NAND Logic Gate with Transistors

Creating a NAND gate using transistors is a fundamental exercise in digital electronics. In this post, I will guide you through the process of building a NAND gate with transistors, explaining each step in detail.

 

What is a NAND Gate?

A NAND gate is a digital logic gate that outputs FALSE or LOW (0) only when all inputs are True or High. If any or all of the inputs are False or Low (0), the output is True or HIGH (1). The truth table for a two-input NAND gate is as follows:

Input A

Input B

Output

0

0

1

0

1

1

1

0

1

1

1

0

 

Components Needed

To build a NAND gate, you will need the following components:

  1. NPN Transistors: BC547 (2 Nos.); Q1, Q2.
  2. Resistors: 10K (2 Nos) - R1, R2.
  3. Resistors: 470E (1 No) - R3.
  4. LED: 5mm Red (1 No) – L1, for output indication.
  5. Switches: Tact Switches/ push buttons (2 No) – S1,S2.
  6. Power Supply - a DC source, +5V, Vds.
  7. Breadboard and Wires - for assembling the circuit

 

Understanding the Circuit

In a NAND gate circuit with NPN transistors, the transistors are connected in series. Each transistor represents an input. The output is taken from the Collector of the first transistor. The base of each transistor is connected to the input through a resistor, which limits the current and prevents damage to the transistors.

 

 

Transistor Basics

  • Emitter: The terminal through which current leaves the transistor.
  • Base: The terminal that controls the transistor's switching.
  • Collector: The terminal through which current enters the transistor.

For an NPN transistor, when a small current flows into the base, it allows a larger current to flow from the collector to the emitter.

 

Circuit Diagram


Building the NAND Gate

Step 1: Connect the Power Supply

  1. Connect the positive terminal of the power supply (+5V) to the positive rail of the breadboard.
  2. Connect the negative terminal (GND) to the negative rail.

 

Step 2: Place the Components on breadboard (As per your design)

  1. Place two NPN transistors (Q1, Q2).
  2. Place two switches (S1, S2).
  3. Place Red LED.


Step 3: Connect the Resistors (Use jumping wires for connections if required)

  1.  Connect resistor (R1) between the positive rail (+5V) and the input of S1.
  2.  Connect the output of S1 to the base of Q1.
  3. Connect resistor (R2) between the positive rail (+5V) and the input of S2.
  4.  Connect the output of S2 to the base of Q2.
  5. Connect the Collector of Q1 to the one end of Resistor R3.
  6. Connect another end of Resistor R3 to the Positive rail.
  7. Connect the Emitter of Q1 to the Collector of Q2.
  8. Connect the Emitter of Q2 to the Ground rail.

 

Step 4: Connect the LED (To show the OUTPUT visually)

  1. Connect the anode of the LED (L1) to the Collector of Q1.
  2. Connect the cathode of the LED (L1) to the GND.

 

Step 5: Testing the Circuit

  1. Apply different combinations of input voltages to A and B.
  2. Press neither S1 nor S2, LED should be ON. 



  1. Press S1, LED should be ON.


                


  1. Press S2, LED should be ON.





  1. Press S1 & S2, LED should be OFF.


 

Note: Value of resistors can be adjusted as per the requirements.

 

 

I hope you liked the information.

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