Load switch for sound card. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Load switch for sound card. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Computers

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A computer-controlled stereo or multichannel sound card load relay switch allows you to select either headphones or speakers as load without switching cables and connectors.

At present, it has become fashionable to connect "home theater" speaker systems to a computer, however, they are distinguished from standard desktop computer speakers not only by an increased number of speakers and a stereo processor, but also by the absence (in most cases) of a headphone jack, which entails inconvenience when changing phones to speakers and vice versa. Simultaneously connecting them to one socket, as well as constantly switching plugs in a sound card, is not always convenient for various reasons. To facilitate the manipulation of speakers and headphones, this load switch was assembled. The device is a relay switch that connects the selected load to the output of the sound card. The device is located in the system unit and controlled via the COM port. The scheme of the device is shown in Fig.1.

Load switch for sound card. Switch Diagram
Fig. 1

The signal is switched using two electromagnetic relays K1 and K2. The signal goes to connector XS2 "Input" and, depending on the states of the relay, goes to connector XS1 "Output 1", designed to connect headphones, or XS3 "Output 2" - for speakers. In this design, it is of fundamental importance to which output the speakers are connected, since in the absence of a signal, it is better to connect the input of their amplifier to a common wire in order to avoid noise, this is provided for output 2. The voltage is supplied to the relay through the transistor VT1. The outputs of the COM port have the following voltage levels: log. 1 corresponds to a voltage of -5 ... -15 V, and the log. 0 - voltage +5 ... 15 V. The electronic key, assembled on the transistor VT1 and resistors R1 and R2, must be controlled by a voltage of 0 / + 5 V; to eliminate the voltage of negative polarity, the diode VD1 is included. Diode VD2 protects the transistor VT1 from self-induction EMF when the relay is turned off. I used 1N4007 diodes, but more popular ones, such as KD522B, are also suitable. The switch uses imported NHG type P-12 relays with two groups of switching contacts. During installation, two unused outer terminals are shortened. Instead of these relays, small-sized 12 V electromagnetic relays with two groups of switching contacts are suitable. From the diagram in Fig. 2 shows that the device is suitable for sound cards with STEREO output, and most modern sound cards have four or more channels. Therefore, it was decided to supplement the design with two more channels for rear speakers and a subwoofer. The diagram of the additional node is shown in fig. 2.

Load switch for sound card. Scheme of an additional node
Fig. 2

and printed circuit boards of the device - in fig. 3, 4, respectively.

Load switch for sound card. Printed circuit board
Fig. 3,4

The windings of the KZ and K4 relays must be connected in parallel to K1 and K2 (this connection can be made detachable), but it should be noted that the current limit through the KT315A transistor is 100 mA. If the total supply current of the relay exceeds 80 mA, a more powerful transistor should be used, for example, KT972B, but at the same time changing the resistance of the resistor R1 to 3,9 kOhm and R2 to 1,5 kOhm. If the device is assembled correctly, then adjustment is not required. However, you will need to install the software required to manage the device. Such a program consists of two files: the directly executable AUDIO Switch.EXE and the info file Config.bak; both of them should be in the water folder. If you open the last file with Notepad, you can find two lines in it; we will be interested in the second. By default, it contains text like "\\.\COM1"; this means that the device is connected to the COM1 port. To use another port, you need to change the contents of the string, for example, for COM2 it will look like this: "\\.\COM2". The program was written under Windows XP and was not tested under other operating systems. After launch, it immediately minimizes to tray, which saves shortcut space on the desktop, and when it is activated with the right mouse button, you can select the listening method - through headphones or speakers. A screenshot of the information panel is shown in fig. 5.

Load switch for sound card. Dashboard screenshot
Fig. 5

Download Switch Management Software

Author: N. Nistratov, Rostov-on-Don; Publication: cxem.net

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