I’ve had some down time project wise lately, most of my SMPS research is on hold, since I need to order even more parts (namely diodes and switches)… so, I’ve taken the time to draw.
One of my ‘back burner’ projects is a whole-house audio distribution system. So, this past week or so, I set out to design the heart of the system, a matrix switch. Originally I had tried to go “too big”, designing a system with four inputs and eight outputs. However, routing the signal lines and control lines was too much of a challenge, even using a two layer board. So, scaling things back, I wound up with a 4×4 matrix that was barely manageable.
There is the schematic for the latest ‘stable’ version. I call it a stable version, because it passes all of the DRC tests and all the nets are routed. I have another version in design, which adds ESD protection to the external audio connections in the form of high speed schottky diodes which will clip any voltage coming in that rises above or falls below Vcc and Vee.
Control will be provided by either a serial / parallel connection to the audio server PC, or from a “controller board” powered by a microcontroller. I haven’t really figured out how complex I want to make the interface yet. Control of the switch itself is fairly straight forward. Two 74HC595 latching registers are daisy chained together, forming a sixteen bit register. This register is used to provide the eight bits which tell the multiplexers which connections to make. Each multiplexer has a two bit interface, A and B, which selects one of the four IO channels to be connected to the COM channel. I selected the 4052 multiplexer which is internally divided into two sets of four channels each, ideal for stereo I thought. So the first eight bits of my 595 register are connected to the eight control lines on the 4052’s. Second, to provide a “hardware mute” or “output enable” feature, another four bits of my 595 register are connected to the /Inhibit (enable) control lines on the 4052’s. This allows the output of each 4052 to be electrically disconnected from all the sources. The final four bits of the register are used to power four “general purpose outputs”, perhaps to control relays, external indicators, whatever. The control lines connecting to the GPO header have inline resistors, to limit current draw to a safe level, so not to damage the 595 in the event of a short or connection to an unsuitable load. The four bits connected to the inhibit lines are also connected to four LEDs, so I can see which zones are active.
The 74HC4052 is an “analog” multiplexer, in that it allows voltages other than digital 1’s and 0’s to pass through. It is also a bidirectional multiplexer, allowing current to flow in either direction. This allows the multiplexer to handle the “AC” nature of an audio signal. In order to allow a true bipolar sine wave to pass, the 4052 requires a dual rail voltage supply. For simplicity, I will be using a five volt supply with positive and negative outputs.
Here is the ‘bare’ pcb layout, without the top and bottom pours rendered. This is a composite, showing both the top layer and bottom layer at the same time. It’s easier to follow the traces this way I think.
Adding in the ground planes or copper pours, it makes the board look mostly purple. I tried to maximize the size of the pours, to help minimize any interference or crosstalk. Although, at such low frequencies, I don’t really expect much trouble.
Finally, the board with just the silkscreen or layout layer selected. This again, is a composite, showing parts on both the top and bottom of the board, which makes it look like some of the parts are overlapped.
There is a lot more to come on this subject, but it will be a while. Overall, this was just a fun project to draw, and hopefully one day it will actually get fabricated. I need to figure out how complex I want the interface to be, and what options I have for interfacing with the audio server.
