Basic Latch Circuit...

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

  1. NFA Fabrication

    Thread Starter Active Member

    Aug 12, 2012
    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


    Mar 14, 2008
    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

    AAC Fanatic!

    Jun 22, 2012
    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),r:10,s:0,i:100
  4. Wendy


    Mar 24, 2008
  5. Papabravo


    Feb 24, 2006
  6. NFA Fabrication

    Thread Starter Active Member

    Aug 12, 2012
    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!