NOR Gate using transistor
Updated: Nov 18, 2020
The NOR gate is simply the inverse of the OR Gate. Also, we can say that the NOR #Logicgate is the combination of an OR Gate and a NOT Gate together. The logical symbol for a NOR Gate will be:
So, the boolean expression for the NOR gate is Q = (A+B)’ i.e. (A+B) inverse. From the boolean expression, we can say that NOR Gate is the inverse of the OR gate. The same can be proven when we see the truth table for NOR Gate. The truth table of NOR Gate is as follows,
0 0 1
0 1 0
1 0 0
1 1 0
From the truth table, it can be said that for a NOR Gate when both the inputs are LOW the output is HIGH and for all the other combinations of inputs the output is LOW.
NOR Gate using transistors
To build a NOR Gate using transistors, we would need to build an OR gate first and then combine that OR gate circuit with a NOT Gate circuit. Also, while building the NOR Gate we need to make sure that whatever the hardware circuit we build, its output should be the same as the truth table of the NOR gate mentioned above.
Let’s start with the components and circuit diagram first and then we would see the implementation & working.
The list of components required to build a NOR gate using transistors are mentioned below:
Two NPN transistors. (You may also use PNP transistors)
It is a low-power amplifying transistor. A common NPN Bipolar Junction transistor (#BJT). This transistor is usually used for Switching purposes as we have used in our circuit.
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2. One power supply(5V preferable).
This #module is specially used for providing voltage supply to circuits designed with components on a small scale. The module has a Barrel jack the can be connected to the DC supply directly. The Voltage is regulated to supply 6-12 V DC. This regulated voltage is then supplied to the circuit built.
The power channel can be configured to supply 3.3V, 0V, or 5V. It has a control button that can be operated as per our requirement to start and stop the power supply. It has an on-board LED which indicates when the power is supplied through the module. For this circuit, the power supply can be 5V or 6V.
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3. Two 10K Ohm resistors and one 4-5K Ohm resistor.
Resistors are passive devices that restrict the flow of current or divide the voltage through the circuit. The resistors used for this circuit are 10k Ohm and 5k Ohm resistor. 10k Ohm Resistor is connected before the transistor, i.e. the input power passes through these resistors and then to the transistors. The 5k Ohm resistor is connected before the LED so that a lesser amount of voltage is diverted to the LED.
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4. One LED-Light Emitting Diode. (To see the output)
Light Emitting Diode is a commonly used light source. It is a semiconductor that emits light when current flows through it.
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5. One breadboard.
The breadboard is the basic component of any circuit building process. All components, be it input sensors or output display devices are connected to the power supply, microcontroller using wired connections through a breadboard. The holes in the breadboard are in series. There are various sizes like full-sized, half-sized, and mini breadboards.
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6. Connecting Wires.
These are the main components that are used to establish the connections between different devices of the circuit.
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7. Two Push Buttons.
PUSH Buttons are simple devices used for switching control. It is easy to use and starts or stops the function when connected in a circuit.
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In case, you don't have you can use push buttons just like I did in this tutorial. But you can use them for easy understanding and implementation. Also, I would explain to you how and where you can use the push button in the circuit. So, let’s see the circuit diagram for making a NOR Gate using transistors.
NOR Gate using NPN Transistors
The circuit has two transistors connected in parallel mode with each other and is very much similar to the circuit we used to build an OR gate using transistors. The two transistors, T1 and T2 have the collector terminal connected to a +5V power supply using a resistor R2 (2K – 4K Ohms), and the base terminal of each transistor is connected with inputs A & B through a resistor R (10K Ohms).
Finally, the emitter terminal of both the transistors is connected to GND (ground) completing the parallel connection between T1 and T2. Now to check the output of the circuit, Q is pulled from the point connecting both the transistor’s collector terminal with the resistor R2. This whole circuit makes a NOR Gate using two NPN transistors.
The same can be represented on a breadboard using the below image -
In the circuit, we have connected the two #transistors in parallel connection with each other and the output is connected to the collector terminal of both the transistors. The OR gate has two inputs and one single output and both the inputs A & B are connected to the base of the first and second transistors respectively. Now, depending upon the value of the input at the base of the transistor the output would change and if the output (with its respective input) is the same as in the truth table, then we have successfully build a NOT Gate.
The working of the circuit can be explained using 4 cases according to the truth table:
Case 1: Input A = 0 and B = 0.
When both the inputs A and B are 0 at the base of the transistors, then there will not be any connection between the collector and the emitter terminal of both the transistors. So, the power supply would reach only the collector terminal of both the transistors and since, the collector terminal of both the transistors is also connected with the LED or the output, so the #LED would have a high input. As a result, the LED will turn ON when both inputs A and B are 0 for a NOR Gate.
Case 2: Input A = 0 and B = 1.
For this case, the first transistor input A = 0 and the second transistor input B = 1. Now, when a 5V supply reaches the collector terminal of both the transistors, their current will only pass from the second transistor have input B. The first transistor has input A which is 0, so there is no connection between the collector and emitter terminal for this transistor while on the other hand, the second transistor makes a direct connection with its collector and emitter terminal.
So, the current would pass from the second transistor emitter terminal to the further circuit, but the emitter terminal is connected to the ground. This makes a 0V potential difference between the output terminal and the ground for the whole circuit. As a result of which, the LED is OFF due to 0 input from the output terminal.
Case 3: Input A = 1 and B = 0.
This case is just the inverse of the previous case where instead of A = 0 and B = 1 here we have A = 1 and B = 0. The output would still be 0, i.e. the LED would be OFF only but the only difference between these two cases is that in this case, the first transistor would have to switch ON i.e. the first transistor would make a connection between its collector and emitter #terminal.
Case 4: Input A = 1 and B = 1.
The LED still would be OFF in this case also even if both the inputs are high i.e. both A = 1 and B = 1. What happens here is, when a 5V supply is passed in the circuit, the current reaches the collector terminal of both the transistors. Now, both the transistors have high input, which means, the switch is ON for both the transistors and there is a connection between the collector and the emitter for both transistors.
So, the current would pass from the emitter terminal of both the transistors but still, it would reach the ground which would make a 0V potential difference. Hence, the output here is 0 and the LED, in this case, is OFF.
The results of all 4 cases are similar to the truth table of the NOR gate and hence, we can say that the circuit we built is a NOR Gate circuit.
To see the practical implementation and explanation of How to build a NOR Gate using transistors? watch the video below.