This TV sound transponder infrared transponder has the advantages of simple structure, easy production, no interference and low noise.
working principle
The transponder consists of two parts: transmitting and receiving. Figure 1 (a) is the principle of the transmitting part of the circuit. The audio (audio) signal after frequency discrimination is amplified by the triode VT and pushes the infrared emission tube. Since the emission intensity of the emission tube is proportional to the current passing through it, the infrared light emitted by VD1 and VD2 is modulated by the audio signal. In order to prevent distortion, VD1, VD2 must be set to a certain offset. Figure 1 (b) is a schematic diagram of the receiving part. The circuit uses an audio amplifier IC LM386. VD is infrared receiving tube. When the infrared light modulated by the audio signal is irradiated to the VD, an electrical signal with the same change rule as the audio signal is generated at both ends, and is coupled to the IC via C1 for amplification. Because the IC has power amplification function, it can be used for listening to 1-4 pairs of headphones at the same time.
Component selection and production Transistor VT uses 8050 medium power tube, P CM = 300mW, I CM = 500mA, R2 power is not less than 1 / 4W, VD is infrared receiver tube (photodiode cannot be used, otherwise the receiving effect will be affected by visible light interference ).
The commissioning of this transponder is very simple. The transmitting part only needs to adjust R1 so that the quiescent current of VT is about 30mA. As long as the receiving part is installed correctly, it can work without debugging.
The transmitting part can be installed inside the TV and powered by the 12V power supply inside. The signal input terminal A and B are connected to both ends of the volume potentiometer. For telephones that use integrated circuit DC volume control, you can connect both ends of A and B to both ends of the speaker, but you need to add a switch to turn off the speaker (using the headphone output jack is more convenient). Adjust the volume so that the forwarding distance is the farthest without distortion.
When the two launch tubes are installed, the radiation range should be considered, so that their effective radiation angles partially overlap.
The design sensitivity of this transponder is slightly lower, the purpose is to increase the anti-interference ability. After actual measurement, the effective forwarding distance is not less than 3m, which can fully meet the needs of ordinary families.

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