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Project Notes

#061 SRLatch

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

About LEAP#61 Logic GatesBJT
Project Source on GitHub 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.