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Project Notes

#061 SR Latch

Set/Reset latch with BJTs.

Here’s a quick video of the circuit in action:

RSFlipFlop demo


This is a classic Set-Reset latch. See LEAP#062 SRLatchFlipper for a version of this circuit but with Arduino control and monitoring.

In this circuit:

  • Switch S1 acts as “Set” (S)
  • Switch S2 acts as “Reset” (R)
  • Out1 is output state Q
  • Out2 is inverted output state -Q

The behaviour is summarised here:

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

Set/reset are edge-triggering pulses, and pulses only need to be long enough for the transistors to react. Repeated pulses cause no change to the output.

A power supply from 5-9V is fine.

Here’s a great animation of the circuit from wikibooks:


Ben Eater has a great video explaining SR latch behaviour:


Latch of Flip-flop?

Although terms are often used interchangeably, this circuit is strictly speaking a “latch”.

  • a latch is asynchronous, and the outputs can change as soon as the inputs do
  • a flip-flop, on the other hand, is edge-triggered and only changes state when a control signal transitions high to low or low to high


The Breadboard

The Schematic

The Build

Credits and References

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 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.