How can I build a SR flip flop using op amps? (for a hand made 555 timer) Transistors may be used. I was thinking of using a hysteric op-amp and putting a schmitt trigger on the input of comparator 2. However, my solution fails in simulation. Here's my current circuit.

circuit output

It works with the LTSpice model of a SR Flip-Flop so I know everything else is working as desired.

circuit1 output1

  • \$\begingroup\$ I don't understand why one needs to be built out of op-amps. There's actually several low-priced chips that do exactly what you need. One is a 4013 CMOS flip-flop with data input, clock input, set input and reset input. You can wire it as an SR flip-flop by tying clock and the data input to ground. \$\endgroup\$ – user116345 Oct 22 '16 at 21:25
  • \$\begingroup\$ It's for a circuits project. We can't use a SR Flip Flop for the 555 timer. We must build one using op amps. \$\endgroup\$ – IHideKeys Oct 22 '16 at 21:26
  • \$\begingroup\$ but the thing that's odd is that op-amps are meant for analog applications (like audio amplifiers) and you're trying to use it for a purpose it's not intended for. \$\endgroup\$ – user116345 Oct 22 '16 at 21:27
  • \$\begingroup\$ From the assignment: You have your circuit kit but you notice that you do NOT have a 555 timer IC chip or a SR flip flop which would make this task easy. You’ll have to find a way to create them on your own. Though you have a transistor to test your ideas, you cannot use it directly in your final circuit. Find a replacement component that will provide a similar function as a switch. In a preliminary circuit, describe a transistors function and characterize it using the Discovery Board. \$\endgroup\$ – IHideKeys Oct 22 '16 at 21:29
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    \$\begingroup\$ An LT1016 isn't an opamp, it's a very fast comparator. \$\endgroup\$ – EM Fields Oct 23 '16 at 5:21

Have a look at the Vo-Vi curve of a hysteresis cycle:


simulate this circuit – Schematic created using CircuitLab

If you are able to force Vi above the positive threshold, the output will go high and will stay there when Vi returns to 0. If you you force Vi below the negative threshold the output will go low and stay there forever.

You can achieve this (well, actually the inverse) with


simulate this circuit

  • \$\begingroup\$ And what happens if you press both SET and RESET? :) \$\endgroup\$ – next-hack Sep 25 '17 at 18:56
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    \$\begingroup\$ Nice fireworks :) Which can be avoided putting resistors (say 1K) in series with the pushbuttons. If you make them different value, you can even choose which one "wins" in case of simultaneous press (and release) \$\endgroup\$ – Petrus Sep 27 '17 at 8:29

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