4 FM bug: simple transmitter

This simple fm transmitter can be tuned to standard fm receiver and has a range of around 300 meters, thus suitable for monitoring purposes. It is designed as simple as possible with minimal parts without compromising its stability. The transmitter is very similar to wireless microphone circuit. Click diagram to enlarge.

simple FM transmitter schematic

How to make the FM transmitter:

Gather the following materials:

R1 - 120 ohm resistor 

R2 - 120K ohm resistor 

R3, R5, R6 - 10K  ohm resistor 

R4 - 1.2Meg ohm resistor 

R7 - 1K ohm resistor 

C1, C2 - 100nF capacitor 

C3 - 10nF capacitor 

C4 - 4-80pf variable capacitor / trimmer cap

C5 - 4.7pf capacitor 

Q1, Q2 - 2N2222 transistor or any general purpose transistor

L1 - 8 turns of #22AWG  wire wound around a pencil or pen

antenna - 1.5ft solid wire

Electret Microphone

9V battery

2N2222 pinout

How the FM circuit works:

Sound is converted to electrical energy by the electret mic, then this energy is feed to transistor Q1. Transistor Q1 serves as an amplifier of the signal. The heart of the fm transmitter is the oscillator circuit which is the trimmer capacitor C4,inductor L1 and transistor Q1. C4 and L1 combination provides the value of frequency on which the transmitter is to be tuned. Materials listed above are not critical, you can replace it with its nearest value.

 *see LC calculator to select the inductor L1 and capacitor C4 values for the tuning frequency of your choice.

FM Transmitter testing:

1.Turn on  fm receiver and look for frequency that there is not used

    by any fm channel or fm station in your area.

2.Turn on the fm transmitter and gently adjust the trimmer capacitor C4

   until the fm receiver pick up the signal from your transmitter.

3.If ever the fm receiver cant pick the transmitter signal dispite of trying 

   many times, try squeezing or stretching the inductor and repeat step 2