The Making and Experience of Wireless Headphones

I would like to share with you my experience of making and using several wireless headphones for your reference.

Solution 1 is an FM wireless transmitter made as shown in Figure 1. The total current of the whole machine including the light-emitting diode is only 5 mA. It can be stably received within a range of 20 meters using any FM radio and is not affected by wall obstruction. Adjusting the transistor emitter resistor can improve modulation distortion, and adjusting the fine-tuning capacitor to make the signal emitted by the transmitter within 87-108 MHz, while avoiding the local FM station and not interfering with TV programs. The signal source is taken out from the audio output jack of the TV or DVD or VCD with a lotus plug. At first, I listened to it with an ordinary head-mounted hand-tuned AM/FM radio, and the effect was very good. Later, I switched to a Sony SRF-MH22 head-mounted digital storage stereo radio, which is extremely convenient to use, and the sound quality is greatly improved, without background noise.

　　Solution 1 cannot receive stereo signals. According to the introduction of "Electronic News", an FM stereo transmitter was made with BA1404. The total current of the transmitter is only 4 mA when powered by 1.5V, and it can receive stably within a range of 10 meters. It was made according to the regular circuit diagram when it was first made. After replacing four BA1404s in succession, it still could not work properly. After careful analysis of its circuit, it is estimated that the RF oscillation circuit is short-circuited to the ground by the 15pF capacitor at the 9th foot because there is no power amplifier stage. The circuit can work normally only after disconnecting this capacitor, as shown in Figure 2. This should be paid attention to when making it. The receiver still uses Sony SRF-MH22 for reception, with obvious mono effect and good sound quality. However, when receiving stereo signals (that is, the receiver stereo is turned on), obvious background noise appears. I don’t know whether it is caused by the 19kHz pilot signal of the transmitter or the receiver. At the same time, the stereo signal separation of this machine is not thorough, and the use effect is general.

　　In order to receive stereo sound with high quality, I bought a set of Blue House brand infrared wireless stereo headphones. After analysis, the structure of this machine is similar to the IR900 infrared stereo headphones introduced in the 7th issue of "Electronics News" this year. The left and right channels of the transmitter use a CD4046 to modulate the signal and then transmit it through two infrared diodes. The left and right channels of the receiver are respectively received by infrared, amplified by transistors, and selected by frequency. After demodulation and amplification, they are passed through the chip integrated block A007B and drive the headphones. The diameter of the headphone speaker is 25 mm. After listening, its stereo separation is very good, the sound is clear, and the sound quality is very good. The disadvantage is that this machine uses 2 No. 7 batteries for power supply, and the battery consumption is too fast.

　　After using it, I think that if the main purpose is to receive mono signals, the scheme in Figure 1 can fully meet the requirements. If the main purpose is to receive stereo signals, an infrared wireless stereo system should be used for listening, and the FM stereo system is best not to be used because of its poor signal-to-noise ratio and insufficient stereo separation. Of course, using two mono FM transmitters to transmit left and right channel signals respectively, and then using two FM receivers to receive them respectively is also a good method.