ara technologies

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our objective is to design an apparatus with the ability to wirelessly transmit voice and audio from a computer 
over an FM frequency to any FM receiver (radio, walkman etc.) the audio could range from mp3 files, voice 
input or Internet radio. the frequency would be designated by the user and range from 87.5-107.9 MHz. the 
transmission range would not exceed approximately 100 feet in order to comply FCC and CSA regulations.

progress report #1		progress report #2		final report

our proposed device design

the main system will have two inputs.  the first input, identified as audio in, will represent the signal or message to be modulated and transmitted.  the other input will be the power from a 9V battery that would allow us to power up the required subsystems. 

the main system will have only one output.  this signal will represent the modulated signal being transmitted over the air, up to a maximum distance of 100 feet.

the main system will consist of six subsystems:

1. power:  the purpose of this subsystem is to regulate and distribute the necessary power to energize all the subsystems

2. FM transmitter:  this subsystem would allow us to modulate the input signal as well as to select the frequency carrier.

the device chosen for this application is the BH1415F manufactured by Rohm.  this device is a FM transmitter IC which consists of a stereo modulator for generating stereo signals and a FM transmitter for broadcasting FM signals.  the transmission is maintained stable due to the built-in phase-lock loop (PLL) system.  PLL input data comes from a serial port that can be controlled by a micro-controller.  two parameters that can be set through this serial port are the chipís carrier frequency (by means of setting a program counter) and the mode of transmission operation (monaural or stereo operation.)

 BH1415F pin diagram from Rohmís specifications


3. micro-controller:  this subsystem will consist of an 8-bit microcontroller with 20 I/O pins that would allows to control all other subsystems (8 pins for LCD, 4 pins for the FM transmitter, 2 pins for LED's and 3 pins for the user buttons.)  the clock speed for the microcontroller will be 4MHz because most LCDís cannot handle faster processing speeds.  based on these requirements, we have chosen the PIC 16F872, manufactured by Microchip.  

PIC16F872 pin diagram from Microchip's specifications

4. LCD:  will provide visual feedback to the user.  data to be display consist of: frequency carrier selection and battery life level.

5. LEDs: will also provide visual feedback to the user.  one LED will be used to display the ON state of the system and another LED will be used to indicate the ACTIVE state of the system, i.e. transmission is operating.

6. user buttons: will give the user a way to select the frequency carrier to be used.  the carrier frequency range covers the standard FM radio spectrum with intervals of Ī200kHz.

prototype design pictures

front view
back view
isometric view