I've seen this in several places and am perplexed. First, I thought that the output of an SR latch was Q, and the inverted output, or 'not Q' is shown as either Q with a bar over it or denoted with a circle at the output.

But as I try to understand the 555 timer, I'm confused! Several diagrams show Q with the circle, and Q bar without. And some show Q used as output but others show Q bar. Which is it, and what explains the confusing notation of the SR outputs?

enter image description here

Source https://www.electronics-tutorials.ws/waveforms/555_timer.html

Above, this one seems to have two inverting outputs only.

enter image description here Source https://www.build-electronic-circuits.com/how-does-a-555-timer-work/

This one (above) has outputs that make sense but also has a separate input to the SR latch called reset, which baffles me.

Is there a diagram that has the Q, Q bar, little circle, and output correct that I should be working from?

  • 1
    \$\begingroup\$ I wouldn't get too hung up on all this. Just consider it a broad hand-wave at a basic behavioral idea that you should be able to follow without getting stuck on trivia. One of them shows the reset line going to the SR block with a bubble and the other one doesn't show the bubble. Either way, RESET has to be LOW to be active because that's how all 555 ICs do things. Also, one shows only /Q (and RESET) and the other shows Q, /Q, and RESET. So ignore the differences. You can go argue with the drafters, I suppose. But why? \$\endgroup\$
    – jonk
    Dec 4, 2022 at 5:06

1 Answer 1


Technically, neither of the images you posted are correct. Here is an image grab that matches the simplified internal schematic of the original NE555 (still produced by TI). This matches the complete schematic if you take the time to dig through it.

As you can see, only the -Q output of the flipflop is used. It drives the Discharge transistor directly, and is inverted to produce the effect of a Q output at the device output on pin 3.

Internal Diagram of 555 timer IC

(Image source: Mechatrofice - Internal Diagram of 555 timer IC)

The 555 was not patented, and there have been many unlicensed and unofficial second sources over the decades. Not all of them are functionally identical. How the Reset input interacts with the flipflop and its outputs can vary from one manufacturer to the next.

Examples of different behavior from different 555 timer ICs

(Image source: Tony van Roon - 555 Timer/Oscillator Tutorial {Internet Archive link} {Mirror})


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