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TV sound Automatic switch the transmitter

Published on Dec 20 2010 // Wireless Transmitter Circuits

TV sound automatically switch the transmitter 1

The main circuit is a common machine-chip FM stereo radio transmitter IC BAl404, the circuit has been very mature for beginners made easy, the coil data also make an introduction: L1, L2 are 0.3mm in diameter with enameled wire. Around 5 turns. Coil inner diameter 3mm, length of 8mm. Click here to coil winding data. Firing frequency of approximately 1 8 MHz. FM radio reception in the general context. Other components

The data shown.

Automatic switch circuit diagram of the upper left part,

The principle is that the TV turned on. Around the TV had a strong electromagnetic field, the electromagnetic fields mainly by the TV picture tube line scanning system produces. TV line scan frequency in China is 15.625kHz. Therefore, compared with the AC mains. Or higher, as long as able to detect the electromagnetic field. And open the broadcast system, the power switch. Even if the job done.

To make the circuit, gU-simple without adding too much cost, electronic switch is a FET as VT. Because FET drive power required is minimal. Basic Do not drive current. Absorbed by the magnet coil L produced the TV Open TV electromagnetic energy around. The D1, D2 doubler rectifier. Accumulated in the capacitor C will charge, when the voltage is gradually increased to FET threshold voltage VT (about 2V above 1. FET turn-on, 3V power supply through current limiting resistor R, VT power supply for the broadcast system. The system uses a .5 V voltage, so the red light-emitting diode LED as a regulator

Tube, but also doubles as a work light.

Selection of components: a small capacitance C required leakage; 12V regulator is to prevent the adoption of strong pulse calls. FET protection designed; IOM resistance is to switch off the TV after the broadcast system can automatically cut off power after 10 seconds. If not the resistance. Would automatically after a long time off. This will put to more power.

This machine is the only device to be self-made bar magnet coil L, can use an ordinary bar magnet on the radio wave diameter of 1 cm magnetic bar. Do not select short bar magnet. Short bar magnet lighter. The key is the coil number of turns. TV on the different sizes, the line can be different number of turns warm, to guarantee a success. Or more around a few times, available in a diameter of 0.1mm magnet wire around a bar magnet on the Law of around 1000-1500 with the mess left circle

Right. Then with a soft wire welding thread, and then wrap tape paper, you can test the device connected to the whole circuit.

Bar magnet coils around the TV to the appropriate location on, LED light emitting diode, the circuit to work properly, and the position determined by the test. The location of the strongest electromagnetic field around the top of the TV after. When the windings enough, on TV can be anywhere around. Circuit is working properly. Note that the wires together for about a meter antenna, FM radio and then search around in the 100MHz, tuning indicator light, indicating a signal is issued, the TV’s audio output will be terminated in any one channel of this circuit can be input In the FM radio broadcast was heard on television.

I started doing tests, using ordinary NPN and PNP type transistor to do electronic switch, the rectified voltage of the transistor circuit is not activated. Turn-on voltage is not enough? I disconnect the circuit. Open circuit voltage up to near 20V, I discovered after testing a short-circuit current, short circuit current is only about 30 microamps, it is difficult to promote the common transistor. Later selected MOSFET. The FET circuit is used to happen to some. Other similar models can be used. Figure 2 shows the internal block diagram BAl404.