Fork me on GitHub

Project Notes

#556 555 SR Latch

Set/Reset latch with a 555 timer.

Build

Here’s a quick demo..

clip

Notes

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

555_simplified_schematic

Construction

Breadboard

Schematic

Build

Credits and References

About LEAP#556 555 TimerLogic Gates
Project Source on GitHub Project Gallery Return to the LEAP Catalog

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

LEAP is my personal collection of electronics projects, 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 (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.