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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Microprocessor-based audio switch
Encyclopedia of radio electronics and electrical engineering / Audio equipment Using the switch, you can easily connect sound sources to various sound reproducing devices in any combination. The number of household sound reproducing equipment is increasing in our apartments every year. A new music center, home theater or computer speaker leads to an increasing number of interconnects. At a certain moment, wireless headphones are added to the whole variety of equipment, exclusively for the peaceful coexistence of home in the evening hours, when the question arises: what is more important for spiritual development - the twentieth series of a reality show or listening to your favorite album on CD. To watch a DivX movie on a PC, it can be useful to connect a powerful home theater speaker system to your computer. At such a moment, you inevitably have to figure out the intricacies of wires in a cramped dusty space behind the back of a TV or computer. The cardinal way out is the purchase of a switch, which, by lightly pressing the buttons, would allow you to connect sound sources to one or another sound-reproducing device in any combination. The problem is that such switches are found exclusively in professional equipment and are quite expensive. To switch the output of a computer sound card, a simple microprocessor-based audio signal switch was developed. It solves quite specific tasks, namely: it connects either active 2.1 speaker systems, or wireless headphones, or a headset with a microphone to work in the Skype program to the computer. The switch has a 1-3 structure, i.e. switches the signal from one input to one of the three outputs. Since the switching element is a passive bidirectional key, it becomes possible to use the switch for the opposite task - to connect one of the three signal sources to active speakers. Of course, this switcher can be used with any other sound-reproducing equipment.
Schematic diagram of the microprocessor switch audio signals The circuit diagram of the switch is shown in the figure. An analog switch on the CD4052 chip acts as a switching element. To switch bipolar signals, it is necessary to apply a negative voltage to the Vee pin, not less than the amplitude of the analog signal applied to the input. The negative voltage is generated using a 561LA7 microcircuit, on which a rectangular pulse generator is assembled, and a diode rectifier according to the voltage multiplication circuit. The current consumption on the Vee line is negligible, so the generator on the 561LA7 consumes only 100 μA on the power line. The operating frequency of the generators is selected in the region of 35 kHz, i.e. outside the audio range. A control node is assembled on the PIC16F84 microcontroller, which polls the switching direction selection button S1, generates control signals for the CD4052 microcircuit and indicates the current state using the HL1 LED. The clock frequency of 3,5 MHz is not critical and can be selected within any range from 3 to 4 MHz. The microcontroller firmware files are available on the magazine's website. A high-quality stereo headphone amplifier is assembled on the TPA6110A2 chip. The need for an amplifier is due to the fact that the signal from the output of the computer's sound card is not always able to provide sufficient volume in passive headphones. The amplifier operates with a gain of 1, however, if desired, the gain can be easily increased by reducing the resistance of resistors R9 and R10. The amplifier is designed to work with headphones with an output impedance of at least 16 ohms. The coefficient of harmonic distortion of the amplifier is less than 0,03% at a frequency of 1 kHz. The TPA6110A2 amplifier IC (Texas Instruments) can be replaced by the LM4881 IC (National Semiconductor). When assembling the device, special attention should be paid to the correct wiring of the power and ground lines. When the device is turned on, audio output 1 always becomes active. By successively pressing the S1 button, you can connect other audio outputs "in a ring": 12-3-1-2-.. . Connecting the next output is accompanied by a single sound indication, while the number of sound signals given corresponds to the number of the output. The LED constantly indicates the number of the connected output by the corresponding number of short flashes with a pause between series. For ease of use, the S1 button can be moved outside the case and placed in a convenient place, while the switch itself can be placed closer to the place where the connectors of the switched audio equipment are concentrated. A free USB port on the computer is used as a power source. The total current consumed by the circuit does not exceed 10 mA. This switch is easy to upgrade, expanding its capabilities. The presence of free ports of the microcontroller makes it possible to complicate the indication scheme, for example, by placing individual LEDs to indicate the connection of each output. To increase the number of switched outputs to 4, it is enough to add a connector and connect it to lines 11 and 4 of the CD4052 chip (with a corresponding change in the program). The current consumption can be significantly reduced by eliminating the LED indication and using the interrupt mode by pressing the S1 button. If you abandon the amplifier on the D3 chip and optimize the operation of the generator (turn it on for a short period to charge C7), then you can build a battery-powered switch. In this case, a set of three AA alkaline batteries will last for several years of continuous operation of the switch. Another way to improve performance is to replace the S1 button with control using an IR remote control or via a radio channel. The computing resources of the microcontroller will be enough for all the above improvements. Quartz 3,5 MHz
Firmware:
Author: Oleg Pushkarev, Omsk; Publication: cxem.net
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