Display Any Words on Loop Using Basic Logic Circuits

In this project, I will be making a seven-segment display controller using basic logic gate circuits. The word I have selected to be displayed is "BEEF HIGH", This is important since the logic diagram and circuit diagram completely depends upon it. One display shows these letters in a loop. No microcontroller or coding was used. This was possible since I have used different logic gate techniques and other simplifications and simulations to execute properly.

This was actually made for an undergraduate-level Digital Logic Design course. This article will guide you to learn about how digital circuits inside microcontrollers work and help you to build your own display

List of tools needed:

1 x 74HC04 NOT gate

2 x 74HC08 AND gate

1 x 4072BE 4 input OR gate

1 x 74HC32 2 input OR gate

2 x 4027BE JK flip flops

1 x NE555 timer IC

1 x 7805 5V regulator

1 x seven segment display(cathode)

4 x 10k resistor

1 x 10uF capacitor

4 x 470 ohm resistor

1 x 100 ohm resistor

3 x blue led

1 x 100k ohm potentiometer

2 x breadboards

Breadboard connecting wires

It basically displays the 8-letter word B E E F H I G H on a loop! You can control the speed of the loop by varying the potentiometer.

I made this block diagram to explain how the project works. Basically, the LM7805 IC converts 9V to 5V and supplies the whole circuit.

NE555 timer is used here in astable mode to continuously produce a square wave which is the clock pulse required for the asynchronous counter. The value of the potentiometer changes the pulse width of the signal thus controlling the speed.

We have 8 letters to represent using binary bits and thus 3 JK flip-flops are used in asynchronous counter configuration to produce the binary bit. It basically outputs the decimal value 0 to 7(in binary), and it increments the value each time a clock signal is given.

The logic circuit is the part that decides what will be displayed. This part can hugely vary for outputting different letters. It is crucial to building the logic diagram first, which is further discussed in the next step. And then I used logisim to simplify and build this part.

The output is connected to a seven-segment display through resistors to limit the current.

Before starting the process learn the basic logic gates. The following three gates are used in this project.

NOT gate: output is 5 volts when input is 0 volts and versa

AND gate: only when all the input are at 5V the output is 5V otherwise 0V

OR gate: If any of the input is at 5V then the output will be 5V.

I recommend you to watch the basics from geeksforgeeks website LINK

This part of the circuit basically provides a signal, based on time. A JK flip-flop is like a memory device that can store a value. It can only store the value when a clock pulse is given. The input of JK is set as the output when the clock pulse is given.

Here the 4027 ICs are used as an asynchronous counter using JK flip flop.

One NE555 timer was used to produce the clock pulse

Two 4027 JK flip flop was used in T flip-flop configuration

One potentiometer of 100k is used to control the output frequency of the clock

The output from the JK flip-flop is directly connected to the input of the combination part. The flip flops produce a sequence of 0 to 7 in binary format. The timing at which the output changes from one value to the other is set by the timer circuit(NE555).

With the theory out of the way, it's time to decide what you want to show. Prepare a truth table as shown, it is quite impossible to make this project without a truth table. Since we have only 8 words to display 3 inputs are required.

3 inputs can show 2^3 = 8 words

4 inputs can show 2^4 = 16 words and accordingly.

If you apply a voltage at the pins of seven segment display the corresponding LED will light up. The common pin is always connected to the ground.

For example, to show the letter B we need all the lead to light up.

To build a circuit automatically, in logisim go to

1) window>combinational analysis

2) set the 3 inputs according to the truth table

3) set the outputs

4) press the cross buttons to set either 0 or 1

5) press build circuit

6) set a name, and preferably select the only two input gates

7) the circuit will be build successfully

Connect the seven-segment display to see if the circuit works properly. You can modify the circuit to make it simpler. Like I removed the connections for e, f, and g since they have the exact same output and connected them together with one wire.

Build the sequential part by taking JK flip-flop from the library. Here the clock pulse is basically our NE555 circuit. The constant is the enable pin which must be set to 1!

And Wala the simulation works just as expected!!!

Denote the pin numbers on simulation this really helps make the circuit run accurately and efficiently. One misconnection can ruin the whole project.

Take some time to build the circuit. Refer to this diagram If you are planning to make the same project then follow this schematic. It will show BEEF HIGH on the loop.

For any different letters, you can copy the sequential portion.

The resistors are used to limit the current going to the display. 100ohm is used since one wire lights up e, f, & g led, thus require more current to get the same brightness.

Build the circuit on a breadboard. Try to keep it clean as much as possible. This will help during troubleshooting.

This is probably a dumb way to display using a seven-segment display! However, through this project, you can actually learn what happens inside a microcontroller and a few decades ago this type of circuit was used to create electronic devices.

Watch the following video to see the project at work.

I hope you learned something from this project! Cheers!