Automatic switch of TV sound transmitter

The main circuit of the whole machine is the common single-chip FM stereo broadcast transmitter integrated circuit BAl404. Its circuit is very mature. In order to facilitate beginners to make their own, the coil data is also introduced: L1 and L2 are both made of wire diameter 0.3mm. Enameled wire. Go around 5 turns. The inner diameter of the coil is 3mm and the length is 8mm. Coil wound according to this data. The transmitting frequency is approximately 1∞MHz. Within the reception range of ordinary FM radios. Other component data are shown in the figure. The automatic switch circuit part is in the upper left part of the picture. Its principle is 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 TV sets in my country is 15.625kHz. So compared with AC mains power. It is still relatively high, as long as this electromagnetic field can be detected. And turn on the power switch of the broadcast system. Even if the task is completed. In order to make the circuit simple without adding too much cost, the electronic switch is performed by a field effect transistor VT. Because the driving power required by the field effect transistor is extremely small. Basically do not drive current. The magnetic rod coil L absorbs the energy of the electromagnetic field generated around the TV when the TV is turned on. Rectified by D1 and D2 double voltage. 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 3V power supply supplies power to the broadcast system through the current limiting resistors R and VT. This system uses 1. 5V voltage, so the red light-emitting diode LED is used as a voltage regulator, which can also be used as a working indicator light. The selection of components: the capacitor C requires small leakage; the 12V voltage regulator is used to protect the field effect tube. The IOM resistor is for the broadcast system to automatically cut off the power after 10 seconds. If the resistor is not used, it will automatically cut off after a long time. This will only consume more power. The main component is the magnetic rod coil L. The magnetic rod can be a medium-wave magnetic rod with a diameter of 1 cm. Do not choose a short-wave magnetic rod. The most important thing is the number of turns of the coil. For TVs, the number of turns of the wire heater can be different. To ensure that it is successful at one time, it is better to wind it a few more times. You can use an enameled wire with a diameter of 0.1mm to wind around 1000-1500 times around the magnetic rod, and then use a soft coil. After welding the wires and wrapping them with tape, you can connect the whole circuit to test the whole device. Place the magnetic rod coil at an appropriate position around the TV. When the light-emitting diode LED emits light, the circuit will work normally. This position can be determined by The test determines that the strongest electromagnetic field is at the back and top of the TV. When the number of coils is sufficient, it can be placed anywhere around the TV. After the circuit is working properly, be sure to add a wire of about one meter. , then use the FM radio to search near 100MHz. The tuning indicator light will light up, indicating that a signal is sent. Connect the audio output terminal of the TV to any channel input terminal of this circuit and you can hear the TV broadcast on the FM radio. When I started the experiment, I used ordinary NPN and PNP transistors as electronic switches. The rectified voltage could not start the transistor circuit. Is the conduction voltage not enough? I disconnected the circuit and the open circuit voltage reached nearly 20V. After measuring the short-circuit current, I found that the short-circuit current was only about 30 microamps, which was difficult to drive an ordinary transistor. Later, I chose a field-effect transistor. The field-effect transistor used in this circuit could be used if I just had it on hand. Figure 2 is the internal block diagram of BAl404.