TV audio infrared transponder circuit

This TV audio 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) shows the circuit principle of the transmitter part. The frequency-identified accompanying sound (audio) signal is amplified by the transistor VT and then drives the infrared transmitter 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 and VD2 must be set to a certain offset. Figure 1(b) is the schematic diagram of the receiving part. Its circuit uses an audio amplifier integrated circuit LM386. VD is an infrared receiving tube. When the infrared light modulated by the audio signal irradiates VD, an electrical signal with the same variation pattern as the audio signal is generated at both ends of the VD, and is coupled to the IC via C1 for amplification. Since the IC has a power amplification function, it can be used for listening to 1-4 pairs of headphones at the same time. Component selection and production The transistor VT uses an 8050 medium power tube, PCM = 300mW, ICM = 500mA, the power of R2 is not less than 1/4W, and VD is an infrared receiving tube (photodiodes cannot be used, otherwise the reception effect will be affected by visible light interference). Debugging of this transponder is very simple. The transmitting part only needs to adjust R1 to make the quiescent current of VT around 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 built-in 12V power supply. Signal input terminals A and B are connected to both ends of the volume potentiometer. For telephones that use integrated circuit DC volume control, you can connect terminals A and B to both ends of the speaker, but you need to add a switch to turn off the speaker (it is more convenient to use the headphone output jack). Adjust the volume so that it forwards the farthest possible distance without distortion. When installing two launch tubes, their radiation ranges must be considered so that their effective radiation angles partially overlap.