Automatic switch circuit diagram of TV sound transmitter

The main circuit of the whole machine is the common single-chip FM stereo broadcast transmitter integrated circuit BAl404. The circuit is very mature. In order to facilitate beginners to make their own, the coil data is also introduced: L1 and L2 are both enameled wires with a wire diameter of 0.3mm. . 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 and can also be used as a work indicator light. Selection of components: The capacitor C requires small leakage; the 12V voltage regulator tube is used to prevent strong pulse incoming electricity. It is designed to protect the field effect transistor; the IOM resistor is so that after the TV is turned off, the broadcast system can automatically cut off the power supply after 10 seconds. If this resistor is not used. It will automatically cut off after a long period of time. This will consume more electricity.

The only self-made component of this machine is the magnetic rod coil L. The magnetic rod can be a medium wave magnetic rod with a diameter of 1 cm from an ordinary radio. Don't choose short wave magnets. Shortwave rod magnets are lighter in color. The most critical thing is the number of turns of the coil. For TVs of different sizes, the number of turns of the wire heater can be different to ensure first-time success. It is better to wind it a few more times. You can use enameled wire with a diameter of 0.1mm to wind around 1000-1500 times around the magnetic rod by random winding method. Then weld the wire ends with soft wires, wrap them with tape, and then connect the whole circuit to test the entire 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 testing. The strongest electromagnetic field is around the back and top of the TV. When the coil has enough turns, it can be placed anywhere around the TV. After the circuit works normally. Pay attention to adding about one meter of wire as an antenna, and then use the FM radio to search near 100MHz. The tuning indicator light will light up, indicating that a signal is sent. Just connect the audio output terminal of the TV to any channel input terminal of this circuit. The TV was heard on the FM radio.

At the beginning of the experiment, ordinary NPN and PNP transistors were used as electronic switches. The rectified voltage could not start the transistor circuit. Is the conduction voltage insufficient? The circuit is disconnected. The open-circuit voltage can reach nearly 20V. After measuring the short-circuit current, we found that the short-circuit current is only about 30 microamps, which is difficult to drive an ordinary triode. Later I chose field effect tubes. The field effect transistor used in this circuit happens to be available. Other similar models are available. The picture below shows the internal block diagram of BAl404.

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