NOT Gate using a Transistor
Updated: Nov 30, 2020
The most ingenious 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 Gate, NAND Gate, etc. are also built using NOT Gate. So, how we build a NOT Gate using a transistor?
Also known as an #Inverter gate, the NOT Gate has a triangular shape with a bubble (o) present at the top. The same can be seen in the below image.
The bubble (o) present after the gate symbol can also be present before the gate symbol when an Active-low input is required. Here are few examples of representing a NOT Gate.
The logic of the NOT Gate is solely 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 called the Negation Gate. Truth Table below represents the logic between the input and the output for a NOT gate.
Where Q = not A or Ā. From the truth table it can be stated that, for a NOT Gate when the input is HIGH, the output will be LOW and vice-versa for the LOW input.
The list of components needed to construct a NOT Gate using an n-p-n transistor is:
NPN transistor (You can also use a PNP transistor if available).
It is a Bipolar Junction transistor that is most commonly used in circuits. NPN transistor is built by sandwiching a P-type semiconductor between two N-type semiconductors (Vide-versa for PNP Transistor). NPN and PNP transistor has three terminals: Base, Emitter, and Collector.
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2. 10KΩ resistor & 4-5KΩ resistor.
Resistors are passive devices that restrict the flow of current or divide the voltage through the circuit. The resistors used for the proper functioning of this circuit are 10k Ohm and 4-5k Ohm Resistor.
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3. LED (Light Emitting Diode)
Light Emitting Diode is a commonly used light source. It is a semiconductor that emits light when current flows through it. Here in this project, it is used to check the output.
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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 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 breadboard.
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5. Power supply.
This is a simple 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.
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6. PUSH button.
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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7. Connecting (Jumper) Wires.
These are the main components that are used to establish the connections between different devices of the circuit.
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As discussed, the logic between the input and the output of a NOT Gate is inversing. So to build a NOT Gate using a transistor, we have to build a #transistor circuit that has an output which is the inverse of the input value. The transistor will be used as a switch to build a NOT Gate and for this tutorial, the transistor used is BC547 NPN Transistor.
The circuit for building a NOT Gate using an NPN transistor (Q1) is easy to understand. A +5V power supply is connected to the Collector terminal of the NPN transistor using a 5KΩ (R1) resistor. The transistor has a #base terminal 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.
For a NOT Gate, the output Ā is taken from the #collector terminal of the transistor and is then connected to an #LED to detect the output value. The same circuit can be seen in the circuit image below.
The circuit on the right side (red color) 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 you are using a PNP transistor then from the above circuit just interchange the connections of the #Emitter with the Collector and the rest of the things should be connected in the same manner.
All these components should be connected in the same way as shown in the below schematic, to build a NOT Gate.
The working of the circuit of NOT Gate using a transistor can be understood more clearly when implemented practically. So, the whole breadboard circuit shown above when connected practically would execute hardware as shown below:
The working of a NOT gate can be understood using two cases from the truth table mentioned above. So, when we take an input according to the truth table of a NOT Gate and get the respective output, for both the cases then we would be successful in making a NOT gate using a transistor.
Let’s see the two cases one by one and also understand how the NOT Gate using transistor circuit works.
Case 1: Input A = 0
If you have made the exact connection 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, you can 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 terminal to the emitter terminal. So, the 5V power supply connected to the collector terminal can’t pass current to the emitter terminal, being the base value 0.
Now, the output is drawn from the collector terminal and since the 5V power supply 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 will be turned ON.
If you want to increase the brightness of the LED, then you might need to reduce the resistance value of R1 i.e. 5KΩ resistor to around 1k-2kΩ and see the difference.
Case 2: Input A = 1
In the second case, the input is HIGH i.e. A = 1 and so, the base would allow passing the electrical signal from the collector terminal to the emitter terminal of the transistor. Now, when a +5V supply is passed 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. So, the LED receives 0 input and would be OFF when input at the base is HIGH.
In both cases, we got the same 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.