• Short demo: https://youtube.com/shorts/3NDd_IlYnws?feature=share
• Code repository: https://github.com/alx-sima/car-modulator

• This project consists of building a portable FM modulator, to be used for music playing in cars which don't have Bluetooth connectivity.
• This idea occured to me when I had a long drive in my Dacia Logan, having no CDs or Bluetooth speakers, and barely any radio signal.
• Features: Bluetooth pairing, frequency selection (using the potentiometer), information display, audio play from SD memory.

List of components:

• Arduino UNO R3
• Breadboard + dupont wires
• 3.5mm jack wire
• KT0803K FM transmitter (I2C)
• MH-M28 BLE stereo receiver
• LCD 1602 (I2C)
• Push button
• 10K potentiometer
• 1K resistor

Pin connections:

• KT0803K:
  • VCC - 5V (Arduino)
  • GND - GND (Arduino)
  • SDA - PC4 (Arduino)
  • SCL - PC5 (Arduino)
  • 3.5mm Jack - 3.5mm Jack (KT0803K)
• MH-M28:
  • VCC - 5V (Arduino)
  • GND - GND (Arduino)
• LCD 1602:
  • VCC - 5V (Arduino)
  • GND - GND (Arduino)
  • SDA - PC4 (Arduino)
  • SCL - PC5 (Arduino)
• Push button:
  • Vin - PD2 (Arduino)
  • GND - GND (Arduino)
• Voltage divider (1K resistor + 10K potentiometer)
  • Vin (resistor) - 5V (Arduino)
  • Vout (between resistor and potentiometer) - PC3 (Arduino)
  • GND - GND (Arduino)

External libraries used:

• marcoschwartz/LiquidCrystal_I2C for easier interaction with the LCD.
• FMTX for controlling the FM transmitter. Copy included in the lib/ folder.

Lab concepts used:

• Lab 0 - GPIO
• Lab 2 - Interrupts
• Lab 4 - ADC
• Lab 6 - I2C

Arduino GPIO configuration:

• PC3 - input (for ADC)
• PD2 - input & pull-up (for button)

Synopsis: A button is pressed to toggle the frequency selection. While in frequency selection mode, the ADC is continuously running and reading values from a voltage divider containing a potentiometer (knob), while the LCD is displaying the current selected value (read from the ADC on interrupts) and the transmitter is playing on the previous established frequency. While not selecting, the ADC is turned off in order to save energy, and the transmitter is playing on the established frequency (selected when the mode changes), while the LCD displays it.

On setup, several registers are configured:

• DDRC, DDRD and PORTD for GPIO configuration (as above);
• EICRA and EIMSK for enabling interrupt on falling edge for pin PD2, in order to detect button presses.
• ADMUX, ADCSRA and ADCSRB for setting up the ADC with the following configuration: AVcc (5V) reference, interrupt enable, auto-trigger enable, free-running mode and channel 3 (input on ADC3/PC3).

Then, using library calls, LCD and FM transmitter are set up. A default transmitting Frequency of 90.0Hz is set, and the region is EUROPE.

Two interrupt handlers are set up, INT0 and ADC:

• INT0: notify that the button has been pushed (after debouncing the input);
• ADC: update the selected frequency value.

The main execution loop converts the ADC raw value into a transmitting frequency (between 87.5MHz and 108MHz) and displays it on the screen if in selection mode.