Introduction to the SN74LS90N Decade Counter IC

The SN74LS90N is a widely-used integrated circuit (IC) that operates as a 4-bit binary or BCD (Binary-Coded Decimal) decade counter. This IC is based on the TTL (Transistor-Transistor Logic) technology and can be used in various applications like counters, frequency dividers, or timers. One of the key features of the SN74LS90N is its ability to count in either a mod-10 or mod-12 configuration, which makes it versatile for different kinds of DIY projects.

In this article, we will explore a DIY project to create a simple digital counter using the SN74LS90N IC. The counter will display numbers from 0 to 9 on a seven-segment display, making it an ideal starting project for beginners looking to delve into the world of digital electronics.

Required Components

To build this project, you will need the following components:

· 1x SN74LS90N Decade Counter IC

· 1x SN7447 BCD to Seven-Segment Display Decoder IC

· 1x Seven-Segment Display (Common Anode)

· 1x 555 Timer IC (for clock signal generation)

· 2x 10μF Capacitors

· 2x 1kΩ Resistors

· 1x 470Ω Resistor (for the display)

· 1x Pushbutton Switch

· 1x 10kΩ Potentiometer (for adjusting clock frequency)

· Breadboard and Jumper Wires

· 5V Power Supply

Understanding the Components

Before diving into the construction of the circuit, it’s important to understand the function of each component:

SN74LS90N: This IC is a 4-bit counter that can count up to 10 (0 to 9) in its BCD mode. The output is provided in binary form, which is then converted to a form that can drive a seven-segment display.

SN7447: This IC is responsible for converting the binary output from the SN74LS90N into a format that can directly drive a seven-segment display. It simplifies the process of showing the count on the display.

Seven-Segment Display: This display consists of 7 LEDs arranged in a figure-eight pattern. When certain segments are turned on, they form numbers from 0 to 9.

555 Timer: This versatile IC is used in the astable mode to generate a clock signal that will increment the counter at regular intervals.

Capacitors and Resistors: These are used to filter, stabilize, and control the timing of the 555 timer circuit.

Circuit Design and Schematic

The SN74LS90N has several pins that control its operation. Below is a brief description of key pins:

· Pin 14 (Vcc): Supply voltage (5V).

· Pin 10 (GND): Ground.

· Pin 15 (CLK A): Input clock signal.

· Pins 11, 12, 9, 8 (Q0, Q1, Q2, Q3): The binary outputs representing the current count.

· Pin 2, Pin 3: Used for resetting the counter to zero.

· Pin 6, Pin 7: Used for modifying the counter’s modulus (count range).

The 555 Timer will be configured to work in astable mode, producing a square wave output that will act as a clock signal for the SN74LS90N. By varying the potentiometer, you can control the frequency of the clock signal, thus changing the speed at which the counter counts.

The SN7447 decoder takes the binary output from the SN74LS90N and translates it into signals that can drive the seven-segment display.

Circuit Diagram

To create the counter, follow the schematic outlined below:

555 Timer Circuit (Astable Mode):

o Pin 1 (GND): Connect to ground.

o Pin 8 (Vcc): Connect to 5V.

o Pin 2 (Trigger) and Pin 6 (Threshold): Connect together.

o Pin 7 (Discharge): Connect to one end of the 10kΩ potentiometer.

o Pin 3 (Output): Connect to Pin 15 of the SN74LS90N (clock input).

o Connect the capacitors and resistors as per the standard astable configuration.

SN74LS90N Decade Counter:

o Pin 10 (GND): Connect to ground.

o Pin 14 (Vcc): Connect to 5V.

o Pin 15 (CLK A): Connect to the output of the 555 Timer (Pin 3).

o Pins 11 to 8 (Q0 to Q3): Connect to the respective input pins of the SN7447 decoder IC.

SN7447 Decoder to Seven-Segment Display:

o Connect the output pins of the SN7447 to the appropriate pins of the seven-segment display.

o Use the 470Ω resistor between the display’s common anode and the 5V supply to limit the current.

Building the Circuit

Step 1: Setting Up the 555 Timer

Begin by assembling the 555 Timer circuit in astable mode on a breadboard. Connect the necessary resistors, capacitors, and the potentiometer as per the astable configuration. The output (Pin 3) will generate a square wave that will act as the clock pulse for the SN74LS90N.

