storing the output transition of a comparator

Discussion in 'General Electronics Chat' started by raj_mkrj, Jan 23, 2010.

  1. raj_mkrj

    Thread Starter New Member

    Jan 23, 2010
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    0
    hello everyone,
    will anybody help me by recommending the best way to store/latch the output -Vsat to +Vsat transition of a comparator circuit (designed by IC741)....:confused:
    whenever there is a change in the input of the comparator from the reference value the comparator output will switch from
    -Vsat to +Vsat..
    I want to store or latch this transition(duration may be few milliseconds)for indefinite
    period of time..how can i do that using simple circuit components..:confused:


     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    One solution would be to use a latching comparator such as the MAX912.

    hgmjr
     
    Last edited: Jan 23, 2010
  3. Wendy

    Moderator

    Mar 24, 2008
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    Are you using a 741 IC? That is most definitely not a comparator.

    The problem is power supply, a 741 needs some pretty high voltages to do it's job.

    If you want to sample and hold a comparator (such as a LM339) a simple flip flop would work.

    We need more information though, where is the voltage the comparator is using and what is it comparing it to?
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Heh, OK, you beat me this time. Interesting circuit, didn't know it existed.
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. raj_mkrj

    Thread Starter New Member

    Jan 23, 2010
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    Sir Bill Marsden,
    I have designed this comparator circuit using LM741..the circuit is attached with this post..
    our aim is to detect the change in the output voltage of the LDR circuit by comparing it with a pre-selected reference value...From the knowledge of the calibration of LDR,we know the output from the LDR in presence of light from the LED ,without obstruction and with obstruction between them.As soon as an obstruction is there between the LED and the LDR, the comparator will switch from -Vsat to +Vsat...I want to latch is this short duration transition for infinite time and may use this to initiate an alarm or something like that..

    now what to do?? A sample and hold circuit or a Simple Hold circuit(basically a low loss capacitor) can store this transition but only for a finite period of time say 1 hour or more...but for infinite time,,what to do???:confused:
     
  7. raj_mkrj

    Thread Starter New Member

    Jan 23, 2010
    3
    0
    Sir bertus,
    I have designed the circuit with the only available power ±5/±12V power supply kit manufactured by a local vendor..That is the only available ready-made power supply in our college LAB..they have mentioned the current as 1.5A..
    you have suggested the using of an SR latch in this case..But I have already tried that for repeated times..But the NAND gate latch(made by using IC 7400) is not working..in our previous semesters we have used the same IC in our Digital electronics lab, the only difference is that in that time we are working on specially manufactured digital trainer kit(some thing like this) with in built supply and toggle switches..

    Now when we are trying to implement that on the bread-board there is no result..i wonder what is the problem:confused:...according to your sugg i must connect the output of the op-amp based comparator(pin 6) to the SR latch..but my question does the out-put current of op-amp in this case is sufficient to drive the SR latch?...the volate is surely LOGIC 1 when transition occurs..but what do with the current??

    how to work with the IC7400 in this condition...it is not even following the truth table when i am trying to test it by placing it on the bread-board and using that 5V supply as logic 1 and ground as LOGIC 0.....
     
  8. bertus

    Administrator

    Apr 5, 2008
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