Automatic signal switch. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television, video equipment

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A significant portion of inexpensive models of televisions and computer monitors in use do not have the number of inputs required in modern conditions for connecting external signal sources, and outdated television receivers are mostly equipped with only one composite input for supplying a video signal. This creates inconvenience if you need to connect several video playback devices (video disc player, satellite tuner, game console, mobile player or video camera) to a TV or computer monitor equipped with one video input and a PAL/NTSC decoder. Frequent switching of connectors ultimately leads to their failure.

To significantly reduce these troubles, it is recommended to assemble a relatively simple automatic signal switch, which will simplify the procedure for using numerous video equipment and increase the reliability of its operation.

Rice. 1 (click to enlarge)

The schematic diagram of the device is shown in Fig. 1. It is an electronic signal switch with two video and audio inputs. Its outputs are connected respectively to the composite video input of the TV (monitor) and the audio input, but you can also connect to the SCART connector via an adapter. The device is built on NJM2233BS integrated circuits with two inputs, selectively switched to one output. The maximum operating frequency of the microcircuits reaches 10 MHz.

The DA1 chip contains an audio signal switch. When there is a low voltage level at its control input SW1, the OUT output of the microcircuit receives the signal coming through the Audio 1 input to its IN1 input. At a high level at the control voltage input SW1, a signal from the “Audio 2” input will pass to the output of the switch. Capacitors C2 and C3 prevent radio frequencies from penetrating both inputs of the microcircuit, including video signals if they are mistakenly supplied to the audio inputs of the switch.

The video signal switch implemented on the DA2 chip works similarly. The video signal "Video 1" passes to the output when the voltage level at the control input is low, and the video signal "Video 2" - when the voltage level is high. It should be borne in mind that if there is no video signal "Video 1", a high level at the control input is always present. This will be discussed in more detail later.

Resistors R1 and R2 ensure matching of the input impedance of the switch with the output impedance of the video signal sources. Zener diodes VD1-VD4 and diodes VD7-VD10 protect the inputs and outputs of microcircuits from high voltage pulses that can occur when connecting signal cables to devices operating on AC power. Typically, the difference voltage between the common wires of two “ungrounded” devices with switching power supplies does not exceed 600 V, but sometimes due to the peculiarities of the circuit design, for example, in the absence of discharge resistors between the primary and secondary circuits of the switching power supply, this voltage can reach 10 kV or more . A typical indicator sign of this is the spark jumping between the pin and socket parts of the connector as they approach.

The supplied pairs of (audio and video) signals can be switched automatically. If the video signal is supplied only to the “Video 2” input, then both switches pass to the outputs the signals arriving at the IN2 inputs of the DA1 and DA2 microcircuits. This happens because, as mentioned above, a high level voltage (about 5 V) operates at the control inputs of both microcircuits. If a video signal is also supplied to the “Video 1” input, then regardless of whether there is a video signal at the “Video 2” input or not, both microcircuits switch and pass the signals arriving at their IN1 inputs.

Switching of switches is controlled by a unit assembled on transistors VT1 and VT2. The video signal arriving at the “Video 1” input passes through the circuit R12C1 to the base of the transistor VT1, is amplified and, through the isolation capacitor C8, goes to a rectifier made on diodes VD5 and VD6. Rectified voltage ripples are smoothed out by capacitor C9. The constant voltage from it acts through resistor R17 on the emitter junction of transistor VT2, opening it. The voltage between the collector and emitter of transistor VT2 and, consequently, the voltage at pins 7 of microcircuits DA1 and DA2 becomes close to zero. As a result, both microcircuits switch to switching signals arriving at their IN1 inputs.

Therefore, the Video 1 input has priority over the Video 2 input. If, if there is a video signal at the “Video 1” input, you need to switch the device to switching signals from the second inputs, you can use the SB1 button with a lock. When its contacts are closed, signals from its second inputs will pass to the output of the switch.

With this operating logic of the automatic switch, it is advisable to connect the most frequently used signal source, such as a DVD player, to its second inputs, and connect a less frequently used device that will have priority, such as a video game console, to the first inputs.

The switch nodes are powered by a parametric stabilizer assembled on transistors VT3 and VT4. Field-effect transistor VT4 operates as a source of stable current for the zener diode VD11. The VD12 diode protects the device from incorrect connection of the supply voltage. If such a situation is excluded in the finished switch, the diode does not need to be installed. Although the supply voltage of microcircuits can be from 5 to 13 V, the value chosen for them is 10,5 V. This is due to the fact that at a voltage of less than 9 V, deterioration in the quality of synchronization can be observed in old domestic TVs, in which the scan synchronization device is assembled on a microcircuit K174ХА11.

