The FM transmitter is designed to run from a 9-volt battery and is made from readily available parts. The primary use of this simple FM transmitter is as a baby monitor. It is very sensitive, and easily capable of picking up a conversation in any part of a room. The dimensions and values given here will allow static-free reception within the perimeter of most homes.
No license is required for this small transmitter according to FCC. If powered from a 9-volt battery and used with an antenna no longer than 30cm, the transmitter's radiated power will be within the FCC limits (200 kHz bandwidth and its field strength less than 50 μV/m at a distance of 15 meters from the device).
The Circuit
Audio is picked up from the room by an electret microphone and amplified by Q1. Resistors R2-R5 set up the DC operating bias of Q1. Capacitor C3 serves to improve the AC response to the audio voltage, and C2 blocks the DC bias and couples the AC to the next stage, where the RF action takes place.
The amplified AC voltage from Q1 is routed to the base of Q2. Transistor Q2 and associated circuitry (C5 and the inductor) form an oscillator that operates in the 80-130 MHz range. The oscillator is modulated by the audio voltage that is applied to the base of Q2. Resistor R6 limits the input to the RF section, and its value can be adjusted as necessary to limit the volume of the input. That will help control the amount of distortion you have on very loud inputs.
Resistors R7-R9 set the DC-operating bias of Q2, another 2N2222 that's used as the oscillator and modulator of the transmitter. Capacitor C5 is a 6-50 pF trimmer capacitor that's used to tune the oscillator tank circuit, and C4 routes the RF from the oscillator to ground to prevent unstable operation.
How to build it
The FM transmitter prototype is built on a piece of perforated construction board with 0.1-inch hole spacing. Component spacing is not critical, but placement is. Generally, you must make the transmitter as small as possible. A piece of perfboard that is 12 holes wide and 30 holes long, will give you plenty of room to work with, but still produce a small unit. During assembling make sure that the ground lead of the microphone is soldered to the ground.
The inductor is made by winding a 24-gauge wire (aprox. 0.5mm wire diameter), around a pencil (aprox. 5mm in diameter) six times. The C7 capacitor must be placed as close as possible across the L1 -Q2, R9, to reduce the amount of RF feedback you'll get into the rest of the circuit. The antenna (24gauge wire) should be soldered to the coil you made, about 2 turns up from the transistor side, and should be about 8-12 inches long.
Operation
To use the transmitter, set up a radio in the area at least 10 feet from the project. Find an unused frequency and turn the radio up so you can hear the static. Connect a 9-volt battery to the transmitter and listen to the radio. Slowly adjust the tank capacitor (C5) until you "quiet" the receiver; this is the tuned spot.
Note that when you remove your hands from the transmitter, you will detune the circuit somewhat. It is usually best to leave it detuned, and tune the radio in to get the best reception. If you cannot get the tuning range you desire, you can squeeze the coils in the tank circuit closer together to lower the frequency, or pull them apart just a little bit to raise it.
The circuit works best when powered by a battery, but if a wall derived supply is needed, make certain that the ripple voltage is as low as possible, or you will get hum in the receiver.
