Automatic switch of TV sound transmitter

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The main circuit of the whole machine is the common monolithic FM stereo broadcast transmission integrated circuit BAl404. Its circuit is very mature. To facilitate beginners to make their own, the coil data is also introduced: L1 and L2 are enamelled wires with wire diameter of 0.3mm. 5 turns. The inner diameter of the coil is 3mm and the length is 8mm. Coil wound according to this data. The transmission frequency is about 1 ∞ MHz. Within the reception range of ordinary FM radio. Other components
The data is shown in the figure.
The automatic switch circuit is at the top left of the figure,
The principle is that after the TV is turned on. A strong electromagnetic field is generated around the TV, which is mainly generated by the line scanning system of the TV's picture tube. The line scan frequency of TV is 15.625kHz in China. Therefore, compared with AC mains power supply, it is still high. As long as the electromagnetic field can be detected and the power switch of the relay system is turned on, the task will be completed.
In order to make the circuit gU simple without increasing too much cost, the electronic switch is operated by the field effect transistor VT. Because the driving power required by FET is very small. Do not drive current basically. The magnetic rod coil L absorbs the energy of the electromagnetic field around the TV set generated by turning on the TV set. After D1 and D2 voltage doubling rectification, a certain amount of electricity is accumulated on the capacitor C. When the voltage gradually increases to the opening voltage of the field effect tube VT (about 2V above) 1. The field effect tube is connected, the 3V power supply is through the current limiting resistor R, and VT supplies power to the relay system. The system uses 1.5V voltage, so the red light-emitting diode LED is used as the voltage stabilizer
Tube can also be used as a working indicator light.
Selection of components: capacitance C requires small leakage; 12V voltage stabilizing tube is used to prevent strong pulse incoming. To protect field effect transistor; The IOM resistor is designed to automatically cut off the power supply of the relay system 10 seconds after the TV is turned off. If the resistance is not used. It will be automatically cut off after a long time, which will consume more power.
The device to be made by this machine is magnetic rod coil L. The magnetic rod can be a medium wave magnetic rod with a diameter of 1cm on an ordinary radio. Do not choose the shortwave magnetic rod. The shortwave magnetic rod is lighter in color. The most important thing is the number of coils. For televisions of different sizes, the number of coils of wire heating can be different to ensure success. It is still necessary to wind several more circles. The enamelled wire with a diameter of 0.1mm can be wound on the magnetic rod by random winding method for 1000-1500 circles
Right. Then use flexible wire to weld the wire ends, and then wrap them with adhesive tape to connect the whole machine circuit to test the whole device.
Place the magnetic rod coil at an appropriate position around the TV. When the LED lights up, the circuit will work normally. This position can be determined by the test. The position with the strongest electromagnetic field is about the upper back of the TV. When there are enough coils, they can be placed anywhere around the TV. After the circuit works normally, pay attention to add about one meter of wires as an antenna, and then use FM radio to search near 100MHz. The tuning indicator lights up, indicating that there is a signal sent. Connect the audio output terminal of the TV to any channel input terminal of the circuit, and you can hear the broadcast of the TV in the FM receiver.
When I started the test, I used ordinary NPN and PNP triodes as electronic switches. The rectified voltage could not make the triode circuit start. Is the conduction voltage insufficient? I disconnected the circuit. The open circuit voltage can reach nearly 20V. After measuring the short circuit current, it was known that the short circuit current is only about 30 μ A, which is difficult to push the ordinary triode. Later, we chose FET. The FET used in this circuit is available at hand. Other similar models can be used. Figure 2 shows the internal block diagram of BAl404.