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NPN Transistor NOR Gate Circuit

  • Concept of Operation

    An electronic NOR gate performs the digital logic NOR function. The output is only high when both of the two inputs are low. When either or both inputs are high, the output is low.

    The example transistor NOR gate circuit is implemented here using PN2222A NPN transistors (a variant of the 2N2222A) but many common NPN bipolar junction transistors could be substituted.

    The gate is built from two transistor switch circuits wired in series. The inputs to the NOR gate are two manual push button switches each of which drives a transistor. This circuit is nearly identical to the transistor OR gate. However, by designing the circuit such that the output is taken at a voltage divider between the transistor and the positive voltage, the NOR circuit has an inverting component. When both of the transistor switches are off, the resistance across the two switches is large, the voltage at the output is high and the LED is lit. When either transistor switches are on, the resistance across them becomes very small, the voltage at the output is low and the LED is off.

  • Schematic

  • Breadboard Diagram

  • Parts

    • PN2222A - NPN General Purpose Transistor (x2)
    • 10K Resistor (x2)
    • 330ohm Resistor (x1)
    • Green 5v LED (x1)
    • Tactile Pushbutton Switch (x2)

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

  • Truth Table

    In Out
    0 0 1
    0 1 0
    1 0 0
    1 1 0
  • PN2222A Pinout

  • 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).