Basic Latch Circuit...

Discussion in 'General Electronics Chat' started by NFA Fabrication, Sep 20, 2012.

  1. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
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    I have found a few diagrams, and simply can't get anything to work right. I have been working with many different circuits lately, and have been able to figure out almost everything with out issue. I am now losing my mind...

    I am looking to use the momentary output output of an lm339 comparator to latch a circuit. I have an LM339 circuit that already has an output based on something else I am doing (< 1sec.). I have been trying to do this with 555 timers, but simply can not get anything to work. At the end, I am basically trying to turn on a relay, and keep it on until another pulse is sent from the LM339 to turn it off. The circuit is run off of 11.1V lithium batteries if you need a voltage reference (9-12.6V roughly between full/dead). What is the simplest way to make a latch circuit that works?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    The easiest is probably to use a Flip-flop such as the CD4013 D type. Connect the /Q output to the D input and connect the clock input to the LM339 output. The 4013 will alternate states for every pulse input. Make sure the LM339 has enough hysteresis (using a small amount of positive feedback) so you only get one pulse per transition.

    You will likely need to add a buffer transistor at the flip-flop output to drive the relay coil. Make sure you add a diode transient suppressor across the relay coil (cathode to positive side of coil).
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Like this circuit here, put the clock (pin 3) to your op-amp output, make sure you have a pull-up resistor on your op amp output( remove all the other components on pin 3)

    http://www.google.co.uk/imgres?q=40...220&start=0&ndsp=16&ved=1t:429,r:10,s:0,i:100
     
  4. Wendy

    Moderator

    Mar 24, 2008
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  5. Papabravo

    Expert

    Feb 24, 2006
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  6. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    I built this after you posted it, and it is almost exactly what I am looking for, but I was hoping that it could be triggered by a high or low input only, where as this one needs to be triggered from another part of it's own circuit. Any way to alter it for that?

    I will look at other options posted here as well, but I just happen to have a ton off 555's, so I was hoping I could utilize them. I also liked the above circuit because it uses very basic components that I have tons of!
     
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