Iowa State University – Senior Design
Grant Blythe Luke Erichsen
Tony Hunziker Jacob Sloat
Dr. John W. Lamont
Prof. Ralph E. Patterson III
FCC: Federal Communication Commission
LCD: liquid crystal display
MP3 player: digital music player, (i.e. ipod)
Satellite radio: subscription radio signal sent via satellite,
(i.e. XM radio, Sirius radio)
Transmission frequency: The frequency at which the
device is transmitting the FM modulated signal to the FM
Wall wart: AC power transformer designed to plug into a
standard wall outlet
General Problem Statement
The objective of this project is to design a FM
transmitter that will:
Connect to a standard headphone output jack of
an mp3 player
Transmit a minimum of 12ft
Transmit between frequencies of 88MHz to
Have 4 programmable preset buttons
Receive power from a cigarette lighter/power
Automatic on/off function
General Solution Approach
The FM transmitter will modulate the signal, send it through an
amplifier, and finally through an internal antenna.
General Solution Approach
Internal memory will store four programmable station
presets from the frequency band.
A LCD screen will display the current transmission
frequency. This LCD will be backlit for easy night use.
The transmission frequency and presets will be
accessible through a button/knob interface that may also
include a lighting system.
There will be a microcontroller to access memory,
automatic turn off/on function, possible lighting, display
frequency, and any other controlling concerns.
The finished device will operate within a personal vehicle
or a household room that could be exposed to:
Intended User(s) / Use(s)
The intended user for this product is anyone owning
MP3 players or satellite radio devices. It does assume
the amount of operating knowledge associated with MP3
player/satellite radio users.
The FM transmitter is intended to make personal music
players more accessible to listen to through home and
The transmitter will be used for all varieties of personal
The transmitter will be used in a variety of environments
including varying temperatures, humidity, seismic
conditions, and electromagnetic noise.
The transmitter will be used at all hours of the day.
Similar products will come directly from the personal
music players producers as competition.
The device will be used with standard North American
FM radio equipment.
The input audio signal will consist of standard music with
a frequency range of 20 Hz to 20 kHz.
The device design will implement solid state electronics.
The user will have access to a steady power source able
to supply the rated voltage and frequency (if AC) within
a 10 percent tolerance.
The cost to purchase this product shall not become
The transmitter must conform to FCC regulations.
– The FCC has regulation broadcast strength of .1kW. The
transmitter must not exceed this strength.
– The device shall conform with FCC rules Part 15 concerning
unlicensed FM broadcasting.
– The RF field strength 3 meters from the device should not
The transmission frequency band must stay within 88-
The device shall be capable of obtaining power from
both a 120 V AC and a 12 V DC source.
The form of the device must be manageable for ease of
transportation and storage.
There must be at least 4 programmable preset
The device must transmit at least 12 feet.
Transmission frequency must be adjustable.
Transmission frequency must be displayed.
The size shall not exceed 6 in. by 6 in. by 3 in.
The weight shall not exceed 1 lb.
The device shall be compatible with both digital and
analog tuned radios.
The device case will be made of plastic
The case will allow for easy hand manipulation and
The device will implement an LCD screen displaying the
The user input interface will consist of six buttons.
– “up” and a “down” button to adjust transmission frequency
– 4 buttons will each access a programmable preset frequency
Pushing and releasing a preset button, in less than 3
seconds, will adjust the transmission frequency to the
stored frequency assigned to that button.
Pushing and holding a preset button for greater than 3
seconds will assign and store the current transmission
frequency to that preset button.
Both the LCD display screen and the buttons may be
backlit for use in low ambient light environments.
This device shall come with an adapter for use with an
American standard wall outlet. This will be a simple wall
wart device. The output from this adapter will be a male
Detailed Functionality Continued…
The transmitter will also come with an adapter for the
standard cigarette outlet for automobile use. The output
of this adapter will be a male power jack.
Both adapters will connect to the device through a
common female jack. The adapters will be designed so
that the device will receive the same power input
regardless of which adapter is being used.
An instruction manual will also be included.
The device and accessories should be deliverable by
This includes the different systems within the design, the
parts currently considered to implement these systems,
and preliminary prices of these components.
These inputs allow the user to adjust and fine tune the
transmission frequency manually.
There will be two buttons for the manual input function.
– one to adjust the transmission frequency up and one
button to adjust the transmission frequency down.
Adjustment of the transmission frequency will happen in
increments of 200 kHz.
