PCB - Revision I

A project log for Chronio

Low power Arduino based (smart)watch

maxkMax.K 08/26/2016 at 14:342 Comments

This was my first custom PCB design, apart from a single sided milled PCB that I had to create for an assignment. Because most of the suppliers in Europe are really expensive, I chose OSH Park. They only charge 5$ per square inch, which is especially great for tiny projects like this. I have no idea how they can produce and ship boards around half the globe at that price.

Is started with the schematics in Cadsoft Eagle. I basically just searched for components and then connected them. For the display I am using a ZIF connector. The Atmega needs an external 16MHz crystal to run at the normal clock rate of an Arduino. An additional led is for testing the board with the blink-sketch. For programming the Atmega I used the footprint of a 5-pin header. The cables could later be soldered on. As I had no Idea of how to include the buttons yet, I just put in some SMD pads.

For the board layout, I did a quick mockup of the PCB in Fusion 360 and tried to find the right size, that would still fit inside the case. I ended up with a 34mm by 28mm rectangle with rounded corners. A small cutout helps the display's flex cable bend within the case. I placed four mounting holes in the corners for good measure. Holding the coin cell was another challenge. The standard SMD holders have a high profile. I put two pads next to the side of the coin cell and decided to make a custom mount.

The routing took me a few hours. Mainly because I used relatively thick traces I ended up with dozens of vias. Also, the angles were all over the place. When the board arrived two weeks later, I had very low expectations. But there were no shorts or major design flaws.

The next step was to populate the board. I ordered the components from Farnell. Very fast service and insane packaging even for low quantities like the two Atmegas.

I soldered the board by hand. Thin solder and a flux pen are absolutely necessary for parts like the ZIF connector. For the coin cell mount I cut the metal strip of an old folder to the right length and soldered it to the board.

After burning the bootloader, I used an old Arduino Uno and removed its chip to program the board. It basically worked on the first try without any modifications:

After adding the screen and uploading the code, the watch was already working. To connect the PCB and display, I created a separate printable part.

Although it is functional, the first board has it's problems. The current consumption in standby is still at 450 uA. Also the programming header is only a temporary solution.


jaromir.sukuba wrote 09/04/2016 at 10:10 point

Farnell is known for insane packaging. Once I received 3kg parcel containing nothing but dozen of TQFP parts and a lot of packaging material.

Lovely project, by the way.

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Max.K wrote 09/04/2016 at 10:39 point

Thanks! I ordered from Farnell multiple times and it's always like that.

Their packaging is now the main source of silica gel for my 3D printer filament.

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