FM transmitter in a matchbox

Almost every electronics enthusiast has the ambition to use radio, whether to remotely control an airplane or to communicate with the outside world, they all express the desired signals they transmit.

Here I introduce a pocket transmitter, which is very suitable for beginners. The circuit is simple and easy to make, the cost is low, the output power does not exceed 5-8mW, and the transmission range can reach about 300 meters in the residential area. It can be received by an ordinary FM radio, showing that its sensitivity and clarity are excellent. The most challenging part of the circuit design is that it only needs a 3V power supply and a half-wave antenna to have such a transmission capability. In addition, since the circuit requires very few parts, it can be placed in a matchbox (larger than the ordinary matchbox in China), and can be placed in the baby room, gate or corridor to monitor the actual situation. In addition, it can also be used as a night security device.

The current consumption of the circuit is less than 5mA, and two dry batteries can provide continuous operation for 80 to 100 hours.

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

Through this production, you can learn about FM transmission and understand its advantages, especially that it produces a very high quality signal without noise, and it is easy to achieve a good range even when using low power transmission.

How the Circuit Works

As can be seen from the circuit in Figure (1), it is divided into two stages, an audio amplifier and an RF oscillator.

There is actually a FET hidden inside the electret microphone. If you like, you can think of it as a stage. The FET amplifies the change in capacitance of the diaphragm in front of the microphone. This is why the electret microphone is very sensitive.

The audio amplifier stage is performed by the emitter transistor Q1, with a gain of about 20 to 50, and sends the amplified signal to the base of the oscillator stage.

The oscillator stage Q2 operates at a frequency of about 88MHz, which is adjusted by the oscillator coil (5 turns in total) and the 47pF capacitor. The frequency is also determined by the transistor, the 18pF feedback capacitor and a few bias components, such as the 470Ω emitter resistor and the 22K base resistor.

When the power is turned on, the 1nF base capacitor is gradually charged through the 22K resistor, while the 18pF is charged through the 470Ω resistor of the oscillation coil, but faster. The 47pF capacitor is also charged (although only a small voltage is obtained at its two ends), and the coil generates a magnetic field.

As the base voltage gradually rises, the transistor turns on and effectively connects the internal resistance to the 18pF. When the 1nF capacitor is charged to the operating voltage of the pole, several chaotic cycles will occur, so we assume that the discussion is close to the operating voltage.

The base voltage continues to rise, and the 18nF capacitor tries to prevent the emitter from moving. When the energy in the capacitor is exhausted and can no longer prevent the emitter from moving, the base-emitter voltage decreases, the transistor is cut off, the current flowing into the coil stops, and the magnetic field collapses.

The magnetic field collapses, generating a voltage in the opposite direction. The collector voltage rises from the original 2.9V to more than 3V, and charges the 47pF capacitor in the opposite direction. This voltage also affects the charging of the 18pF capacitor and the voltage drop on the 470Ω emitter resistor, causing the transistor to enter a deeper cutoff.

When the 18pF capacitor is charged, the emitter voltage drops to a certain level, at which point the transistor begins to conduct, and current flows into the coil, counteracting the decaying magnetic field.

The voltage on the coil reverses, causing the collector voltage to drop. This change is transmitted to the emitter through the 18pF capacitor. As a result, the transistor enters a deeper conduction, short-circuiting the 18pF capacitor, and the cycle begins to repeat.

Therefore, Q2 forms an oscillation here, generating an 88MHz AC signal. The amplified audio signal is fed into the base of Q2 through a 0.1uF capacitor, changing the oscillation frequency and generating the required FM signal.