How to make a simple wireless headset

The reason why I introduce this circuit to you is that it not only reflects the foundation of analog electronics, but also reflects the typical high-frequency circuit. The frequency is high, so it can exercise everyone's hands-on ability! "If the circuit is strictly calculated and tested, the general assembly success rate is above 99%." Why not try to make one yourself (see the figure below), you will definitely feel a sense of accomplishment, and the effect is indeed quite good. It is named "wireless headset" because of its high sensitivity. When it is quiet, even the "tick-tock" sound of the watch can be heard. This circuit is actually an FM wireless microphone. The circuit is divided into two parts: one part is an audio amplifier, and the other part is a high-frequency oscillator. The components are very easy to buy. The following is an introduction to the production method of this wireless headset.

The audio amplifier is composed of VT1, etc. The amplified audio signal is coupled to the base of the high-frequency oscillation circuit VT2 through C2 (0.01uF). The operating frequency of the high-frequency oscillation circuit VT2 is 88-108MHz, which is determined by the oscillation coil L1 and the capacitor C4 (47P). It is modulated by the audio signal of VT1. When the audio signal is coupled to the base of VT2 through C2, the oscillator frequency is changed to generate the required FM (frequency modulation) signal, which is transmitted to the air through the antenna.

When making it, the microphone MIC uses a high-sensitivity electret microphone, and the shell is connected to the negative pole. VT1 model is 9013H, the magnification is greater than 150, VT2 model is 3DG130D, blue dot (magnification is greater than 120), ft is greater than 600MHz, and 9018 can be used here as a substitute. L1 oscillation coil (see the right picture) uses Φ0.71 enameled wire to wrap 4 turns on the round handle of the 3mm drill bit, and then it is cast. The antenna can take a 1.2m soft wire, and the transmission distance can be about 200m. If the transmission distance is not required to be very far, then the antenna can use a 10cm wire at one end, and the transmission distance can be about 30m. I only experimented with the latter method, which can achieve the expected effect, but the stability of the antenna has a great influence on the stability of the frequency. After the experiment, it is better to use a thin hard copper wire directly soldered on the circuit board to make an antenna. When working normally, the circuit current is about 5mA. When the power supply voltage is different, the current of the whole machine will be different.

During debugging, use an FM radio to receive. Pull away about 5-10m, and you can usually receive the sound from the transmitter. You can use a mechanical watch close to the microphone for testing. If you can't receive the signal, you can slightly adjust the distance of L1, or reduce the capacity of the 47P capacitor. Adjusting R1 can adjust the sensitivity of the microphone.

How is it? Are you happy? If you increase the power, you can also make a small FM transmitter.