How to Build a NAND Gate with Transistors?

A NAND gate is a fundamental digital logic circuit that outputs a low signal only when all of its inputs are high. In this breadboard-based project, the logic is implemented using two 2N2222 NPN transistors arranged in series. The circuit is powered by a 9V battery and controlled using two push buttons that act as the digital inputs (Input A and Input B). Each push button is paired with a 10kOhm resistor to pull the transistor base to a default low state when the button is unpressed. When you press a button, it applies a high voltage to the base of its respective transistor through a 220Ohm current-limiting resistor, allowing current to flow. Because the two 2N2222 transistors are wired in series between the output node and ground, electricity can only flow completely through both transistors to complete the path to ground if both Input A and Input B are activated simultaneously. A 470Ohm resistor is used on the output line to protect the attached LED indicator. When both inputs are HIGH, the series path conducts, pulling the output voltage down and turning the LED off. If either input (or both) is LOW, the series path is broken, forcing the output high and lighting up the LED. This behavior perfectly mirrors the inverse of an AND gate, creating the universal NAND logic function entirely out of discrete analog components.