Dec 4, 2011

Halloween Build Part 4: Board Layout

Are we still talking about Halloween? The tryptophan from Thanksgiving has worn off and Christmas is right around the corner yet this blog seems to be stuck in October. I am not one to live in the past so let’s wrap this project up and move on to bigger and better things. Two more entries on my Halloween project and then it’s on to RC circuits like I promised back in September.

If I had any confidence in my printed circuit design abilities, I would have a custom PCB fabricated through SeedStudio or BatchPCB like the one pictured to the right. Despite having written tutorials on how to use Eagle software for PCB design, I actually have limited experience when it comes to generating boards. I have played around with programs like Eagle, DesignSpark, Altium Designer, and DipTrace in the past, but haven’t ever taken the time to learn one thoroughly (my EAGLE tutorials were pretty lacking). Instead, I go to Radio Shack each time and pick up some overpriced protoboard to solder all my components together. This project alone is enough to make me want to avoid doing that ever again. My next goal is now to design and fabricate my first solo PCB.

From my previous posts, you know by now that there aren’t that many functional pieces to this circuit. There is power, shown in the red box in the picture, the PIC microcontroller, in blue, and the LEDs in orange. For some circuit designers, they like to layout schematics the way their board will eventually be laid out. Others prefer having all their inputs on one side and all the outputs on the other. Personally, I don’t see a benefit with either approach because outside of simple boards your final board design won’t look much like the schematic even if you try. In my case I did try and follow the schematic pretty closely in terms of the layout, but things never turn out quite how you expect when it comes to these projects. 

It’s one thing to draw connecting lines on a schematic and know how the circuit should function, but it’s quite another to realize a physical design. For instance, I wanted to be able to replace the battery easily when it died so I soldered a heavy duty 9V snap to a JST connector and mounted the female end to the board (highlighted in purple in the picture). JST connectors are really nice for easily disconnecting batteries from the system and you can find them a lot in RC car designs and similar battery powered toys. The downside is that the heads of the connector are only two terminals, which means anything that needs to be tied directly to power or ground has to end up connected to the JST in some fashion. If you look at the soldering on the underside of the board, you can see that none of the copper pads are connected to each other. For this reason, it becomes pretty difficult to create a bus line without using a lot of bridged solder joints. Since I had no other choice, I went that route and ended up with a bunch of large solder blobs and wires running along the top connecting several areas of my board.

One other problem I ran into was the placement of the LEDs. Originally I had intended to use four LEDs, but when I started soldering on the first three I realized I wasn’t going to be able to fit a fourth and still have them in a semi-cluster. Because I was anticipating a fourth LED on the left side of the microcontroller, I mounted it in the middle of the board and that meant that I had to bend the LEDs awkwardly to fit through the cutout in the top of paper bowls (shown in Part 1 of this build log).

In the end, all the boards worked as expected and I was able to finish out the project. As much as I am in this for the electronic design, I have become really interested in the form versus function aspects of my final products. Companies like Apple have made a fortune off the way their products feel when people hold them and have gone out of their way integrate form and function as much as possible. I am still trying to get a handle on that concept in my own designs, but I have come up with some new ideas to try in the future. Look to wrap up this build log in my next post with the bill of materials (BOM).


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