NOT Gate from Transistors

Build a NOT gate (inverter) using a single transistor—the simplest logic gate that outputs the opposite of its input.

Overview

The NOT gate is the simplest logic gate—it outputs the opposite of its input. When input is HIGH (on), output is LOW (off), and vice versa. Understanding how a single transistor can invert a signal is fundamental to digital electronics. Every computer processor contains billions of transistors performing logic operations just like this one!

Components Needed

  • 1x Switch
  • 2x 10kΩ resistor
  • 1x LED
  • 1x 2N2222 transistor
  • severalx Wires

Instructions

  1. Gather Your Components

    This project uses a switch for input, a 2N2222 NPN transistor, two 10kΩ resistors, an LED for output indication, wires, a breadboard, and a 5V power source. The image shows components for multiple logic gate projects—you'll use a subset for this NOT gate.

  2. Build the NOT Gate Circuit

    Wire the circuit: Connect one 10kΩ resistor from 5V to the transistor collector (this is the pull-up). Connect the LED from collector to ground through a current-limiting path. Connect another 10kΩ resistor from the switch to the transistor base. Connect the emitter to ground. The switch connects between 5V and the base resistor.

  3. Test: Switch OFF → LED ON

    With the switch in the OFF position (no input signal), the transistor base has no current, so the transistor is OFF. The pull-up resistor pulls the output HIGH, turning the LED ON. This demonstrates the inversion: input LOW produces output HIGH.

  4. Test: Switch ON → LED OFF

    Flip the switch to the ON position. Now current flows into the transistor base, turning it ON. The transistor conducts and pulls the output LOW, turning the LED OFF. Input HIGH produces output LOW—the NOT gate is working!

Challenges

  • Chain two NOT gates together—what happens to the output?
  • Measure the actual voltage levels at the output with a multimeter
  • Calculate the transistor base current and verify it's enough to saturate
  • Build the same gate using a PNP transistor instead—how does the circuit change?
  • Use this NOT gate as a building block for other gates (NAND = AND + NOT)