AND gate using a voltage comparator monoshot

Discussion in 'General Electronics Chat' started by devalvyas, Jun 24, 2009.

  1. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    i have made the following circuit for two voltage comparator monoshot, whose output comes to gether as AND gate and the output of AND gate goes to NC switch of 555.

    When both the comparators are triggered, the output goes high, in turn closing both the transistor switch Q1 and Q2. This closes the NC switch of 555.

    When either of the voltage comparator goes low, the the corrosponding Q1 or Q2 opens there by opeaning the NC switch of 555 timer and triggering the timer. i want the 555 timer to be trigged when any one of the voltage comparator goes low after being triggered.

    kindly let me know your views especially on the component values.....

    (please excuse the poor connecting line from Q1Q2 to 555 timer NC switch, the lines are ment to be wires and i could not include them in the rimu schematics software that i am using, hence i did a draw job)
     
  2. DrNick

    Active Member

    Dec 13, 2006
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    2
    I think there may be some wiring/design errors.

    The way the comparators are configured, they will always have low outputs. The non-inverting inputs are pulled down to ground thru 1M resistors. The inverting is biased to 6V (.5 * 12V). When you close one of the switches that is capacitively coupled to the non inverting input, this will only raise the voltage, thus doing nothing.

    Furthermore, when Q1 and Q2 turn on (if they ever would), nothing will happen. Q1 will do nothing as its emitter is tied to the positive voltage rail of the 555 circuit. Q2 will do nothing because it will try to pull C3 to ground, where it already is pulled down thru R2.
     
  3. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    actually i have to check the entire ckt. i have checked only one voltage comparator and its working fine. I check the entire ckt and post the reply.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    I wonder if the circuit actually needs those transistors after the comparators?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Just a slight "clean-up" of the OP's schematic, and extracted it from the Word document so it's easier to view.

    In addition to DrNick's comments...
    I don't know why he'd use clamping diodes on both the inverting and noninverting inputs of the comparators.

    If Q2 could ever turn on, it would attempt to charge B1 from B2.

    R9 and R10 aren't necessary, since R3 and R7 already limit current.

    R3 and R7 should be 1.5k.

    In the 555 portion, the most that one would see on pin 3 is Vcc-1.3v. Since Vcc=5v, pin 3 would be limited to 3.7v, which means 370 Ohms. However, there is no point in putting 100mA current through a transistor's base when you'll probably saturate it at (desired collector current / 20).
     
  6. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    I am sorry about the schematics...you were right...the switch is at wrong terminal. The switch is on non-inverting terminal of comparator.

    Also, the diodes are not really doing anything...i removed the diodes and still the circuit is working fine....

    i have posted the revised schematics..(this time without any draw job..but in word format...) please let me know your views..i m yet to test the entire circuit..

    i will again post after checking the entire circuit..

    Also is the optocoupler switch really necessary...can i directly connect the emitter of Q2 and collector of Q1 to the input of 555 and use it as trailing edge trigger...as shown in second schematic in the same word file..

    actually my intension is to to trigger the 555 on trailing edge of the pulse....


    Also, as beenthere is suggesting...how can i avoid the transistors...? can you please explain...??
     
    Last edited: Jun 25, 2009
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    As it is now, both transistors have to be on to pass current through the optocoupler. Both comparators are in the same stae - one comparator with the capability to source or sink enough current could replace both transistors and simplify the circuit.

    If both switches have to be made, use isolation diodes so they make a diode AND gate to trigger the comparator.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Please post your schematics in .PNG format. It makes them much easier to view as compared to Microsoft Word format.

    You do not have any current limiting for the optocoupler's LED. This will cause the optocoupler to be destroyed as soon as both Q1 and Q2 are turned on.

    A 620 Ohm resistor between the collector of Q2 and V+ will limit the optocoupler's current to around 20mA. Note that Q2's base current (around 7mA) will flow through the optocoupler's LED as well. If you want that to be less confusing, move Q2 below the optocoupler's LED.

    C5 is indicated as a polarized capacitor. It must be non-polarized, as otherwise it will be destroyed.
     
  9. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    can you please explain with schematics.

    i have done the changes as sgtwookie suggested...attaching the schematics as .png...but the clarity is not good...i am copying the schematics in paint and then saving it..
     
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