– For example 102.7 MHz will increase to 102.9 MHz or decrease
to 102.5 MHz in response to the received input.
The four programmable preset frequencies shall be
stored in non-volatile memory in the device.
The transmission frequency shall be adjusted to any of
these frequencies by pushing one of the four
A preset shall be programmed to the current frequency
by pushing and holding the corresponding button for
greater than 3 seconds.
These four buttons will be backlit for use in low light
The device shall receive its power input from a
12 V DC cigarette lighter/power socket in an
automobile or a standard 120 V AC wall outlet.
An adapter will be provided for each of these
– Both adapters will provide a common 5 V DC output
to the device.
– The device will have one power input jack that will be
capable of connecting to either adapter.
Power Supply Continued…
The adapter for the cigarette lighter will be a single
stage adapter that plugs directly into the standard
cigarette lighter/ power outlet of an automobile.
– The adapter will convert the 12V output of the cigarette lighter
to the 5V DC input needed for the device
The source from the wall outlet will be a wall wart
– Converts the 120V AC to a 5V DC output
– Provided by the EE/CprE Senior Design
The input signal may come from an mp3
player, or a satellite radio system.
The audio signal input is expected to
consist of audio frequencies within the
range of 20Hz to 20 kHz.
The device shall accept a stereo input
audio signal through a standard 3.5 mm
Microcontroller Design Requirements
It will take in the manual and preset inputs and tune the
device to a transmission frequency depending on which
buttons are pushed.
It will allow the device to store transmission frequencies
into the preset buttons.
It will also implement the auto turn on/turn off function
with respect to the input signal.
The LCD display will also be controlled by the
The PIC processor PIC16F873A was chosen along with
the LM555 clock.
– It contains a 28 pin IO interface and the memory and processing
power needed for our application.
– PIC processors will give us the necessary computing power to
control all components of the device including the LCD.
– At the same time PIC processors are inexpensive and will fall
within the project budget.
– PIC processors also possess the non-volatile memory necessary
to store the preset stations.
Microcontroller Solution Continued…
Auto On/Off Implementation
The microcontroller will be in a continuous loop awaiting
the input in order to control the peripherals.
– Upon detection of an input signal, the device shall power on and
begin transmitting within 1 second.
– After detecting no input signal for 1 minute, the device shall
power down within 1 second.
– This count down is a period to wait for signal before actually
powering down. This prevents premature power down.
After examining the available technologies, an integrated
solution was found to provide the modulation,
amplification, and transmission.
The device will implement a Rohm Electronics BH1415F
Wireless Audio Link IC to provide stereo modulation and
The BH1415F consists of a stereo modulator for
generating the stereo composite signal and a FM
transmitter for broadcasting an FM signal on the air.
The IC transmits on a frequency range of 88 MHz to 108
MHz. The transmission frequency is set by the
Composite Stereo Signal
The composite stereo signal is then modulated with a
– This carrier signal is the frequency you tune to with an FM radio.
The carrier signal is set on the BH1415F by the
microcontroller using a data packet.
Data Packet to BH1415F
Carrier Frequency Data
For Example: in the case of 99.7 MHz carrier frequency.
99.7 MHz / 100 kHz (fref) = 997 3E5 (HEX)
There are two outputs from the device.
An LCD screen that will display the frequency that the
device is currently transmitting at.
The other output is the FM audio output transmitted by
LCD Display Requirements
When a frequency change occurs, the microcontroller
will send the appropriate control signals to the LCD
Each of the 4 digits plus the decimal point displayed will
be controlled individually, and will only need to be
refreshed when the frequency is change.
The format of this display will be “XXX.X” (i.e. “102.7”).
The display is also backlit.
LCD Display Solution
The LCD chosen was the VIM-404-DP-FC-S-HV
LCD manufactured by Varitronix.
It is a transflective backlit display
The way it accepts inputs is dependant upon its
programming making it a versatile choice.
Its 20 I/O pin interface is typical for LCD
components on the market.
LCD Display Solution
LCD Display Solution
Pin COM1 COM2 COM3
1-3 N.C. N.C. N.C
4 1B 1C 1P
5 2B 2C 2P
6 3B 3C 3P
7 4B 4C ---
8 --- --- COM3
9-10 N.C. N.C. N.C
… … … …
The device will use a small wire antenna that will remain
inside the device case. Because the case is plastic and
the transmission range is small, there should not be a
need for an external antenna.
Interfacing with microcontroller
Rohm BH1415F Wireless Audio Link IC
Phase Lock Loop