Posted: June 25th, 2021

FM Transmitter

FM Transmitter
Here is the schematic, PC board pattern, and parts placement for a low powered FM transmitter. The range of the transmitter when running at 9V is about 300 feet. Running it from 12V increases the range to about 400 feet. This transmitter should not be used as a room or telephone bug.

Part
Total
Qty.
Description Substitutions

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C1
1
0.001uf Disc Capacitor
 –

C2
1
5.6pf Disc Capacitor
 –

C3,C4
2
10uf Electrolytic Capacitor
 –

C5
1
3-18pf Adjustable Cap
 –

R1
1
270 Ohm 1/8W Resistor
270 Ohm 1/4W Resistor

R2,R5,R6
3
4.7k 1/8W Resistor
4.7K 1/4W Resistor

R3
1
10k 1/8W Resistor
10K 1/4W Resistor

R4
1
100k 1/8W Resistor
100K 1/4W Resistor

Q1, Q2
2
2N2222A NPN Transistor
2N3904, NTE123A

L1, L2
2
5 Turn Air Core Coil
 –

MIC
1
Electret Microphone
 –

MISC
1
9V Battery Snap, PC Board, Wire For Antenna
 –

L1 and L2 are 5 turns of 28 AWG enamel coated magnet wire wound with an inside diameter of about 4mm. The inside of a ballpoint pen works well (the plastic tube that holds the ink). Remove the form after winding then install the coil on the circuit board, being careful not to bend it.
C5 is used for tuning. This transmitter operates on the normal broadcast frequencies (88-108MHz).
Q1 and Q2 can also be 2N3904 or something similar.
You can use 1/4 W resistors mounted vertically instead of 1/8 W resistors.
You may want to bypass the battery with a. 01uf capacitor.
An antenna may not be required for operation.

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