Step 2: Wiring the SN74LS90N

After setting up the timer, wire the SN74LS90N counter. Connect Pin 15 of the SN74LS90N to the output of the 555 timer to receive clock pulses. The ground and Vcc pins should be connected to the power rail of the breadboard (5V and GND).

Step 3: Connecting the Decoder and Display

Next, connect the outputs of the SN74LS90N (Q0 to Q3) to the inputs of the SN7447 decoder. Wire the output of the decoder to the corresponding pins of the seven-segment display. Ensure that the 470Ω resistor is in place to protect the display from excessive current.

Step 4: Adding a Reset Button (Optional)

For added functionality, you can introduce a reset button that will bring the counter back to zero. This can be achieved by connecting a pushbutton to the reset pins (2 and 3) of the SN74LS90N. When pressed, the counter will reset to 0.

How the Circuit Works

The 555 Timer generates a clock signal, which is fed into the SN74LS90N. With each clock pulse, the SN74LS90N increments its count by 1. The binary output of the SN74LS90N is sent to the SN7447 decoder, which translates the binary number into signals that can light up specific segments of the seven-segment display.

The counter starts at 0 and counts up to 9, and then resets back to 0. The speed at which the counter increments depends on the frequency of the clock signal from the 555 Timer, which can be adjusted using the 10kΩ potentiometer.

Testing and Troubleshooting

Once you have assembled the circuit, connect it to a 5V power supply and observe the counter in action. The seven-segment display should show numbers incrementing from 0 to 9 at the rate controlled by the 555 Timer.

If the counter is not working as expected, check the following:

1. Power Connections: Ensure that the ICs are correctly powered and that there are no loose connections on the breadboard.

2. Clock Signal: Use an oscilloscope or multimeter to check the output of the 555 Timer. The output should be a square wave.

3. Reset Button: If the counter resets unintentionally, check the reset pins of the SN74LS90N for proper connections.

Expanding the Project

Once you’ve built the basic counter, there are numerous ways to expand the project:

1. Add Multiple Counters: You can cascade multiple SN74LS90N ICs to create a counter that counts into the hundreds or thousands.

2. Up/Down Counter: Modify the circuit to count both up and down by adding a toggle switch and additional logic.

3. Frequency Counter: Use the counter in combination with other components (e.g., frequency dividers) to build a frequency counter for measuring clock speeds or other signals.

Further Improvements and Enhancements

After completing the basic digital counter project, you can explore several ways to enhance it, making the design more functional and versatile. These additional modifications will not only expand the counter’s capabilities but also help deepen your understanding of digital electronics.

1. Multiple-Digit Counter

One natural progression is to expand the project into a multi-digit counter. By cascading two or more SN74LS90N ICs, you can extend the counter’s range to handle larger numbers. For example, by connecting two ICs, you can create a two-digit counter that counts from 00 to 99. Each SN74LS90N can drive its own seven-segment display, with the first IC controlling the least significant digit (units) and the second controlling the tens digit. This involves using the overflow output from the first IC to trigger the clock input of the second, ensuring that it increments only when the first IC reaches its maximum count (i.e., 9).

2. Up/Down Counter Configuration

Another enhancement is to convert the design into an up/down counter. By adding an extra logic circuit, you can allow the counter to increment or decrement based on the position of a toggle switch. This involves using additional logic gates (like the 7404 NOT gate or 7400 NAND gate) to manipulate the clock signal fed into the SN74LS90N. With this modification, the counter will increment when the switch is in one position and decrement when the switch is in the other. This makes the project more dynamic and useful for applications where bidirectional counting is required, such as in elevator control systems or industrial counters.

3. Interruptible Clock Signal

You can also modify the project to include a pause or hold function. This feature is especially useful when you want to stop the counter at a specific number and resume counting later. You can achieve this by incorporating a switch that temporarily disables the clock signal going into the SN74LS90N. By opening the circuit to the clock input, the counter will "freeze" at its current value until the clock signal is restored.

Conclusion

Building a digital counter using the SN74LS90N decade counter IC is an excellent project for beginners in electronics. Not only does it demonstrate the practical use of counters, but it also introduces key concepts like clock generation, binary counting, and interfacing with a seven-segment display.

This project is relatively simple and requires only a few components, making it a great way to learn how digital electronics work. Once you've completed the project, you can further enhance it with additional features and functions, expanding your knowledge and skills in the process. Happy building!