BH1404 Automatic switch TV audio repeater (FM band)

BH1404 Automatic switch TV audio repeater

The main circuit of the whole machine is the common single-chip FM stereo broadcast transmitter integrated circuit BA1404. Its circuit is very mature. To facilitate beginners to make it by themselves, the coil data is also introduced: L1 and L2 are both made of enameled wire with a wire diameter of 0.3mm, wound 5 turns, the inner diameter of the coil is 3mm, and the length is 8mm. The transmission frequency of the coil wound according to this data is about 100MHz. It is within the reception range of ordinary FM radios. The data of other components are shown in the figure.

The automatic switching circuit part is in the upper left part of the picture. Its principle is that after the TV is turned on, a strong electromagnetic field is generated around the TV. This electromagnetic field is mainly generated by the line scanning system of the TV's picture tube. The line scanning frequency of the TV in my country is 15.625kHz, so it is still higher than that of AC mains. As long as this electromagnetic field can be detected and the power switch of the broadcasting system is turned on, the mission is completed.

In order to make the circuit as simple as possible without increasing the cost too much, the electronic switch is performed by the field effect tube VT. Because the driving power required by the field effect tube is extremely small, basically no driving current is required. The energy of the electromagnetic field generated by turning on the TV is absorbed by the magnetic bar coil L, and is rectified by D1 and D2, and a certain amount of electricity is accumulated on the capacitor C. When the voltage gradually increases to the turn-on voltage of the field effect tube VT (about 2V or more), the field effect tube is turned on, and the ?V power supply supplies power to the broadcasting system through the current limiting resistors R and VT. This system uses a 1.5V voltage, so a red light-emitting diode LED is used as a voltage regulator, which can also serve as a working indicator.

Selection of components: Capacitor C requires low leakage; the 12V voltage regulator is used to prevent strong pulse calls and protect the field effect tube; the 10M resistor is designed to ensure that the broadcast system can automatically cut off the power supply after 10 seconds after the TV is turned off. If this resistor is not used, it will automatically cut off the power supply after a considerable time, which will waste more electricity.

The only part that needs to be made by yourself is the magnetic bar coil L. The magnetic bar can be a medium wave magnetic bar with a diameter of 1 cm on an ordinary radio. Do not choose a short wave magnetic bar, which is lighter in color. The most important thing is the number of turns of the coil. The number of turns of the coil can be different for different sizes of TV sets. To ensure success at one time, it is better to wind more turns. You can use a 0.1mm diameter enameled wire to wind about 1000-1500 turns on the magnetic bar in a random winding method, then weld the wire ends with a soft wire, and then wrap it with tape, and then you can connect the whole machine circuit to test the whole device.

Place the magnetic bar coil at an appropriate position around the TV. When the LED is on, the circuit is working properly. This position can be determined by experiment. The position with the strongest electromagnetic field is approximately above and behind the TV. When the number of turns of the coil is sufficient, it can be placed anywhere around the TV. After the circuit is working properly, pay attention to add a wire of about one meter as an antenna, and then use an FM radio to search around 100MHz. The tuning indicator light indicates that a signal is sent. Connect the audio output of the TV to any channel input of this circuit, and you can hear the TV broadcast on the FM radio.