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Simple FM Transmitter
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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. 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.
Ask your question about this circuit in the forum.
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