This circuit is a simple two transistor (2N2222) FM transmitter. No
license is required for this transmitter according to FCC regulations
regarding wireless microphones. If powered by a 9 volt battery and used
with an antenna no longer than 12 inches, the transmitter will be within
the FCC limits.
The microphone is amplified by Q1. Q2, C5, and L1 form an oscillator
that operates in the 80 to 130 MHz range. The oscillator is voltage
controlled, so it is modulated by the audio signal that is applied to
the base of Q2. R6 limits the input to the RF section, and it's value
can be adjusted as necessary to limit the volume of the input. L1 and C6
can be made with wire and a pencil. The inductor (L1) is made by
winding two pieces of 24 gauge insulated wire, laid side by side, around
a pencil six times. Remove the coil you have formed and unscrew the two
coils apart from each other.
FM Transmitter Circuit Diagram
One of these coils (the better looking of
the two) will be used in the tank circuit, and the other can be used in
the next one you build. The antenna (24 gauge wire) should be soldered
to the coil you made, about 2 turns up from the bottom, on the
transistor side, and should be 8-12 inches long. To make C6, take a 4
inch piece of 24 gauge insulated wire, bend it over double and,
beginning 1/2" from the open end, twist the wire as if you were forming a
rope. When you have about 1" of twisted wire, stop and cut the looped
end off, leaving about 1/2" of twisted wire (this forms the capacitor)
and 1/2" of untwisted wire for leads.
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