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12-16-2009

FM Telephone Bug

Here is a simple transmitter that when connected to a phone line, will transmit anything on that line (execpt the dial tone) to any FM radio. The frequency can be tuned from 88 to about 94Mhz and the range is about 200 feet. It is extremely easy to build and is therefore a good, useful beginner project.

Schematic

FM Telephone Bug

Parts

Part

Total Qty.

Description

R1

1

180 Ohm 1/4 W Resistor

R2

1

12K 1/4 W Resistor

C1

1

330pF Capacitor

C2

1

12pF Capacitor

C3

1

471pF Capacitor

C4

1

22pF Capacitor

Q1

1

2SA933 Transistor

D1, D2, D3, D4

4

1SS119 Silicon Diode

D5

1

Red LED

S1

1

SPDT Switch

L1

1

Tuning Coil

MISC

1

Wire, Circuit Board

Notes

  1. L1 is 7 turns of 22 AWG wire wound on a 9/64 drill bit. You may need to experiment with the number of turns.
  2. By stretching and compressing the coils of L1, you can change the frequency of the transmitter. The min frequency is about 88 Mhz, while the max frequency is around 94 Mhz.
  3. The green wire from the phone line goes to IN1. The red wire from the phone line goes to IN2. The green wire from OUT1 goes to the phone(s), as well as the red wire from OUT2.
  4. The antenna is a piece of thin (22 AWG) wire about 5 inches long.
  5. All capacitors are rated for 250V or greater.
  6. The transmitter is powered by the phone line and is on only when the phone is in use. S1 can be used to turn the transmitter off if it is not needed.
  7. If you have problems with the LED burning out, then add a 300 ohm 1/4W resistor in series with it.

aaroncake.net

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