Featured Project

# Phone Audio FM Transmitter

Team Members:

James Wozniak (jamesaw)

Madigan Carroll (mac18)

Dan Piper (depiper2)

# Problem

In cars with older stereo systems, there are no easy ways to play music from your phone as the car lacks Bluetooth or other audio connections. There exist small FM transmitters that circumvent this problem by broadcasting the phone audio on some given FM wavelength. The main issue with these is that they must be manually tuned to find an open wavelength, a process not easily or safely done while driving.

# Solution

Our solution is to build upon these preexisting devices, but add the functionality of automatically switching the transmitter’s frequency, creating a safer and more enjoyable experience. For this to work, several components are needed: a Bluetooth connection to send audio signals from the phone to the device, an FM receiver and processing unit to find the best wavelength to transmit on, and an FM transmitter to send the audio signals to be received by the car stereo.

# Solution Components

## Subsystem 1 - Bluetooth Interface

This system connects the user’s phone, or other bluetooth device to our project. It should be a standalone module that handles all the bluetooth functions, and outputs an audio signal that will be modulated and transmitted by the FM Transmitter. Note: this subsystem may be included in the microcontroller.

## Subsystem 2 - FM Transmitter

This module will transmit the audio signal output by our bluetooth module. It will modulate the signal to FM frequency chosen by the control system. Therefore, the transmitting frequency must be able to be tuned electronically.

## Subsystem 3 - FM Receiver

This module will receive an FM signal. It must be able to be adjusted electronically (not with a mechanical potentiometer) with a signal from the control system. It does not need to fully demodulate the signal, as we only need to measure the power in the signal. Note: if may choose to have a single transceiver, in which case the receiver subsystem and the transmitter subsystem will be combined into a single subsystem.

## Subsystem 4 - Control System

The control system will consist of a microcontroller and surrounding circuitry, capable of reading the power output of the FM receiver, and outputting a signal to adjust the receiving frequency, in order to scan the FM band. We will write and upload a program to determine the most suitable frequency. It will then output a signal to the FM transmitter to adjust the transmitting frequency to the band determined above. We are planning on using the ESP32-S3-WROOM-1 microcontroller given its built-in Bluetooth module and low power usage.

## Subsystem 5 - Power

Our device is designed to be used in a car, so It must be able to be powered by a standard automobile auxiliary power outlet which provides 12-13V DC and usually at least 100W. This should be more than sufficient. We plan to purchase a connector that can be plugged into this port, with leads that we can wire to our circuit.

# Criterion for Success

The device can pair with a phone via bluetooth and receive an audio signal from a phone.

The Device transmits an FM signal capable of being detected by a standard fm radio

The Device can receive FM signals and scan the FM bands.

The digital algorithm is able to compare the strength of different channels and determine the optimal channel.

The device is able to automatically switch the transmitting channel to the predetermined best channel when the user pushes a button.