NOT Gate with NPN transistors

Overview

Secondary School

Physics, Mathematics, Computer Science, Engineering

Quantum Computing

English

Overview

Keywords: Transistor logic, NPN, resistors, LED, Inverter, Boolean algebra, gate, NOT
Age group: 14-17
Required knowledge/skills: Basic electronics, circuit design, working in tinkercad.com virtual environment
Time frame: 1-2 hours: assembling the circuit in Tinkercad

Authors: Astrinos Tsoutsoudakis (GR)

Content

Conceptual introduction
Required materials
Tasks for teachers/students

Summary

An NPN transistor can be used as a digital switch to implement a NOT gate (inverter). By controlling the voltage at the base, the transistor switches between blocking and conducting current. A LOW input results in a HIGH output, while a HIGH input produces a LOW output. This simple circuit demonstrates how transistors perform basic logical operations in digital electronics.

Quantum Computing From Bits to Quantum Leaps teaser

Conceptual introduction Conceptual introduction

The NPN transistor acts like an electronic switch controlled by the voltage at its base. The circuit typically consists of a power source, a current-limiting resistor for the base, the NPN transistor itself, and a load (like an LED with a resistor) connected to the collector. The emitter is connected to ground.

Input is LOW

When a LOW voltage (or 0V) is applied to the base of the transistor, no significant current flows from the base to the emitter. This effectively keeps the transistor "off," or in its cutoff state. Since the transistor is an open circuit between the collector and emitter, the current cannot flow from the power source through the load to the ground. This causes the full voltage to drop across the load, making the output HIGH.

Input is HIGH

When a HIGH voltage (e.g. 5V) is applied to the base, a small current flows from the base to the emitter, "turning on" the transistor. The transistor then acts as a short circuit between the collector and emitter, putting it in a state of saturation. This allows a large current to flow from the power source, through the load, and to the ground. Since the transistor now has a very low resistance, almost all the voltage from the power supply drops across the load. This causes the output voltage, measured at the collector, to be very close to ground, which is considered LOW.

A NOT gate, or inverter, inverts a single digital input – it flips it. If the input is ON, the output is OFF, and if the input is OFF, the output is ON. We're going to build this basic logic gate using an NPN transistor, which acts like an electronic switch.

© Science on Stage
NPN transistor circuit

© Science on Stage
NPN transistor symbol

Required materials

Software: Arduino IDE
Online tool: tinkercad.com

Hardware

Name	Quantity	Component
U1	1	Arduino Uno R3
U2	1	MCP23008-based, 32 (0 x 20) LCD 16 x 2 (I2C)
S1/S2	2	Slide switch
R1 / R2	2	10 kΩ Resistor

Tasks for teachers

Set up Tinkercad classrooms, provide Arduino code templates, circuit diagrams.
Introduce binary arithmetic (Introduction to Binary) and logic gates (Introduction to Logic Gates).
Help students place components correctly: transistors, resistors, switches, LEDs.
Encourage inquiry and discussion.
Use short quizzes, discussion, or mini presentations to check comprehension.