparators give monoshot output for 10 seconds.

    What i want is that only when both the comparators are on, the output of the NAND gate should be low and the led will not glow, otherwise the output will be high and LED will glow.

    the gate is working fine, but with one problem. The ON time is getting Added...

    What i mean is that if i trigger both the comparator at time t=0 seconds the output of both the comparators should be high for 10 seconds, hence the output of NAND Gate should be low for 10 seconds (LED IS OFF) and then go high again (LED glows),

    the sequence is woking fine, except that the output of NAND Gate changes after t=20 seconds...can you please help?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I can't clearly read the values on all of the components.

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The most likely explanation for the discrepancy between your predicted timeout and the actual time out is the due to two issue. One is the leakage in the timing capacitor and the other is the comparator's very small input current draw.

    hgmjr
     
  4. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    attaching another file...

    the time out is happening correctly if i test it using only the comparators...i mean both the comparators are giving right timeout independantly...but when i connect them to NAND gate as above...the output of NAND gate is not giving correct timeout...

    the output of NAND gate should change as soon as any output of any one of the comparators change...somehow the nand gate is not allowing both the comparators to go low....
     
  5. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Are both transistors getting fully saturated?
     
  6. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
    80
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    i also changed the values of timing RC for the comparators...

    i changed C1=C3=100 micro farad and R4=R8=100K..

    Still i have the same problem.
     
  7. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
    80
    0
    yes...the transistors are getting saturated..the swithing acting is fine..the LED turns on and off correctly...but the timing is way too long...
     
  8. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    If you have a DVM, I would be interested in the voltage at the junction of the two 1M resistors in each comparator circuit. It sounds like the voltage is not exactly half the supply as you are intending it to be.

    hgmjr
     
  9. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
    80
    0
    i changed the values C1=C3=1000 micro farad and R4=R8=4.7K

    I removed the 10 k resistance from base of the transistors...and its working fine now...i guess the current from output of comparator was not good enough to drive the transistors...

    but there is some flickring in the LED's..so i am thinking of using 7401 NAND gate ic inplace of transistors...will that be better than this?


    i will do it and post the responce..
     
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