How to Build an XOR Gate with Transistors?

At its core, an exclusive-OR (XOR) gate produces a high output only when an odd number of inputs are high—meaning when input A and input B differ. To achieve this behavior at the transistor level without using integrated circuit logic chips, the circuit utilizes a network of five 2N2222 NPN bipolar junction transistors combined with 10kΩ and 220Ω resistors. The push buttons serve as the two independent binary inputs, pulling the respective input stages to high or low states when pressed or released. When neither button is pressed, or when both buttons are pressed simultaneously, the internal transistor switching arrangement causes the output node to remain low, keeping the indicator LED unlit. However, when exactly one button is pressed—either input A alone or input B alone—the configuration of the NPN transistors routes current through the circuit to drive the output high, illuminating the LED. The 10kΩ resistors act as base resistors to limit current and set proper biasing thresholds for the transistors, while the 220Ω resistors limit current through the LED and active transistor legs to safe operating levels. Powered entirely by a 9V battery on a standard breadboard, this analog and discrete component setup demonstrates how fundamental Boolean logic functions can be synthesized entirely from individual semiconductor switches.