Overview:
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.
Parts:
| Part |
Qty. |
Description |
Substitutions |
| 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 |
| L1,
L2 |
2 |
5
Turn Air Core Coil |
|
| MIC |
1 |
Electret
Microphone |
|
| MISC |
1 |
9V
Battery Snap, PC Board, Wire For Antenna |
|
Notes:
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.
Lie
Detector