Homemade FM wireless microphone for teaching

The wireless FM microphone made by the author is based on Q5337, plus a low-frequency amplifier and RF power amplifier circuit (which can improve the sensitivity of the microphone and the RF transmission power). This FM microphone has been used for classroom teaching in the 250m2 classroom of my unit for 6 years with good results.

The voice clarity of this microphone is high. Several measures are mainly taken: the signal output by the MIC is first sent to the BG1 tube for amplification, where R1 and C1 are additional high-frequency pre-emphasis circuits. C2 and C3 are the input and output coupling capacitors of the BG1 tube, and their values ​​are small, in order to attenuate the bass and enhance the mid-high frequency. The circuit of diodes D3 and D4 connected in reverse parallel at the output end of the BG1 tube and C4 and R7 uses the characteristic that the internal resistance of the diode decreases when it is forward-conducted to limit the strong signal, while the signal of normal strength is not affected. At the same time, it also has a good inhibitory effect on the howling caused by the positive feedback between the microphone and the speaker. After the microphone signal is amplified by BG1, it is added to the variable capacitance tube inside the IC through L5, and the high-frequency signal is FM modulated to obtain a larger frequency deviation. C7, C8, C9, and L1 form an FM signal tuning circuit, and its operating frequency is between 88MHz and 108MHz. The high-frequency signal outputted from the pin 1 of the IC is tuned and selected by L2 and C10, and then sent to C11 and coupled to the BG2 tube for RF amplification (BG2 can use a general ultra-high frequency tube), and then radiates the FM microphone signal into space. The whole machine is installed in the shell of a pocket semiconductor radio. The MIC is led out with an 80cm long single-core shielded soft wire, and this microphone lead also serves as a transmitting antenna. The high-frequency signal outputted by C13 is isolated from the ground by inductor L4 and connected to the outer layer of the shielded wire. The MIC is installed in a suitable latex tube, and then fixed to the latex tube with a tie clip. When using it, clip the microphone to the chest near the collar, and hang the machine on the belt to expand the microphone line, so that the transmission effect is best. L1, L2, and L3 are wound 5 times on a round rod with a diameter of 5mm with an enameled wire of about ∮0.5mm, and there is a tap on L2. L4, L5, and L6 can use ordinary small color-coded inductors. When debugging, first adjust the tightness of L1 so that the radio can receive the signal transmitted by the FM microphone in the FM band, and then adjust C16 to make the signal stronger. Finally, shorten the radio antenna and adjust L3 to maximize the transmission distance. If there is a simple field strength meter to cooperate with the debugging, the best effect can be achieved.

The frequency of this machine is stable, and it will not drift after being adjusted once for several months. When I use 4.5V power supply, the transmitting-receiving distance is within 25 meters without directionality. When I listen to it with an FM radio, it feels like an FM radio station.