So I have one unused OpAmp left in my circuit and I would like to use it as "latching comparator" (for lack of a better name), but I'm having hard time wrapping my head around it. Here is how I want it to behave: "latching comparator" behaviour

Desired properties are:

  • Fairly high input impedance
  • Works without inputs reaching rail volatges
  • Speed doesn't matter.

Closest thing that I found so far is this. But I wasn't able to figure out how can adapt it to my application.

I hope this makes some sense. I'm trying to keep it abstract(quick to read), but if more detail is needed or this turns out not to be possible - I'll add in more info.

Ok, so to be more precise of what my goal is:

I'm using LM3419 as bar-graph driver chip and I need a circuit that would latch when top graph LED turns on and stay untill bottom LED turns off. I use output of that to enable/disable some other chips.

LM3914 pulls constant current through LEDs, but I don't know what LEDs at what current I will use, hence arbitrary "high" and "low" levels. I have one free LM358 available and I wonder if I can make it work as described.

Wanted to add schematic representation here, but it doesn't let me (need more reputation). I hope it's understandable :P

  • \$\begingroup\$ It seems you need just a hysteresis configuration. \$\endgroup\$ Jun 6, 2017 at 18:10
  • \$\begingroup\$ Google Schmidt trigger \$\endgroup\$
    – user76844
    Jul 23, 2017 at 0:58

1 Answer 1


Try this; given the circuit will latch, with +-0.5 volt overdrive to provide robust 1-bit-storage, are diodes from your Set and Reset inputs all you need to add?


simulate this circuit – Schematic created using CircuitLab

  • 6
    \$\begingroup\$ Although the circuit is sound I would use 10x or even 100x smaller resistor values. With these 10 Mohm and higher resistors 1) the voltage drops as soon as you connect a multimeter (which will be 10 M ohm input impedance) and 2) the circuit will be disturbed when you sneeze at close range. I'd use 100 kohm and 220 kohm instead and enjoy a less sensitive circuit. \$\endgroup\$ May 3, 2017 at 12:08
  • \$\begingroup\$ Yes. The OP needs to provide the real specs required, eh? \$\endgroup\$ May 3, 2017 at 12:16
  • \$\begingroup\$ I've just tried it in breadboard with 100x smaller resistors and it doesn't seem to work (when input2 goes high, output falls back down). \$\endgroup\$
    – PTD110
    May 3, 2017 at 12:47
  • \$\begingroup\$ My input "high" sits at 2.8V and "low" is 1.6V, I guess it will become more clear if I append what it is that I'm trying to do. \$\endgroup\$
    – PTD110
    May 3, 2017 at 12:54
  • \$\begingroup\$ What opamp are you using ? The suggested TL081 is unsuitable for 5 V. You could increase the supply to 15 V and see if that works or use an opamp more suitable for a 5 V supply. \$\endgroup\$ May 3, 2017 at 14:21

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