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I'm new to electronics and have been studying the 555 timer. One of the key components within the 555 timer is the "Flip-Flop", at least that's how every resource on the internet refers to it.

Here is a diagram of a 555 timer that is commonly found across the internet. enter image description here

You can clearly see the component is labeled a "Flip-Flop". If you look at the Wikipedia page for a 555 timer it states it uses a "SR Flip-Flop".

As we know, a "Flip-Flop" uses a Latch as an integral component. So what is the difference between a Latch and a "Flip-Flop"? Every resource I find, describes a "Flip-Flop" as having one of these two defining characteristics:

  1. It is a Gated Latch
  2. It is enabled through a clock signal

So, the problem is, in a 555 timer, every diagram shows the "Flip-Flop" only taking 2 inputs, and has no enable pin, therefore, it is not gated. Thus, it must be a simple Latch.

Again, I'm new to electronics, but my question is:

Does a 555 timer actually have a "Flip-Flop" or is it just an SR Latch?

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  • \$\begingroup\$ If you really want to look at it carefully: righto.com/2016/02/555-timer-teardown-inside-worlds-most.html (includes an interactive die shot, so you can click on parts of the die, and it'll describe what part of the schematic (also shown) is implemented by that part of the die (so if you're willing to work at it a bit, you can trace through the entire schematic from the die shot). \$\endgroup\$
    – Jerry Coffin
    Dec 3, 2020 at 7:09
  • \$\begingroup\$ You don't need every article. You just need one article to clarify what is latch and what is flip flop.en.m.wikipedia.org/wiki/Flip-flop_(electronics) \$\endgroup\$
    – Mitu Raj
    Dec 3, 2020 at 7:33
  • \$\begingroup\$ Even if the 555 didn't include a circuit that you would define as a "flip-flop" that doesn't matter. The diagram you found is a block diagram and from that engineers learn what the behavior of the IC is. It does not matter and the engineers don't care how it is implemented as long as the block diagram accurately describes the functionality. Which it does. Often in these "vintage" ICs, some transistors have shared functions making the individual parts harder to distinguish, only IC designers need to be able to understand that. \$\endgroup\$
    – Bimpelrekkie
    Dec 3, 2020 at 8:55

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It's a level-sensitive (level-triggered) latch (or level triggered flip-flop). I don't think it really helps to get bogged down into the definitions and taxonomy in this kind of thing unless you have some authoritative source to point at that is respected as such. Terminology evolves and the 555 is an ancient IC.

An R-S flip-flop is the same thing as an SR latch etc.

The Wikipedia entry actually notes this evolution:

Recently, some authors reserve the term flip-flop exclusively for discussing clocked circuits; the simple ones are commonly called transparent latches.[1][2] Using this terminology, a level-sensitive flip-flop is called a transparent latch, whereas an edge-triggered flip-flop is simply called a flip-flop. Using either terminology, the term "flip-flop" refers to a device that stores a single bit of data, but the term "latch" may also refer to a device that stores any number of bits of data using a single trigger. The terms "edge-triggered", and "level-triggered" may be used to avoid ambiguity.[3]

Even if some boffin attempts to encrispen the definitions, there's no guarantee the rest of us are going to cooperate, and those old datasheets are not going to revise themselves even if it was desirable.

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  • \$\begingroup\$ You say the 555 timer is ancient IC. Is there anything that is its successor? \$\endgroup\$
    – Gary Holiday
    Dec 3, 2020 at 17:55
  • \$\begingroup\$ CMOS versions eg. LMC555, TLC555, which are "only" 30 or 40 years old (the NE555 is 50 years old) may occasionally be useful, but usually it's better to use a microcontroller to get that kind of timing function and add any external parts necessary to operate from whatever supply is available and/or to switch power at the output. \$\endgroup\$
    – Spehro Pefhany
    Dec 3, 2020 at 19:02
  • \$\begingroup\$ Is it always practical to use a microcontroller over a 555 timer? What if cost is a factor, it seems a 555 timer would be more cost efficient. \$\endgroup\$
    – Gary Holiday
    Dec 4, 2020 at 1:28
  • \$\begingroup\$ Sure, the parts are still sold in quantity today. Sometimes it’s cheaper because the 555 operates from high voltage, has high current output, and does not require the programming step in manufacturing. I have a novelty neon sign- a neon hockey player shape- that uses an NE555 for the high voltage DC-DC. But small micros are very cheap, pennies, so if there is anything else for the micro to do (say the sign had an auto shut-off mode after several hours) or if more precision is required a micro and support parts will be superior. \$\endgroup\$
    – Spehro Pefhany
    Dec 4, 2020 at 5:01
  • \$\begingroup\$ As a handy rule of thumb in figuring out what to use: CMOS TLC555 costs 3x more than NE555, but consumes >10x less power, and is >10x faster. LMC555 consumes >10x less power than the TLC, is 2x faster, and costs 2-3x more. This is an easy to remember progression of performance increase factors :) Prices as of today, based on buying direct from TI. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Nov 9, 2022 at 19:03

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