Main Features
- Frequency range: 95-105MHz (can be extended with component changes; see text)
- Transmission range: 40m with folded dipole antenna transmitter to typical FM receiver
- Current consumption: 10mA
- Distortion: typically 3% at 200mV audio signal level
- Separation between channels: typically 45dB at 1kHz
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There are many applications for an FM transmitter,
particularly if it can broadcast in stereo. You can broadcast stereo
signals from your CD player or any other source (stereo or mono) to an FM tuner
or radio.
The FM MiniMitter uses a single IC and a few other
components and fits in a small plastic case. It broadcasts on the FM band
(ie, 88-108MHz) so that it can be received by any standard FM tuner or portable
radio.
We published our first version of the MiniMitter in October
1988 and it has been a very popular project ever since. So why are we presenting
it again? The main reasons are to extend its transmission range (a frequent
request over the years), to make it easier to tune and to make it easier to
operate the input level controls.
Our new version of the FM MiniMitter operates from 3V rather
than 1.5V and this can come from two AA cells or you can run it from a 6V DC
plugpack. Running from two AA cells gives more than double the battery life of
one cell since the circuit will continue to operate even when the supply voltage
drops below 1V (although the power output and range will be much reduced).
The RCA input sockets and DC socket are all PC-mounted so there
is virtually no wiring to do.
Setting the input levels is now much easier and the
adjustable coils have been spaced further apart to minimise interaction and
allow easier tuning. However, the biggest change is in the antenna circuit. You
can now use a simple wire antenna or a 300Ω dipole antenna
for extended range.
