Disclosure: I’m using the iCircuit app to model the question below

When I manually construct (in iCircuit, bottom half of image)the synchronous SR flip flop it works as expected. My expectation is that when holding clock HIGH, pressing the S button should illuminate the LED (and stay on) until the R input button is pressed.

When I insert what should be an equivalent IC (labeled SR flip flop in iCircuit, top half of image) I cannot cause the output Q to change state, no matter how I push those buttons or rewire it. I attached the image as I would expect it to work. Any advice? I am strictly attempting to learn, there is no project in mind.

enter image description here

  • \$\begingroup\$ When the buttons are not pressed the associated inputs will be floating and have undefined logic levels. You need to add pullup resistors between the switches and the gates. \$\endgroup\$ Commented Dec 26, 2021 at 20:34

1 Answer 1


The circuit above is a SR Flip-flop. The circuit below is a SR Latch.

They are not equivalent.

The triangle in the SR Flip-flop means it requires a clock edge (most likely rising) to operate, it does not work with only a static level at the clock terminal.

The SR Latch just has an enable input so it requires only a correct level to be enabled, no edge is necessary.

  • \$\begingroup\$ That was it, thank you. If I trigger the s or r pins whilst a clock signal comes in it works. Thank you for turning me towards the rising edge factor. \$\endgroup\$
    – exaustgas
    Commented Dec 26, 2021 at 22:24

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