Both device microcircuits are powered by a stable voltage through filters L1C10C12 and L2C11C13, which prevents the appearance of moire and other similar defects in the image. LED HL1 signals the presence of supply voltage. The device consumes 30 mA current when the input supply voltage is 12 V. When using the switch, no deterioration in video signal quality was noticed.

Fig. 2

All parts of the device, except for the SB1 button, all “Tulip” type sockets and the HL1 LED, are mounted on a mounting plate measuring 78x65 mm in a hinged manner. Their referenceThe exact location can be seen in Fig. 2. The reverse side of the board is not shown, since most of the connections are made with the leads of the parts themselves, and the necessary jumpers are made with MGTF-0,03 wire. The common wire (negative power wire) of the switch is made on the board in the form of a crossed out letter Ш with a stranded mounting wire with a diameter of 1 mm.

The resistors in the device can be any small ones. Non-polar capacitors - K10-50 or analogues. In the switch assembled by the author, resistors R16 and R17, as well as ceramic capacitors C10, C11 and C18, are used in a surface-mount design. Oxide capacitors - K50-35, K53-19, K53-30, etc. Chokes L1 and L2 can be installed with any small-sized, low-power ones with an inductance in the range of 10...100 μH. The author used surface mount chokes from an old computer hard drive. All these elements are located on the reverse side of the board.

Diodes 1N914 (VD5-VD10) can be replaced with 1N4148 or KD503, KD521, KD522 with any letter index, and MUR120 (VD12) with any of series 1 N4001 -1N4007, KD105, KD208, KD243. Instead of the BZV55C-6V2 (VD1-VD4) zener diodes, you can use any of the 1N4735A, 1N4734A, BZV55C-5V1, TZMC-6V2, and instead of the BZV55C-11 (VD11) zener diode - any of the TZMC-11, 1N4741A, KS211Zh.

Transistors STS9014 (VT1, VT2) are interchangeable with BC547 or SS9014, as well as KT3102, KT6111 with any letter indices, and transistor 2SD2058 (VT3) with any of KT817, KT819, KT863. Instead of the KP303V (VT4) field-effect transistor, any of the KP303, 2P303, KP307 series with an initial drain current of 2.3 mA is suitable. When replacing, you should take into account the differences in the pinouts of the mentioned series of transistors.

The NJM2233BS chips are housed in a SIP-9 package. They can be easily replaced with similar NJM2233BL in a SIP-8 package with a similar pinout. NJM2233BX microcircuits in packages with double-row pinouts have a significantly different pinout.

The switch's power source can be any unit with a constant output voltage of 12.24 V, capable of providing an output current of at least 30 mA. For example, any antenna amplifier unit will do if you replace the 78L12 stabilizer with a more powerful one and increase the capacitance of the rectified voltage filter capacitor. When embedding the switch into a TV or monitor, you can use a suitable voltage source available there, for example, a 12...18 V voltage source usually operating in light mode, designed to power the UMZCH or LCD screen backlight lamps.

When the device is powered from the same source with a constant output voltage, a resistor can be installed instead of a field-effect transistor (VT4). Its resistance is chosen so that the current through the zener diode VD11 is in the range of 2.3 mA. In this case, capacitor C19 must have a capacity in the range of 47...100 µF. The stabilizer can be assembled on any suitable chip in a TO-220 package, for example LM78M12, and the minimum input supply voltage must be at least 15 V.

Fig. 3

The switch design layout is shown in Fig. 3. On the top cover of the case there are input (left) and output sockets, an SB1 button for switching input signals, sockets for supplying voltage and an HL1 LED.

The flawlessly assembled switch starts working immediately and does not require setup.

This device is convenient to use not only for automatically switching signals from two different sources, but also as a kind of “buffer”, for example, during repairs when connecting a DVD player and a TV, when one of the devices is faulty. This will reduce the likelihood of damage to a working device. Although the switch contains components for protecting integrated circuits, similar protective components may not be present in switched devices. Therefore, it is advisable to connect various equipment even through the described switch while completely disconnecting all involved devices from the electrical network.

If switching of multi-channel audio signals is necessary, then you need to install the appropriate number of nodes assembled on NJM2233BX chips, turning them on in the same way as the DA1 chip of the device under consideration. In this case, it may be necessary to install transistor VT3 on a small heat sink.

Author: A. Butov

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