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What it does :
This easy to build project transmits
high quality stereo sound from your MP3 player, computer, walkman,
discman,... to any household FM radio or car radio
(when you user this kit in the car).
The Circuit
is designed around the BA1404 single chip FM stereo transmitter from
ROHM. The IC requires only a small number of external parts
so it is well suited for hobbyist projects. The chip features excellent
frequency stability, low power consumption & good channel separation.
See datasheet for more details
datasheet
(pdf) (English)
datasheet (pdf)
(Japanese)
The transmitting RF frequency can be set by adjusting the coil (Lx).
This 2 ½ turn coils is paired with a 39 pF capacitor (Cx) to give a
frequency range from 87 MHz ~ 106 MHz.
Antenna : this
can be any piece of soft wire of 1 foot or longer. You may try different antenna
lengths to find the one that gives best result.
The audio plug supplied is a 3.5mm
stereo phone plug that matches with the phone jack of most MP3 players. If the
kit is used with other audio sources such as DVD player, you must change the phone
plug to other matching type (e.g.: almost all DVD players
use RCA jack )
Range :
This transmitter has very limited RF power so the range is expected to be quite
short. In future projects, we may try to include an upgraded version with
boosted RF output that has a longer range.
Tuning & use :
- Turn on the radio and adjust the dial to a
location (frequency) which is not occupied by any broadcast station.
- Insert 2 pcs of AA batteries into the
battery holder and insert the
phone plug to the MP3 player's phone jack. Turn the volume control of the player
to around 30 ~ 40 % of max.
Adjust Lx slowly
until the radio receives a clear sound from the transmitter.
Do a fine
tuning on the radio dial to get the best sound quality.
Next, adjust the volume of the MP3 player to a suitable level so as not to
overload the inputs of BA1404. Distortion occurs if the signal level at each input of
BA1404 are to high. The acceptable signal level is 20 ~ 200mV.
Caution :
- The datasheet
states that the absolute maximum supply
voltage for BA1404 is 3.6V.
Therefore, you should never attempt to increase the range by increasing
the supply voltage to any voltage higher than 3.6V. Doing so will
cause instant & permanent damage to the IC.
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