Signal Corps Trainer is an embedded device for training and decoding Morse code, built around the ATmega328P-XMINI microcontroller and a vintage Soviet telegraph key.

What it does:

• Allows the user to physically send Morse code by pressing the telegraph key, automatically decoding characters in real time
• Provides a Trainer mode in which the system generates random letters and checks whether the user transmits them correctly
• Provides a Decoder mode in which any incoming signal is interpreted and displayed on the LCD screen
• Visualizes the Morse signal waveform on the OLED graphical display
• Provides audio feedback through a piezo buzzer and visual feedback through colored LEDs

Project goal: The project aims to transform a vintage object — the Soviet telegraph key — into a fully functional educational tool. Morse code is not merely history: it is still actively used in amateur radio, civil aviation, and emergency communications.

Starting idea: A fascination with analogue technology and the desire to understand how radio operators communicated before the digital era. A telegraph key forgotten in a drawer became the starting point for a complete learning system.

Why it is useful: Unlike software-based Morse learning applications, this device provides the authentic mechanical feel of a real telegraph key and the characteristic sound heard by operators 70 years ago. The simultaneous physical and audio feedback accelerates the memorization of the code.

The system is organized into four functional blocks that interact in real time through the microcontroller's pins:

Block diagram:

┌────────────────────────────────────────────────────────────────┐
│                    Signal Corps Trainer                        │
│                                                                │
│  ┌──────────────┐    I2C (SDA/SCL)    ┌─────────────────────┐  │
│  │  LCD 1602    │◄───────────────────►│                     │  │
│  │  (0x27)      │                     │   ATmega328P-XMINI  │  │
│  └──────────────┘                     │                     │  │
│                                       │   MCU + mEDBG       │  │
│  ┌──────────────┐    I2C (SDA/SCL)    │                     │  │
│  │  OLED 0.96"  │◄───────────────────►│  PC4=SDA  PC5=SCL   │  │
│  │  SSD1306     │                     │  PC0=A0   D2=INT0   │  │
│  │  (0x3C)      │                     │  D6       D7  D8    │  │ 
│  └──────────────┘                     │  D9(PWM)            │  │
│                                       └─────────┬───────────┘  │
│  ┌──────────────┐                               │              │
│  │  KY-037      │──── AO ──────────────── PC0   │              │
│  │  Sound sensor│                               │              │
│  └──────────────┘                               │              │
│                                                 │              │
│  ┌──────────────┐                               │              │
│  │  Morse key   │──── D2 (INT0) ────────────────┤              │
│  │  Vintage USSR│                               │              │
│  └──────────────┘                               │              │
│                                                 │              │
│  ┌──────────────┐                               │              │
│  │  Mode BTN    │──── D6 ───────────────────────┤              │
│  │  Trainer/Dec.│                               │              │
│  └──────────────┘                               │              │
│                                                 │              │
│                      ┌──────────────────────────┘              │
│                      │                                         │
│              D7 ─────►──── R1(220Ω) ──── Green LED             │
│              D8 ─────►──── R2(220Ω) ──── Red LED               │
│              D9 ─────►──── Passive Piezo Buzzer                │
│                                                                │
└────────────────────────────────────────────────────────────────┘

Module descriptions:

• ATmega328P-XMINI — Central processing unit. Runs the Morse decoding logic, measures pulse durations (dot/dash), manages the I2C displays, and controls all feedback elements. The on-board mEDBG debugger enables direct programming via USB without an external programmer.

• LCD 1602 (I2C, 0x27) — Displays the decoded text character by character and system status messages (active mode, Trainer score, errors).

• OLED 0.96” SSD1306 (I2C, 0x3C) — Visualizes the Morse signal waveform in real time (horizontal scroll) and shows feedback animations for correct/incorrect answers.

• KY-037 Sound Sensor — Monitors the ambient noise level via the analogue output (AO → A0). If the threshold is exceeded, the red LED lights up as a warning.

• Vintage telegraph key — Primary input interface. Connected to D2 (INT0) for hardware interrupt detection. Generates pulses of variable duration (dot < 150 ms, dash > 150 ms).

• Mode selector button (SW2) — Connected to D6, switches between Trainer and Decoder mode on each press.

• Passive piezo buzzer — Generates a 700 Hz tone on D9 (PWM, ``tone()`` function) synchronized with the key signal.

• Green LED / Red LED — Instant visual feedback: green = correct answer in Trainer mode, red = error or ambient noise detected.

• Pull-up: SW1 and SW2 use the MCU's internal pull-up resistors (INPUT_PULLUP). The external 10kΩ resistor is optional.
• Decoupling: Capacitor C1 (100nF) should be placed as close as possible to the MCU's VCC/GND pins for effective power supply noise filtering.
• Power supply: The entire system is powered at 5V via USB from the X-MINI board.

The circuit was assembled on a breadboard using male-to-male and male-to-female jumper wires. The detailed schematic includes:

• The I2C bus with both displays connected in parallel
• Protection resistors R1 and R2 in series with the LEDs
• Decoupling capacitor C1 between VCC and GND
• The telegraph key connected directly between D2 and GND (internal pull-up active)

• The Soviet telegraph key works correctly as a simple switch — detected via INPUT_PULLUP on D2
• The piezo buzzer generates a 700 Hz tone synchronized with key presses, with no perceptible latency
• The OLED display shows the current state and signal waveform in real time
• The shared I2C bus between the LCD and OLED operates stably with addresses 0x27 and 0x3C
• The green/red LED feedback system responds correctly to commands on pins D7/D8

The project demonstrates that vintage technology can be reinterpreted in a modern educational context. The Soviet telegraph key, far from being a mere collector's item, becomes an authentic tactile interface and a bridge between 20th-century analogue communication and contemporary microelectronics.

The primary value of this project lies in the learning experience: the user does not learn Morse code through an abstract application, but through a physical object with history, a distinctive mechanical feel, and the characteristic sound heard by radio operators 70 years ago.

Challenges encountered:

• Correctly configuring the ATmega328P-XMINI board in Arduino IDE (mEDBG driver)
• Sharing the I2C bus without address conflicts
• Calibrating dot/dash timing thresholds to match the user's keying speed

• ATmega328P-XMINI User Guide — Microchip Technology
• SSD1306 OLED Driver Datasheet — Solomon Systech
• KY-037 Sound Sensor Module — Arduino Modules Info

• Adafruit SSD1306 Library — GitHub
• Adafruit GFX Library — GitHub
• Arduino Language Reference — arduino.cc
• Morse Code Timing Standard — ITU-R M.1677-1

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