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.
Title: Simple FM Transmitter
electronic circuit
Source: unknown
Published on: 2005-08-27
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