A Logic Gate is an electronic device that implements a Boolean function, i.e., a logical operation is performed on one or more binary inputs to produce a single binary output. Logical operations are a fundamental part of digital circuits and are based on Boolean algebra.
At any given time, the inputs and the output can have two conditions, true or false. True is expressed as 1 (high), and false is expressed as 0 (low). Depending on the type of Logic Gate and the combination of inputs, the output will differ.
Basic Logic Gates
There are three basic logic gates.
These are: AND, OR, NOT, also we have a few other gates derived from the basic gates NOR, NAND, XOR, XNOR
In the following sections, we look at the principle of AND Gate.
The AND Gate is an electronic circuit that produces a high output if all its inputs are high.
Symbol of AND Gate:
The AND Gate has inputs A and B and output Y. It implements the logical expression Y = A.B. This expression is sometimes also denoted as Y = A*B Y = A&B.
Truth Table
| Input A | Input B | Output Y |
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
Transistor Implementation of AND Gate
A two-input AND Gate can be built with two Bipolar Junction Transistors (BJT).
Circuit Diagram:
This circuit has inputs A and B connected to the base terminals of the two transistors. An LED, which is the output Y, is connected to the emitter of the second transistor. +5V supply (Vcc) is supplied to the collector of the first transistor, which is connected in series with the second transistor.
Working
When both the switches are open, there is no current flowing through the transistors, and hence the output is LOW. In this state, the LED is OFF. When one switch is closed and the other is open, the respective transistor is either ON or OFF. However, the current cannot flow through the transistors as they are connected in series. Hence the output is LOW. When both the switches are closed, both the transistors are ON, and the current flows through them. Hence the output is HIGH, and the LED turns ON.
For detailed instructions on the construction of AND Gate using transistors, visit: https://www.youtube.com/watch?v=I0jgY0N4AMk
IC Package
AND Gates are available in IC Packages. The most common IC Package is 7408 IC. It is a QUAD 2-input AND Gate and contains four independent Gates each of which performs the logic AND function
IC7408
Witblox AND Gate
Witblox AND Gate is an addon Blox. It is a single AND Gate with two inputs and one output. You can connect any input Blox to the AND Blox and build any AND-Logic-based applications.
How it Works
Example applications using AND Logic
Coin-operated vending machine
Here, a coin has to be inserted, AND a button should be pressed for the machine to dispense.
Safety operation in machines
To ensure operator safety the machine door should be closed, AND the start button should be pressed for the machine to run.
Authored by Shreyas S Arbatti (13)
Witblox Maker and Electronics enthusiast