FM wireless headphone transponder

Individuals watching TV, sound will interfere with or influence others to study or rest, can be achieved using a single wireless headset to listen to. The working principle is to take first audio signal from the TV's headphone jack or audio output terminal out. Then connected to a wireless headset nearby dedicated "transponders" audio modulated high-frequency oscillation circuit; to the audio signal into a frequency range 88MHz ~ 108MHz frequency RF signals, shoots out a small antenna. Then by matching the receiver (or an ordinary FM radio) to receive, use headphones to listen to. This method eliminates the use of headphones plugged directly into the cable TV approach to answer the inconvenience and constraints. This also doubles as a wireless headset such as CD, MP3 and other audio wireless transponder, then the radio receiver, for people to listen. Following the Hua Xia card wireless headphones (some measured transponder circuit see photo below) as an example. Simple how it works, for reference.
    
The transponder circuit consists of audio input, high-frequency oscillation, the transmitting antenna and other components. First. Input signal from the input line of dual RCA plugs from the TV audio lotus output (red or white) to obtain: if taken from the TV headphone jack. Need to increase access "RCA/3.5mm" adapters. Then the signal into R2 (R3)-R4 appropriate attenuation and partial pressure match. R4 on the signal and the C1, L3-R5-C2, L2 transport links to Q1 composed of high-frequency oscillation circuit. Modulation / launch: why additional C1, L3-R5-C2-L2 and other components. The aim is to isolate the high-frequency signals. Q1 to prevent high-frequency signal by the addition part of the channeling.
  
Q1 high-frequency part of the total base capacitance for the three-point high-frequency oscillation circuit. This circuit start-up is easy, high frequency, waveform distortion. Ce between the pole tube through Q1 and Q1 capacitor C8 junction capacitance and the tube itself with C4 capacitors and other components. Can form a positive feedback network, resulting in sustained oscillations. Work in the oscillation, the C10-L1 element tank fixed-frequency, frequency stability can fall on a 88MHz ~ 108MHz frequency band, thus corresponding with FM radio receiver. And. Q1 oscillating tube in b-e pole on the audio signal to join. In this way, the audio voltage oscillation will be modulated on the Q1, could eventually form the oscillator Q1 c very strong carrier signal radiation voltage. Since then, the signal for the radiation to space, then Q1 c pole coupling capacitor C11 through the printed board by etching or low sense coil L4, dedicated by the transponder to complete a small telescopic antenna RF transmitter. This is the first to add a sense of the antenna after the launch mode. One is equivalent to L4 can be re-arranged on the radiation on RF antenna current distribution can be. Required to shorten the length of the antenna; another. Sense of the antenna plus antenna for high-frequency oscillator can also reduce the negative impact of reflections. So that the high-frequency oscillation circuit stability can be strengthened.
    
This headphone transponder antenna RF signal radiation range. About 8m or so. The operating current and the corresponding work with lower power dissipation. Even so, the battery may still occur due to long-term use and leakage of electrode corrosion, so the best selection of high-quality batteries. In addition. Sometimes encountered in receiving the noise increases, the signal is weak, and this common for the improper placement of transponders, transponders should be kept away from the metal to reduce its signal attenuation and absorption of radiation; general After adjustment, most of them can resume normal reception. If the adjusted invalid. Common oscillator signal is weak, should enhance the positive feedback.

FM无线耳机转发器电路图FM transmitter

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