Introduction

An SR (Set-Reset) latch is one of the most fundamental sequential logic circuits, storing a single bit of information. The 74LS00 NAND gate IC can be used to construct a basic SR latch using two of its gates. This experiment demonstrates how to configure the 74LS00 IC to behave as an SR latch.

Objective

The objective of this experiment is to configure two NAND gates from the 74LS00 IC as an SR latch. We will use pushbuttons to control the Set and Reset inputs, and an LED to indicate the output state.

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Components Needed

• 1 x 74LS00 NAND Gate IC
• 2 x Pushbutton Switches (for Set and Reset inputs)
• 2 x 10kΩ Resistors (for pull-down resistors)
• 1 x LED (to indicate the latch output)
• 1 x Resistor (220Ω for the LED)
• 1 x Breadboard
• Connecting Wires
• Power Supply (5V)

Circuit Diagram

Procedure

1. Place the 74LS00 IC on the breadboard.
2. Connect pin 14 (VCC) to 5V and pin 7 (GND) to ground.

Step 1: Connecting the SR Latch Circuit

3. Use two NAND gates from the 74LS00 IC to form the SR latch. For example:
   • Connect the output of the first NAND gate (pin 3) to one of the inputs of the second NAND gate (pin 4).
   • Connect the output of the second NAND gate (pin 6) to one of the inputs of the first NAND gate (pin 2).
4. The remaining inputs of the NAND gates (pin 1 for the first gate and pin 5 for the second gate) will serve as the Set (S) and Reset (R) inputs.

Step 2: Connecting the Inputs

5. Connect the Set input (pin 1) to a pushbutton, with a 10kΩ pull-down resistor connecting it to ground.
6. Similarly, connect the Reset input (pin 5) to another pushbutton, with a 10kΩ pull-down resistor connecting it to ground.

Step 3: Connecting the Output

7. Connect the output of the first NAND gate (pin 3) to the anode of the LED. The cathode of the LED should be connected to ground through a 220Ω resistor. This LED will indicate the state of the latch (Q output).

Step 4: Power the Circuit

8. Connect the power supply to the circuit and ensure all connections are secure.

Step 5: Testing the SR Latch

9. When the Set (S) button is pressed, the latch will be set, and the output (Q) will go HIGH, turning on the LED.
10. When the Reset (R) button is pressed, the latch will be reset, and the output (Q) will go LOW, turning off the LED.
11. If both the Set and Reset buttons are pressed simultaneously, the behavior is undefined (invalid condition for SR latches).

Results

The LED will turn on when the Set button is pressed, indicating the latch is in the "set" state (Q = HIGH). When the Reset button is pressed, the LED will turn off, indicating the latch is in the "reset" state (Q = LOW).

Conclusion

In this experiment, we successfully configured the 74LS00 NAND gates to behave as an SR latch. The SR latch is a fundamental memory element used in various digital circuits, and this experiment demonstrates how basic logic gates can be used to store a single bit of data.