His Low-cost Cordless Bug Can Transmit Voice Signals In Fm

FM BUG T

his low-cost cordless bug can transmit voice signals in FM band ( 88-108MHz ) up to about 250 meters. Which can be heard on any ordinary Fm radio. The rated power of the transmitter with operating voltage of 3 volts is about 5mW. One of the most important features of this project is the use of standard components. Since restrictions apply to FM transmitters in some countries, you will have to find out the laws in your locality before commencing construction.

R1 27K

R4 10K

R5 39K C4 VC1 33p 2-22p

R3 1M R2 15K

C6 68p

L1 4T

C2 0.1 C1 47n

C3 1n

Mic Electret microphone

C5 10p

FEATURES

·

Low current consumption.

·

Good transmitting range.

·

Excellent audio quality.

·

High frequency stability.

·

Up to 10 days battery life

SW1 On/Off

R6 150

3V BATTERY L1 = 4Turns, 24swg copper wire , 3mm air core Antenna = 80cm wire

A

·

C8 0.1

C7 15p Q2 BF494

Q1 BC547

ANT

bug is a transmitter which can transmit audio or video. A bug should be small and easy to hide. The purpose of a bug is to hear conversation that you are not suppose to hear. It is not a big deal, half of the American population has been bugged during the last year by several well known intelligence's and also by unknown intelligence's. This miniature transmitter is easy to construct and it's

DEVADAS.K transmissions can be picked up on any standard FM radio. It has a range of up to 1/4-Km (250 meters) or more, depending on the line-of-sight, obstructions by large buildings, etc. It is great for room monitoring, baby listening, nature research, etc The circuit comprises two stages. The first stage Page 1

is a microphone preamplifier built around BC548 transistor. The next stage is a VHF oscillator wired around another BF494. The 27K microphone load resistor determines the gain of the microphone. The 47n and 15K couples the microphone to the input of the first stage. Transistor Q2, capacitors C3,C4,VC1 C5,C6,resistors R5,R6 and inductor L1 form an FM oscillator that operates at a frequency of around 98 MHz Variable -capacitor VC1 allows the oscillator frequency to be adjusted between 88 and 108MHz. The carrier frequency of an oscillator is determined by the capacity of C4, VC1 in conjunction with the inductance of the coil L1. Capacitor C6 helps in frequency tuning and C7 is an antenna coupling capacitor.

FM BUG

PARTS LIST TRANSISTOR

EBC

BEC

BC547

BF494

33

33

Ordinary

NPO

Q1— BC547 Q2— BF494 RESISTOR R1— 27K R2— 15K R3— 1M R4— 10K R5—39K R6— 150Ω CAPACITOR C1— 47n (0.047µF) Polyester C2— 0.1µF Polyester

Test and tune

C3— 1n (0.001µF) NPO Ceramic disk

Connect the Transmitter to the battery. Turn on a nearby FM radio and tune to a quiet spot (no station using that frequency) on the dial somewhere between 90 and 95MHz. Adjust VC1 and L1 until you do hear the bug. First adjust VC1 for the best reception, and then fine tune the radio.

C4— 33P NPO Ceramic disk C5— 10P NPO Ceramic disk C6— 68P NPO Ceramic disk C7— 15P NPO Ceramic disk C8— 0.1µF Ceramic disk

Transmitting range — field test

MIC— Condenser MIC VC1— Trimmer 2-22p

Philips pocket radio — 75m

L1— 4Turns, 24SWG, 3mm diameter air core, Enameled Copper Wire

Nokia cell phone radio — 150m

ANT— 80cm Insulated Copper Wire

Icom IC-R5 wide band receiver — 250m

Battery— Two, UM4 — AAA Cell 1.5V each SW1— On/Off switch One AAA Battery holder Capacitor C7

4.7pF

15pF

Range

*

**

Stability

***

**

47pF

Working voltage

R1

R4

R6

3V

27K

10K

150Ω

***

6V

39K

15K

270Ω

*

9V

47K

22K

470Ω

Working voltage

3V

6V

9V

Range

250m

500m

750m

Page 2

FM BUG

Bug PCB Layout

Bug Component Layout

40mm 15mm

Actual size 40mm x 15mm

Drill hole 0.8mm

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