The complete BulkyMIDI-32 consists of several separate PCB designs, many of them adding entirely separate functionality that can be used with or without the main unit. This specific module, once fully assembled will allow you to switch between six different audio inputs - think of it as the source selection switches on your stereo system.
I didn't want to use yet another MCU for what would account to an extremely trivial task such as this, neither did I want to switch the audio through a chip - so I instead went for simple flip-flops and somewhat exciting relays for each of the six channels.
Building a device such as this assumes you have at least minor experience soldering things together, recommended is that you at the very least have access to a soldering station with adjustable temperature. If currently available, you should probably start with 60/40 rosin core solder as the lead-free alternatives tends to be a bit harder to work with.
With requirements for basic equipment sorted, we need to talk a bit about what you want to achieve while building this project. The reason I ask is that while you can build a functional BulkyMIDI-32 Audio Switch by using this module by itself, but there is certainly the option of building the complete version of it - including front panels, faceplates for the top of the unit and so on.
Now that you know what you want to build, we can get started on the most expensive step in any build process - shopping! So, head on over to the BOM-section for some information on what kind of components that you'll need. While I would have liked to give you specific part numbers for "reputable" electronic part suppliers, I can't afford to shop there so I usually make due with what I can find on sites such as ebay, AliExpress and any similar sites that sell cheap chinese components. The downside to this is that I can't link to a part directly and expect it to stay listed, so you'll need to consider the description listed as suggested search terms.
The construction of this unit should be fairly straight-forward, just take your time in order to make sure everything goes into place - more or less straight. I usually recommend starting by having a good look at the PCBs, try to figure out where all of the listed components go and in which orientation they go in (most of the information should be clearly marked on the silkscreen).
I will usually try to install components sorted by their physical size, doing it this way means you'll have an easier time keeping everything from looking somewhat crooked. This is your board, your build and you want it perfect - don't you? If you said no, then go sit in the corner because I worked hard on creating this for myself and whoever wants to build one.
Here we've already gotten started by installing all of the resistors needed, making sure that we use the correct values for each position on the board. While doing this, it is always a good recommendation to try and keep the colour bands in the same orientation - mostly because it makes it easier to spot mistakes later on. As I'm going to use this with a front panel, I've also taken the time to install the six 470 ohm resistors on the bottom of the board
There are quite a few 1n4148 diodes in several locations on the board, these are polarized and for that reason we need to pay particular attention that the black band of the component matches up with those of the markings on the PCB.
Sockets should be installed for the singe IC needed for the complete unit. With that in place I've also installed the monolithic disc capacitors, these are all unpolarized and for the ones without any specific markings we will install 100nF capacitors (the code 104 will be printed on them). The single capacitor on the left edge of the board has a different value from the others, it is 22nF and will have the code 223 printed on it.
The next component we're going to install are the 2n3904 NPN-transistors, these have a flat side and a round side that needs to be matched to what is printed on the silkscreen - putting them in the wrong way around will usually create small smoke-bombs instead of their intended purpose. While there are components available that have pre-bent legs matching those on the footprint, I've not come across them at a reasonable price - so I just bend the legs outward myself and firmly push down on them until they sit a bit above the board.
Next up are the polarized capacitors, these need to go in the right way for them to function as expected. The stripe on the side of the capacitor denotes the negative side of it, this should be matched up with the filled in area of the footprint. When in doubt, just put the longer pin into the other hole (marked with a "+"). For the most part we won't be anywhere near any relevant voltage rating, but the higher rated ones will generally be physically larger - the one at C2 should be rated at 16v or 25v, mainly as we physically won't have room for anything else.
Here we've added a common DC barrel connector (5.5mm x 2.1mm) to the board, followed by the six HK-19F-DC 5V relays. The relays have a tendency to come with crooked pins and we may need to do a bit of careful bending in order to get them into the board, there is also a little indent on one end that need to matched up with that on the board (similar to those of ICs).
I designed the board for a PJ-307 3.5mm stereo audio sockets, but as I didn't have enough of those in stock I used PJ-317 instead - they're a tight fit, but with the little nubs on the bottom cut off they sit well on the board. I've separated the six channel inputs from the single output by using differently coloured parts.
Depending on your life decisions up to this point, the assembly from this point continues in one or the other direction:
So you resisted the temptation to build the complete version, choosing instead to cut corners where possible? Well, that's OK too. I'm not judging, it was just a general observation.
