CAPTCHA Machine Using Arduino Uno and OLED Display

The CAPTCHA Machine is an electronic project developed using an Arduino Uno, an OLED display, two push buttons, and an LED. The project generates random mathematical questions and asks the user to select the correct answer from three options.

The OLED display shows the question and its possible answers. The user navigates between the options using the NEXT button and selects an answer using the SUBMIT button. If the selected answer is correct, the display shows "CAPTCHA PASSED!" and "ACCESS OK". If the answer is incorrect, the display shows "WRONG ANSWER!" and "TRY AGAIN".

This project demonstrates how an Arduino can generate random questions, process user input, and provide visual feedback.

Deshbot shield

uno nano

oled display

OLED Display

OLED PinArduino Uno Pin

VCC - 5V, if supported by the OLED module

GND = GND

SDA = A4

SCL=A5

Push Buttons and LED

ComponentArduino Uno Pin

NEXT Button = D10

SUBMIT Button =D12

LED Signal = D11

The buttons use the Arduino's internal pull-up resistors through INPUT_PULLUP. Connect each button between its assigned digital pin and GND.

LED note: The code treats D11 as a single LED output. If using a bare LED, connect it through a suitable current-limiting resistor. If using an RGB LED, this code controls only one channel; separate pins are required to control red, green, and blue independently.

The system operates in the following sequence:

1. The Arduino Uno initializes the OLED display and configures the push buttons and LED.
2. A starting message appears on the OLED screen.
3. The Arduino generates a random mathematical question.
4. Three answer options are created, including one correct answer and two incorrect answers.
5. The OLED displays the question and all three answer options.
6. The user presses the NEXT button to move the selection marker between the answers.
7. The user presses the SUBMIT button to check the selected answer.
8. If the answer is correct, the system displays a success message.
9. If the answer is incorrect, the system displays a failure message.
10. After the message disappears, a new CAPTCHA question is generated.

The LED turns on while either button is pressed and turns off when both buttons are released.

Types of CAPTCHA Questions

The program generates five types of mathematical questions:

S. No.Question TypeExample

1

Addition

5 + 7 = ?

2

Multiplication

4 × 6 = ?

3

Number Sequence

3, 5, 7, ?

4

Square Number

5 squared = ?

5

Subtraction

18 − 6 = ?

The numbers change randomly when a new question is generated.

Program Description

generateCaptcha()

This function randomly selects a question type, generates the question, calculates its correct answer, and creates three answer options. The correct answer is placed at a random position.

displayCaptcha()

This function clears the OLED screen and displays the CAPTCHA title, mathematical question, and three answer options. An arrow indicates the currently selected answer.

showSuccess()

This function displays "CAPTCHA PASSED!" and "ACCESS OK" when the user selects the correct answer. After a short delay, the system generates another question.

showFailure()

This function displays "WRONG ANSWER!" and "TRY AGAIN" when the user selects an incorrect answer. After a short delay, another question appears.

setup()

This function initializes the buttons, LED, Serial communication, and OLED display. It also initializes the random number generator and displays the startup message.

loop()

This function continuously reads the button states, updates the LED, moves the selected answer when NEXT is pressed, and checks the selected answer when SUBMIT is pressed.

Button Debouncing

The program uses a debounce delay to reduce unwanted repeated button actions caused by mechanical button contacts.

Expected Output

Startup Screen

The OLED displays:

CAPTCHA MACHINE

Starting...

Question Screen

An example question screen is:

CAPTCHA MACHINE

5 + 7 = ?

Choose answer:

1. 12
2. 9
3. 15

The arrow indicates the currently selected option.

Correct Answer

When the user selects the correct answer and presses SUBMIT, the OLED displays:

CAPTCHA PASSED!

ACCESS OK

Incorrect Answer

When the user selects an incorrect answer and presses SUBMIT, the OLED displays:

WRONG ANSWER!

TRY AGAIN

After a short delay, a new question appears.

The CAPTCHA Machine demonstrates how an Arduino Uno can generate random mathematical questions, display multiple-choice answers, read user selections, and verify responses. The project combines OLED interfacing, push-button inputs, random number generation, and conditional logic in an interactive embedded system.

It provides a useful foundation for developing more advanced quiz systems, electronic games, and simple verification demonstrations. Future improvements could include separate red and green LED indicators, increasing difficulty levels, a score counter, and a limited number of attempts.