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Project Notes

#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, 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






Credits and References

About LEAP#556 555 TimerDigital Logic
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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!).

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