NOT Gate using a Transistor
Updated: Apr 14
Logic gates can be realized using transistors. One of the most ingenious logic gates of all the #logicgates is the NOT gate and is used in many digital circuits where inverted output is required. The Logic Gates like NOR, NAND, etc. can be realized using NOT Gate. Let's learn to build a NOT Gate using a transistor.
To learn more about Logic gates Click here.
Let's learn the procedure of making a NOT gate with a transistor and see how is a NOT Gate realized with the help of a transistor.
The NOT Gate which is also called as #Inverter gate has a triangular shape with a bubble (o) present at the top. The symbol of the NOT gate is shown below.
The bubble (o) symbol present after the gate symbol can also be present before the gate symbol when an Active-low input is required. Few representations of NOT Gate are given below.
The logic of the NOT Gate is inversion i.e. the input gets inverted and becomes the output. This inversion can be represented using the equation A = not(A), where A is the input, and not(A) represents the output. It is also known as Negation Gate. The truth table of logic NOT gate is given below.
A(Input) Q(Output) 0 1 1 0
Where Q = not A or Ā. From the truth table it is clear that, for a NOT Gate when the input is HIGH, the output will be LOW and vice-versa.
The list of components needed to design a NOT gate using n-p-n transistor are:
1. NPN transistor (PNP transistor can also be used)
NPN Transistor is a Bipolar Junction Transistor (BJT) that is most commonly used in circuits. NPN transistor is designed by sandwiching a P-type semiconductor between two N-type semiconductors (Vice-versa for PNP Transistor). NPN and PNP both transistors have three terminals: Base, Emitter, and Collector.
To know more about Transistors click here.
2. 10KΩ resistor & 4-5KΩ resistor
Resistors are passive devices that oppose the flow of current or divide the voltage through the circuit. The resistor values used for the proper functioning of NOT gate using transistor are 10k Ohm and 4-5k Ohms.
To know more about Resistors click here
3. LED (Light Emitting Diode)
Light Emitting Diode is the most commonly used light source. It is a semiconductor, when current flows through it, the LED emits light. Here in this project, it is used to see the output.
The basic component of any circuit-building process is Breadboard. The connections between any components can be made using Breadboard. The input sensors or output display devices are connected to the microcontroller using wired connections using the breadboard. The holes in the breadboard are in series. They are available in various sizes like full-sized, half-sized, and mini breadboards.
This is a power supply board that takes DC power from the normal circuit board and converts its required voltage values and supplies it to the circuit built. Here we are using +5V DC power.
6. PUSH button
PUSH Buttons are simple devices that are used to control switching. It is easy to use. It opens or closes the circuit when connected in a #circuit.
7. Connecting (Jumper) Wires
Jumper wires are the main components that are used to establish the connections between different devices of the circuit.
The logic between the input and the output of a NOT Gate is inverting. So to build a NOT Gate using a transistor, we have to build a #transistor circuit that produces an output that is the inverse of the input value. The transistor is used as a switch to build a NOT Gate and for this tutorial, the transistor used is BC547 NPN Transistor.
To know the working of a transistor
The circuit for building a NOT Gate using an NPN transistor, BC547 (Q1) is easy to understand. The Collector terminal of the NPN transistor is connected to +5V power supply through a 5KΩ (R1) resistor. The #base terminal of the transistor is connected to the input A with a 10KΩ (R2) resistor in between them and the emitter terminal of the transistor is connected to the GND.
The board (red color) on the right side of the image given below is the +5V power supply which is connected to the #breadboard and further, the circuit is connected as explained above. If a PNP transistor is used then just interchange the connections of the #Emitter with the Collector in the above circuit and the rest of the things should be connected in the same manner.
Connect all the components in the same way as shown in the below schematic, to build a NOT Gate.
The working of NOT Gate using a transistor can be understood clearly when implemented practically. The hardware connections for the whole circuit shown above when connected practically is as shown below:
The working of a NOT gate can be understood by taking two cases from the truth table mentioned above. We would be successful in making a NOT gate using a transistor if the output produced is inverted with respect to the given input.
Let’s see the two cases and understand how the NOT Gate using transistor circuit works.
Case 1: Input A = 0
If the connections exactly as shown above, then for providing a LOW input at base terminal A, you would have to remove the wire connected between the 10KΩ resistor and power supply for the base terminal. For more ease, connect a PUSH Button instead of using the wire between the power supply and 10KΩ resistor, and then, you would only need to press the push button to provide HIGH or LOW input.
When we have Input A = 0 connected to the base terminal of the transistor, the electrical signal would not pass from the collector to the emitter terminal. So, the 5V power supply connected to the collector terminal cannot pass current to the emitter terminal, being the base value 0.
Now, the output is drawn from the collector terminal and since the power supply (5V) is ON, the current supply would go directly to the output. The output is further connected to the LED and receiving a HIGH input, the LED turns ON that indicates the output high.
If you want to increase the brightness of the LED, then you may reduce the resistance value of R1 resistor to around 1k-2kΩ and observe the difference.
Case 2: Input A = 1
In the second case, when the input is HIGH i.e. A = 1 and so, the base would allow passing the electrical signal from the collector to the emitter terminal of the transistor. Now, when a +5V supply is given to the collector, the base would allow the current to pass to the emitter terminal which is connected to the ground.
The output terminal connected to the collector terminal gets a LOW value because the collector terminal is connected to the ground through the emitter terminal of the transistor. So, the LED receives a LOW(0) input and would be OFF when input at the base is HIGH.
In both cases, we got the inverted output (with their respective inputs) as in the truth table of the NOT Gate and hence, we have successfully built a NOT Gate using a transistor.
The video below explains the working of NOT Gate using Transistor.
Follow us -
Please do follow us i.e #learnelectronicsindia to get daily updates about new blogs, videos, courses, quizzes, and contests.