Understanding NOT Gate

Logic gates perform a specific logical function, processing one or more binary inputs to produce a single binary output. The NOT gate is a logic gate that reverses the binary input. This means a high output (1) results from a low input (0), and a low output (0) is produced from a high input (1).

A NOT gate is a type of logic gate that inverts the provided input. Also referred to as an inverter, it produces a high output when given a low input and, conversely, a low output when given a high input.

A NOT gate has a single output and only one input. The NOT gate’s Boolean logic is Z= ~A for an input A.

Truth Table

Representation

• The standard symbol for a NOT gate in digital electronics is a triangle pointing to the right, followed by a small circle at the tip. The input is on the left side, and the output is on the right side.
• If you provide a logical ‘1’ (high) at the input, you get a logical ‘0’ (low) at the output, and vice versa.
• The output Z in terms of the input A can be represented as Z = ~A

Characteristics of NOT Gate

1. Like any digital circuit, NOT gates have propagation delays, which is the time it takes for the output to change in response to a change in the input
2. It consumes power when switching from one state to another. Power consumption characteristics vary based on technology (e.g., CMOS, TTL, etc.).
3. Especially useful when a certain action is triggered by a low signal (active low). Connecting a device to the output of a NOT gate can operate in the opposite phase, i.e., active high.

Let’s Understand NOT Gate Using a Real-Life Example

1. A push-to-break button (often found on emergency stop functions in machinery)

When it’s not pressed (input 0), the circuit is complete, and the machine operates. When you push the button (input 1), it breaks the circuit, stopping the machine. The action of the button inverts the operational state of the machine.

2. A light switch

The light is off when in the “off” position (input 0). The light turns on when you flip the switch to the “on” position (input 1). The action of the switch inverts the state of the light.

Let’s See Some Numerical Questions Based on NOT Gate

Q1. Given a digital signal sequence 11001, pass it through a NOT gate. What is the output sequence?

Ans. The output sequence after passing through the NOT gate would be 00110. The NOT gate inverts every bit, so each 1 becomes 0 and vice versa.

Q2. A binary string 10101010 is passed through a series of 8 NOT gates. Will the output string be identical to the input? Why or why not?

Ans. Yes, the output string will be identical to the input. Each time a binary number passes through a NOT gate, it gets inverted. Since there are 8 NOT gates, and 8 is an even number, the signal will be inverted 8 times. Inverting it an even number of times will bring it back to its original state.

Q3. You are given a digital system where a logic ‘0’ is represented by a 2V and a logic ‘1’ by 7V. If you pass the voltage 2V into a NOT gate, what voltage do you expect at the output?

Ans. Given that 2V represents logic ‘0’, after passing through a NOT gate, it will be inverted to logic ‘1’. Therefore, you would expect an output voltage of 7V.

Q4. If the input voltage level representing logic ‘1’ is 5V and logic ‘0’ is 0V in a digital system, what will be the output voltage of a NOT gate when its input voltage is:
a) 5V
b) 0V

Ans. a) If the input is 5V (which represents logic ‘1’), the NOT gate will invert it to logic ‘0’, i.e., 0V.
b) If the input is 0V (which represents logic ‘0’), the NOT gate will invert it to logic ‘1’, i.e., 5V.

Applications of NOT Gate

• The NOT gate’s inverting property is key in CMOS technology, where a combination of p-type and n-type transistors is used to create inverters. These inverters serve logic functions and are essential in generating clock waveforms for digital circuits.
• It can be used in circuits that detect specific temperature thresholds. When the threshold is crossed, the NOT gate can invert a signal to activate or deactivate a heating or cooling element, ensuring the system maintains the desired temperature.
• Found in various integrated circuits, including printers (printer IC) and disk drive controllers (disk read-write IC).
• Microprocessors and microcontrollers are essentially vast networks of interconnected logic gates. NOT gates, alongside AND, OR, and other gates, play a crucial role in these devices’ arithmetic, control, and data management functions.
• Modern machinery and equipment, from industrial automation systems to home appliances, utilize digital control circuits for precision and reliability. The NOT gate and other gates, like AND, help regulate these digitally controlled machines’ on-off states, speed, and other operational parameters.

Thus, NOT gate, or inverter, is a fundamental building block in digital electronics and logic design. From basic digital operations in microprocessors to control systems in machinery, its presence underlines the essence of binary logic in computational and digital systems. It’s easy yet vital operation serves as a foundation of basic logic principles in the modern digital world.