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NPN Transistor SR Flip Flop Circuit

  • Concept of Operation

    The SR flip flop, also known as the SR latch, is a two input logic circuit that retains the most recent input even after the stimulus is removed. The example circuit here uses two momentary switches for input making an excellent demonstration of the functionality. Each switch has a corresponding LED which becomes lit when its button is pressed and will remain lit until such time as the opposite switch is pressed illuminating the other LED. (Pressing the button of the active switch/LED pair has no effect.) At any given time only one LED is illuminated.

    The example SR flip flop circuit has been implemented here using two cross-coupled NOR gates.

    By convention the two inputs are referred to as set and reset, giving the SR input its name. Also by convention the outputs are labeled Q and ~Q (not Q), for logic high and logic low. The set input is associated with Q and the reset input is associated with ~Q. In the logic diagram below, note how the cross-coupling of the gates is reflected in which NOR gate drives Q and which drives ~Q. Also note that because the circuit is symmetrical, the designation of which input/output pair is S/Q or R/~Q is arbitrary.

  • Logic Diagram

  • Schematic

  • Breadboard Diagram

  • Parts

    • PN2222A - NPN General Purpose Transistor (x4)
    • 330 Ohm Resistor (x2)
    • 10K Resistor (x4)
    • Green 5v LED (x2)
    • Tactile Pushbutton Switch (x2)

    (All parts for the project are available in our store.)

  • Notes on Implementation

    1. The LEDs used in the project have a built in current limiting resistor eliminating the need for an external resistor to protect the device. This resistor is shown in the schematic as a resistor with the value “Lim” for completeness.
    2. The circuit was built and tested at 4.8 volts DC (4 AA NiMH rechargeable batteries).