FM wireless headphone circuit

With the emergence of new TVs one after another, equipped with wireless headphones has brought new conveniences to users. In fact, it is not difficult for radio enthusiasts to own a pair of wireless headphones. They can do so by making their own. Here, the author introduces to you an FM wireless headset for those who are interested in experimenting with it. Figure 1 is the transmitting circuit of the wireless headset. The audio signal obtained from the TV headphone jack through the stereo headphone plug pp is amplified by BG1. The limiting circuit composed of D1, D2, C2 and C3 limits the amplitude and then sends it to BG2 and other components. A high-frequency oscillator, the modulated FM signal is coupled to the antenna through C7 and transmitted outward. Figure 2 shows the receiving circuit, with the dedicated FM receiving module IC1 TDA7021T as the core. The demodulated audio signal is output from the pin and sent to the IC2 LM386 for power amplification to promote the sound of the headphones. Transistor BG 9018 and other components form a high-frequency amplification stage to improve receiving sensitivity. Adjusting the adjustable capacitor C23 can change the receiving frequency so that it is aligned with the transmitter's transmitting frequency. Component Selection Since the circuit works at high frequency, all non-polar capacitors are high-frequency ceramic capacitors. The coils L1 and L2 are made of ∮1mm silver-plated wire wound around 5T on a ∮4mm round rod. The transmitting antenna is made of long A 10cm thick copper wire is erected on the transmitter, and the receiving antenna can be wound around the earphone strap ring with a suitable length of gel wire. There are no special requirements for other components. Circuit debugging Circuit debugging is very simple. Adjust the collector current of transistors BG1 and BG2 in the transmitting circuit to 0.5mA, and adjust the collector current of BG in the receiving circuit to 0.3mA to make it work normally. Then adjust C23 to align the frequencies of the transmitter and receiver and put it into use. One thing to note is that if the sound heard in the headphones is distorted, it means that the input signal of the transmitter is too large, and the volume of the TV should be turned down.