In this guide I will show you how to build a AND gate with two NPN transistors.
Using the image above as a reference, we see that our transistor has three (3) pins. A collector (C), base (B), and emitter (E). Functionally, the transistor acts as an electronic switch, with the base input controlling whether or not there will be a flow of electricity from collector to emitter. There are different types of transistors with different purposes; from high speed switching to amplification, but in this guide we are just using the NPN transistor as a switch.
Step 1: Parts List
To build a logic AND gate you are going to need the following parts:
- 1x Breadboard
- 1x LED (Any color, red was used in this project)
- 1x 1K Ohm resistor
- 2x 10K Ohm resistors
- 2x NPN Transistors (I used the 2N2222)
- 2x N.O. Push buttons (Normally Open)
- 5x Wires/Jumper Wires
The only prerequisite to this project is the ability to read and construct a circuit from a schematic.
Step 2: Schematic / Truth Table
Construct the gate of out the schematic above. ^
Every gate has its own truth table stating what the output will be based on the state of its inputs. For the AND gate, the table is as follows:
A B OUT 0 0 0 0 1 0 1 0 0 1 1 1
You see, the only time the output is ON (1), is when both A AND B are ON (1). Any other state results in the output being OFF (0).
That is it for the AND gate. A seemingly simple device, but when arranged in certain combinations with other gates (NAND, OR, NOR, XOR, XNOR, NOT) we get devices from computers to cell phones and rocket guidance systems.