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. 

 

 

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    

1. L1 and L2 are 5 turns of 28 AWG enamel coated magnet wire wound with a 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.
2. C5 is used for tuning. This transmitter operates on the normal broadcast frequencies (88-108MHz).
3. Q1 and Q2 can also be 2N3904 or something similar.
4. You can use 1/4 W resistors mounted vertically instead of 1/8 W resistors.
5. You may want to bypass the battery with a .01uf capacitor.
6. An antenna may not be required for operation.

ref: aaroncake.net/Circuits/fmtrans.asp