DIY Tron Identity Disc – Designing the Electronics
Moving on from the mechanical design, I could finally jump into developing the electronics to go inside the Tron Identity Disc. If you haven’t already, go check out the mechanical design here.
With the mechanical design complete I needed to design the electronics to make the Tron Identity Disc actually cool and do something other than just sit there. I would need to design a few different circuit boards to hold different sections and perform different tasks. Each of these was already determined from the mechanical design. I had already identified and chosen some parts during the CAD process but the schematic can be completed. Following this the circuit board layout. As well as the not planned stuff. Things such as continuous back and froth between CAD packages and models, and datasheets and schematics.
Picking the parts was a mixed difficulty task, some really easy and knew straight away others took a bit of research, digging and asking around. Allocating footprints was easy as I chose parts I could solder by hand making the smallest package 0603 and a few other may just take a bit more patience to get into place.
- SK6812 Addressable RGB LED, in both 5050 package (Light Arc)and side mounted (Main PCB)
- STM32, more specifically aiming for the
- Batteries, 6x
- 9DoF IMU, LSM9DS1
- Magnetic field Sensors,
- Power Regulators, booster and switches to control power around the boards
- Passive components like resistors and capacitors in 0805 or 0603 packages, as that’s the smallest I can easily solder by hand
- and everything else required to work
The schematic was broken down into key sections to make it easier to manage. This was applied to both circuit boards being developed. Each major section of the design was put into it’s own designated schematic to make it easier t o find sections that required modifications later on and made the overall schematic easier to follow, in my opinion.
Now the schematics have been completed it was time to assign footprints and move into laying out the PCBs.
Laying out the PCBs took a little while. I constantly pushed the changes to github and quite often went back and forth between a few layouts trying to find one that worked out well enough to continue. That being said the final layout should work but is probably not the most space efficient or optimised, that will be something for version 2 maybe.
To begin I grabbed the board outlines form the CAD models from Fusion360, then imported the file into KiCAD on the edge cuts layer. I decided to start designing for the standard 2 layer boards. I went the entire way through the design working around the 2 layers, and did not see any major benefits of more layers. The track widths varied around the board depending on the power and location on the board trying to maximise the width to reduce the possibility of failure. I also put in copper fills around the board to help with the complex geometry.
I faced quite a few challenges developing the electronics design for the Tron disc, but argubly way less than the mechanical design.
Design Rules Check (DRC)
The Design Rules Check (DRC) helps check to see if the board parameters meet the specified rules. These specified rules ensure the board is manufacturable, meets the minimum spacing requirements and a few other bits too. This error occurred because of how the minimum distance is calculated and due to the angle of the parts the minimum distance calculated was less than the parameter in the DRC. To get around this i changed the DRC parameter for these parts by -0.01. This was a small enough change such that it was not an issue for manufacturing but large enough to pass the DRC.
Routing in a circle isn’t easy in KiCAD but is apparently possible in some other software packages. This meant that some of the traces are not aesthetically placed around the board. I did attempt using an open source plugin to help create some curved tracks but did not use them very much in the final design. However I did use a plugin that created circular fill regions to help create some nice round copper lines between regions. I also made use of the inbuilt shove routing once a majority of tracks were doing to push tracks into nicer locations.
Providing power to all components was a slight issue. The main problems being reaching to opposite end of the PCB. To solve this issue I used copper fills on most areas of the PCB. KiCAD connected the majority of the components. After this I went in and joined the remaining unconnected pads. This allowed me to quickly get power to the main areas of the board before routing the rest of the tracks.
The last issue was part placement. It wasn’t a major issue, more tedious. The main issues i had was trying to evenly space the parts out at the correct angles and at the right interval. I attempted some of this by hand and that’s how I did the entirety of the alight arc PCB. As for the main PCB I decided to do some research and found a plugin. The plugin allowed me to select parts and arrange them in a circle. It wasn’t perfect but at least rotated the parts to the correct angle. I then placed the parts manually lining them up with where I wanted them.