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#556 555 SR Latch

Set/Reset latch with a 555 timer.


Here’s a quick demo..



A Set/Reset latch is one of the fundamental building blocks in digital logic: essentially a 1-bit memory device (set high with a “set” input, and remaining high until a “reset” input is received; repeated pulses cause no change to the output).

The behaviour is summarised here:

S R Action Q (next)
0 0 Hold State ~
1 0 Set 1
0 1 Reset 0
1 1 Invalid, n/a ~

The 555 timer has an SR Latch as one of its internal building blocks, so it should be no surprise that it is possible to build an SR Latch with the chip.

The trick is to essentially by-pass the “5-5-5” stacked comparators:

  • threshold (4) is pulled to ground, thus it can never activate and reset in the internal SR Latch
  • trigger (2) is the “set” input - normally pulled high, but when triggered (pulled low) will immediately set the internal SR Latch
  • reset (4) directly triggers the internal SR Latch reset when pulled low






Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.