Multifunctional miniature FM transmitter

performance

The output power does not exceed 5-8mW, and the transmission range can reach about 300 meters in residential areas. It can be received by an ordinary FM radio. After testing, its sensitivity and clarity are excellent. The most challenging part of the circuit design is that it can have such a transmission capability with only a 3V power supply and a half-wave antenna. The circuit consumes less than 5mA and can work continuously for 80 to 100 hours with two dry batteries.

The circuit is very stable under normal operation, and the frequency drift is very small. After testing, the receiver still does not need to be adjusted after working for 8 hours. The only thing that affects the output frequency is the condition of the battery. When the battery ages, the frequency changes slightly.

The frequency of the transmitter is around 82.8MHz, and there is less interference from radio stations in this frequency band.

How it works

The circuit is shown in the figure below. It consists of a first-stage audio amplifier and a first-stage RF oscillator.

There is actually a FET hidden inside the electret microphone. The FET amplifies the capacitance change of the diaphragm in front of the microphone, which is why the electret microphone is very sensitive.

Transistor Q1 is used for audio amplification, with a gain of about 20 to 50. The amplified signal is sent to the base of the oscillator Q2. Q2 operates at about 88MHz, and this frequency is adjusted by the oscillator coil (a total of 5 turns) and capacitor C4. The frequency is also determined by Q2, 18pF adjustable capacitor C5 and bias components (such as R5 and R4).

After the power is turned on, capacitor C3 is gradually charged through R4, while capacitor C5 is charged through the oscillation coil and R5, but faster; the 47pF capacitor C4 is also charged (although there is only a very small voltage across its two ends), and the coil generates a magnetic field.

As the base voltage gradually rises, transistor Q2 turns on and effectively connects its internal resistance in parallel across the 18pF capacitor.

The base voltage continues to rise, and capacitor C5 tries to prevent the movement of the emitter potential. When the energy in the capacitor is exhausted and no longer prevents the movement of the emitter potential, the voltage between the base and the emitter decreases, the transistor is cut off, the current flowing into the coil also stops, and the magnetic field decays. A reverse voltage is generated, and the collector potential rises from the original 2.9V to more than 3V, and charges C4 in the opposite direction. This voltage also charges C5 and increases the voltage drop on R5, causing the transistor to enter a deeper cutoff state.

As the back EMF energy on L is consumed, the emitter potential of Q2 drops, and drops to the point where the transistor begins to conduct. Current flows into the coil, causing the voltage on the coil to reverse again, forming a collector potential drop, and is transmitted to the emitter through C5, causing Q2 to saturate and conduct, and the cycle begins to repeat again. This causes Q2 to oscillate and generate an 88MHz AC signal. The audio signal from the pre-amplified stage is fed into the base of Q2 through a 1μF capacitor, changing the oscillation frequency and generating the required FM signal.