trigger circuit for comparator

Discussion in 'General Electronics Chat' started by devalvyas, Aug 10, 2009.

  1. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
    I am trying to build an edge trigger circuit for comparator. in the schematics shown, i want the switch for comparator to be replaced by a trigger circuit which will give only one short pulse to the comparator irrespective of the duration the switch is closed.

    once the switch is opened, the trigger circuit should be ready to trigger again...something like the trigger circuit that is used for 555 circuit in the same schematics...can you please help?
  2. kkazem

    Active Member

    Jul 23, 2009

    I'm trying to help you, but it would be easier if you could tell me what the purpose of the A1,A2,Q1,Q2,& Q4 circuits are for. In order for the 555 to trigger, it needs a relatively short, low-going pulse, as your Q4,C5,R9,& R10 circuit should do. The low-going pulse time for 1-time constant is 1 millisecond and the 555 monostable on timing out of pin-3 is about 1-second, assuming that your Ct cap is reasonably close to 10uF. Most electrolytics have a tolerance of +75%/-20%, which could seriously affect your timing. If you use tantalum or a multi-layer ceramic, you can easily get 10% tolerance on the cap.

    It seems that your trigger circuit (before Q4) will force Q4 to be normally-on when either switch is on or when both are off. When both are on, then Q4 base will be somewhat shunted to ground, but it will likely stay on due to the diode drop of about 0.65 from the diode between Q2 emitter and Q1 collector. When you add this drop to the on-state voltage of Q1 & Q2, which I expect will be around 200-300 mV, you get about 0.95 to 1.0 volts on Q4-base, which is an on-state for Q4 and it will almost certainly not turn off as it seems the designer wanted it to. Therefore, Q4-collector will always be near ground and C5 can never charge-down. Actually, C4 is in backwards if you are going to use a polarized cap since the way it's supposed to work is that when Q4 is off (open), C5 thru R10 will discharge, so that it's ready to generate a pulse when Q4 suddenly and quikly turns on. In that case, the Q4-collector side of the cap will be near ground potential and the R9 side of C5 will be near VCC=12V. Note that the side with the positive charge will be the (minus) end and the (pos) side of C5 will be the grounded end. This will damage any polarized cap; perhaps not immediately, but it will damage it over several uses.

    If you are trying to generate a low-going pulse, use nearly the exact same circuit as the Q4, C5, R9, & R10 circuit, except for 2 things. First, the positive side of the cap should be the side going to R9. And second, you can replace Q4 with a push-on switch. The switch can be pressed for as long as you want to and the pulse generated will be determined only by the timing cap and the timing resistor (R9 in this case). R10 is needed to discharge the cap when the switch is opened to get the ckt ready for the next pulse of the switch.

    IF you need more help, just reply, but try to be as specific as possible and try to address the issues I brought-up about your Q1 & Q2 circuit as it doesn't make sense.

    Kamran Kazem

    In a way, you answered your own original question about how to make short-pulse circuit. You can do it in the same manner as the 555
  3. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
    the entire discussion about this circuit and the help i got from forum is there in my threads on this forum. pl search with my name.

    i know how to make a trigger circuit for 555, but here there is a pull up resistor at pin 2 and the trigger ckt grounds pin 2 for the trigger duration.

    what i want is that that the trigger should give should connect non-inverting terminal of comparator to +Vcc for trigger duration. ...