Introduction

In this experiment, we will use the 74LS90 decade counter to count from 0 to 9 (decimal) with an LED display. The 74LS90 is a 4-bit binary counter with additional divide-by-2 and divide-by-5 outputs, making it suitable for counting and display purposes.

Materials Required

• 74LS90 Decade Counter IC
• Four LEDs (for displaying the 4-bit binary output)
• Resistors (220Ω for each LED)
• Breadboard and jumper wires
• Clock source (555 timer or manual clock button)
• Power supply (5V)

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Circuit Diagram

Below is the circuit diagram for the 74LS90 Decade Counter experiment:

Pin Configuration of the 74LS90

• Pin 1 (R1): Reset 1
• Pin 2 (R2): Reset 2
• Pin 3 (A): Divide-by-2 output
• Pin 4 (B): Divide-by-5 output
• Pin 5 (NC): No connection
• Pin 6 (C): Q2 output
• Pin 7 (D): Q3 output
• Pin 8 (GND): Ground
• Pin 9 (Q0): Q0 output
• Pin 10 (Q1): Q1 output
• Pin 11 (CLK): Clock input
• Pin 12 (NC): No connection
• Pin 13 (VCC): Power supply
• Pin 14 (NC): No connection

Steps for the Experiment

1. Place the 74LS90 IC on the breadboard.
2. Connect the Q0, Q1, Q2, and Q3 outputs to four individual LEDs with 220Ω resistors in series to ground.
3. Connect the clock input (CLK) to a clock source such as a 555 timer configured for clock pulses or a manual push button.
4. Connect the Reset 1 and Reset 2 inputs to ground (disabling reset functionality).
5. Connect the power supply: 5V to the VCC pin and ground to the GND pin.
6. Observe the LEDs light up in sequence, corresponding to the binary count output (0000 to 1001, or decimal 0 to 9).

Explanation

The 74LS90 decade counter is capable of counting in binary from 0 to 9. Each clock pulse increments the binary output by 1, cycling through the following binary sequence:

• 0000 (0 in decimal)
• 0001 (1 in decimal)
• 0010 (2 in decimal)
• 0011 (3 in decimal)
• 0100 (4 in decimal)
• 0101 (5 in decimal)
• 0110 (6 in decimal)
• 0111 (7 in decimal)
• 1000 (8 in decimal)
• 1001 (9 in decimal)

After the count reaches 9, it automatically resets back to 0 on the next clock pulse. The four output pins Q0, Q1, Q2, and Q3 represent a 4-bit binary number, where Q0 is the least significant bit (LSB) and Q3 is the most significant bit (MSB).

Results and Observations

As the clock signal is applied:

• The LEDs light up in a binary sequence, representing the count in decimal from 0 to 9.
• Once the count reaches 9 (1001 in binary), the counter resets to 0 (0000 in binary) and starts over.

Applications of the 74LS90 Decade Counter

The 74LS90 decade counter is widely used in various applications, including:

• Digital clocks
• Frequency division and counting
• Decimal counting circuits
• Binary-to-decimal conversion circuits

Conclusion

In this experiment, we demonstrated the working of the 74LS90 Decade Counter. The counter outputs a binary sequence representing the decimal numbers 0 to 9. This simple counting circuit has a wide range of applications in digital electronics and frequency counting circuits.