TV sound infrared transponder circuit

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The transponder consists of transmitting and receiving parts（Click here to download the schematic diagram）. Figure 1 (a) shows the circuit principle of the transmission part. The accompanying sound (audio) signal after frequency discrimination is amplified by the triode VT to push the infrared emission tube. Since the emission intensity of the transmitting tube is proportional to the current passing through it, the infrared light emitted by VD1 and VD2 is modulated by the audio signal. To prevent distortion, VD1 and VD2 should be set with a certain offset. Fig. 1 (b) is a schematic diagram of a reception section. The circuit adopts an audio amplification integrated circuit LM386. VD is an infrared receiver tube. When the infrared light modulated by the audio signal shines on the VD, an electrical signal with the same change rule as the audio signal is generated at both ends of the VD, which is coupled to the IC through C1 for amplification. Since IC has power amplification function, it can be heard by 1-4 headphones at the same time.
　　Selection and manufacture of components
The triode VT selects 8050 medium power transistor, P_CM=300mW, I_CM=500mA, R2 power is not less than 1/4W, VD is an infrared receiver tube (photodiode cannot be used, otherwise the reception effect will be affected by visible light interference).
The debugging of this transponder is very simple. The transmitting part only needs to adjust R1 so that the static current of VT is about 30mA. The receiving part can work without debugging as long as the installation is correct.
The transmitting part can be installed inside the TV and powered by 12V power supply inside the machine. The signal input A and B ends are connected to both ends of the volume potentiometer. For the telephone with integrated circuit DC volume control, both ends of A and B can be connected to both ends of the speaker, but a switch should be added to turn off the speaker (it is more convenient to use the headphone output jack). Adjust the volume to maximize the forwarding distance without distortion.
When two emission tubes are installed, their radiation range should be considered so that their effective radiation angles overlap.