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Parts, tools and supplies for the electronics experimenter.

NPN Transistor OR Gate Circuit

  • Concept of Operation

    An electronic OR gate performs the digital logic OR function. The output is only high when either of the two inputs are high. When both inputs are low, the output is low.

    The example transistor OR 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 parallel. The inputs to the OR gate are two manual push button switches each of which drives a transistor. This circuit is similar to the transistor AND gate in which the transistor switches are wired in series. However, by connecting the transistor switches in parallel, each forms an independent complete circuit between positive voltage and ground. Therefore either switch can drive the output high.

  • Schematic

  • Breadboard Diagram

  • Parts

    • PN2222A - NPN General Purpose Transistor (x2)
    • 10K Resistor (x2)
    • 1K 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 0
    0 1 1
    1 0 1
    1 1 1
  • 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).