DIY a simple FM wireless microphone

1. Principle of wireless microphone

Electronic microphones first convert various sound signals into audio electrical signals, and then this electrical signal modulates the high-frequency signal generated by the electronic oscillator. Finally, the high-frequency signal is transmitted into the air through the antenna. If the transmitting frequency is designed to be in the FM radio band, then it can be used with any FM radio to receive the high-frequency signal and restore the sound signal from the high-frequency signal, thereby completing various uses.

2.Circuit diagram

The attached figure is a circuit diagram of an FM wireless microphone. The high-frequency transistor Vl and capacitors C3, C5, and C6 form a capacitive three-point high-frequency oscillator. The loads C4 and L of the triode collector form a resonator, and the resonant frequency is the transmit frequency of the FM microphone. According to the parameters of the component in the figure, its transmitting frequency can be between 88 and 108MHz, which just covers the receiving frequency of FM radio. By adjusting the value of L (coil L), the transmitting frequency can be easily changed to avoid FM radio stations. The transmitted signal is coupled to the antenna through C4 and then transmitted out.

R4 is the base bias resistor of V1, which provides a certain base current to the triode so that Vl works in the amplification area. R5 is the DC feedback resistor, which stabilizes the operating point of the triode. The frequency modulation principle of this FM microphone is to achieve the purpose of frequency modulation by changing the capacitance between the base and emitter of the triode. When a sound voltage signal is added to the substrate of the triode, the capacitance between the base and the emitter of the triode will change synchronously with the size of the sound voltage signal, and at the same time, the emission frequency of the triode will change, achieving frequency modulation.

The microphone MIC can collect external sound signals. Here is an electret microphone, which has very high sensitivity and can collect weak sounds. At the same time, this kind of microphone must have a DC bias to work, and the resistor R3 can provide a certain DC bias. , the greater the resistance of R3, the weaker the sensitivity of the microphone in collecting sound. The smaller the resistance, the higher the sensitivity of the microphone. The AC sound signal collected by the microphone is coupled and matched by R2 and then sent to the base of the transistor. The two diodes Dl and D2 in the circuit are connected in reverse parallel and mainly function as a bidirectional limiting function. , the conduction voltage of the diode is only 0.7V. If the signal voltage exceeds 0.7V, it will be shunted by the diode conduction. This ensures that the amplitude of the sound signal can be limited between plus and minus 0.7V. An excessively strong sound signal will cause the transistor to overshoot. Modulation, producing sound distortion or even not working properly.

CK is an external signal input socket. External sound signal sources such as TV headphone sockets or Walkman headphone sockets can be introduced into the FM transmitter through a dedicated connecting cable. The external sound signal is sent to the FM transmitter through R1 attenuation and Dl and D2 limiting. The base of the triode performs frequency modulation. So this kit can not only be used as a wireless microphone, but also can be used as a wireless headset for a TV.

The light-emitting diode D3 in the circuit is used to indicate the working status. It will light up when the FM microphone is powered on. R6 is the current-limiting resistor of the light-emitting diode. C8 and C9 are power supply filter capacitors. Because large capacitors are generally made using the winding process, the equivalent inductance is relatively large. Connecting a small capacitor C8 in parallel can reduce the high-frequency internal resistance of the power supply. This circuit is very common.

Kl and K2 in the circuit are switches, which have three different positions (not shown here, only explained). When turned to the far left, the power is disconnected. On the far right, Kl and K2 are connected for use as FM microphones. In the middle The position is Kl connected, K2 disconnected, used as a wireless repeater.