There's not much that we need to do, mainly just install the single 3mm power LED (the longer LED goes into the round pad). The six switches are standard right-angle switches, the recommended way to install them is simply to solder a single pin then reheat and adjust its alignment before soldering in the rest of the pins.
If you have a faceplate, the board that goes on top of the unit, add some 20mm hex standoffs to each of the four corners before continuing on to the next part of the build.
The Audio Switch Panel have its own set of documentation going into the various steps required to put it together correctly, a finished unit looking like what is shown above. In particular we need to ensure that the hex standoffs have been aligned so that a flat side is lined up with the left and right edges of the board, the nuts on the back needs to have a similar flat side matched to the top and bottom. You may need to trim these slightly with a sharp knife or a file.
Starting with installing 20mm standoffs, making sure that these have a flat side pointing towards the inside of the PCB. With the alignment correct you should be able to insert the panel into the board without too much force, then your selected faceplate is installed on top of the boards using screws. At this point we've not actually soldered them together yet, mostly to ensure that everything comes together correctly before we actually do so. Push the panel firmly so that it sits towards the back of the cutouts, then solder the pins into place - afterwards we just trim the excess parts of the pin headers.
When soldered, everything should stay where it should be and before continuing on we just remove the faceplate again before moving on to the next step.
If you've made it this far, you're almost ready to simply start using it. What's missing in order to test it is the single IC that needs to be inserted into the socket, so just get that out of the way before you forget to do so. The device is powered using a 5v DC barrel jack (center positive). These should be easy to obtain from your favourite electronics vendor, just make sure that it's from a somewhat respected brand.
The only step that's remaining is to just screw the faceplates back on the top of the device, once that's in place we can also add the front panel as well. With those in place we're ready to start planning the next project, quite possibly while playing some music through the audio switch.
The supplied KiCad files should be sufficient as both a schematic and as a starting point for ordering PCBs (basically you could just zip the contents of the export folder and upload that on a fabrication site), the schematic is also available in PDF-format and this is what you'll need to print and work your way through this things don't work as expected after assembly.
Most parts should be easy to get a hold of from your favourite local electronic component shop, but given that I don't have access to such shops where I live so everything was based on whatever I could get cheapest from eBay/AliExpress (free shipping, but plan on waiting 3-4 weeks for delivery). Components in parenthesis can be considered optional for features beyond the more basic functionality, but where's the fun in that? You deserve the complete package.
| Reference | Item | Count | Order |
|---|---|---|---|
| PCB | Fabricate using Gerber files | 1 | PCBWay |
| Panel PCB | Fabricate using Gerber files | (1) | PCBWay |
| C1,C3-C6,C7-C9 | 100nF ceramic capacitor (5mm) | 7 | |
| C2,C11 | 10uF electrolytic capacitor (4mm x 2mm) | 2 | |
| C6 | 470uF electrolytic capacitor (8mm x 3.5mm) | 1 | |
| C10 | 22nF ceramic capacitor (5mm) | 1 | |
| D1-D6,D10-D12,D16-D18 | 1N4148N small signal diode (DO-35) | 12 | |
| D19 * | 3mm LED | (1) | |
| J1 | 2.1mm x 5.5mm barrel plug | 1 | |
| J2,J3,J5,J7-J9 | PJ-307 3.5mm socket (alternatively PJ-317) | 7 | |
| K1-K6 | HK19F-DC5V relay | 6 | |
| Q1-Q6 | 2N3904 transistor (TO-92) | 6 | |
| R16-R18,R25-R27 * | 470 ohm resistor | (6) | |
| R10-R12,R19-R21 | 1k ohm resistor | 6 | |
| R7,R9,R13-R15,R22-R24 | 10k ohm resistor | 8 | |
| R1-R6,R8 | 100k ohm resistor | 7 | |
| SW1-SW6 * | 6x6x5mm right-angle momentary button | (6) | |
| U1 | CD40174BE (DIP-16) | 1 | |
| Mounting **** | Nylon M3 hex standoffs 20mm (M-F) | 4 | |
| Mounting **** | Nylon M3x6mm nylon screws | 4 | |
| Mounting **** | Nylon M3x6mm nylon nuts | 4 |
*) You can choose to install a front panel, in which case you would skip the installation of the 3mm LED as well as the switches. The resistors will be used by the front panel.
****) These are available in the form of kits usually advertised M3 nylon standoff kit which should contain most of what you'd need. I do however recommend that you pick up extra M-F standoffs of lengths 8mm and 20mm, as quite a lot of them will be used when building more than one module.