#061 SR Latch

Set/Reset latch with BJTs.

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

Notes

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:

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

Construction

Credits and References

• LEAP#062 SRLatchFlipper - same project, but with Arduino control and monitoring
• S9013 Datasheet
• SR latch - wikibooks
• Flip-flop - wikipedia
• PCB heaven - demo circuit used as the basis for this project
• ..as mentioned on my blog