Electronics Design

The assignment this week was to reproduce the Fab Lab 'Hello World' circuit board with some modification on the basic design. Since I'm fairly new to circuit board fabrication, and have to get my board from week 2 to be fully functional, I decided try and keep it simple for this week.

The base 'Hello World' board consists of a ATTINY44 micro-controller, a 6-pin FTDI header for powering the board, a 6-pin header for programming the board, a 10K Ohm pull-up resistor connected to the micro-controller, and a 1µF cross-over capacitor.

Following an example tutorial posted by Anna Kaziunas France from the AS220 Fab Lab in Providence RI, I modified the board by adding a LED (and necessary current limiting resistor) to pin PA7 of the micro-controller, and a button (with resistor in parallel) to pin PA3. If my understanding of the micro-controller is correct, this configuration will allow for generating two 'states' at pin PA3, one when the button is pressed and another when it is not, and these states can be reported by different blinking patterns expressed by the LED connected to pin PA7.

A second modification that I incorporated into the board, was a quality control measure recommended by TA Palash Nandy. This consisted of a second circuit in parallel, with a LED and current limiting resistor, acting as an 'On' light for the board. When power is supplied to the board, this LED will light, providing assurance the the board is receiving power.

Following Anna's tutorial and using Eagle PCB design sortware, I was able to layout the schematic of the board, and render the traces. After exporting the traces as a .png file (in monochrome! important), I was able to add some personalization in Photoshop. Who needs 'Intel Inside', when you are Powered by Negri! Now it was time to set tool-paths and start milling.

When setting the tool-paths, thanks to Oliver Bucklin's sharp eye, we noticed a problem with how the traces were running underneath the ATTINY 44. In the original version (below left) there were 3 traces passing under the micro-controller (highlighted in red). Unfortunately, these 3 were squeezed so tightly together that when setting the tool path using Fab-Modules instead of getting 3 paths there was just a pad of copper (no good). Going back to Eagle and editing the traces, I ran one to the outside of the ATTINY44 (below right, highlighted green) and defined paths could be set.

Time to mill. I secured the board stock to the milling surface, loaded the 1/64th" end-mill, set the X,Y,Z min, and off we go! Unfortunately, I ran into another set back, during the milling the board came loose from the milling surface, and the end-mill started moving the board around rather than cutting traces over it. No worries, new board, extra-tape this time to make sure the board is really secure, set the X,Y,Z and off we go again. The second time around the board stayed in place, and traces were all milled cleanly.

Now it was time to cut the board out of the stock. I replaced the 1/64th" end-mill with the 1/32nd" end-mill, reset the Z-axis being sure to keep the X and Y minimums exactly the same. However, despite being sure that the outline path was larger than the dimensions of my internal traces, and being careful about setting the X,Y minimums of the mill cut right through my traces, Damn it!!!!

At this point, I had been at this all day and was getting tired. I knew that I could mill the traces, which is the business end of the operation, and not worry so much about cutting the board down to size. I secured a fresh piece of board stock onto the milling surface (extra tape again), replaced the 1/64th" end-mill, reset the X,Y,Z, and off we go again, again. The milling finished cleanly, and I left it at that. My personalization in the top left did not turn out well, but that was just for aesthetics anyway.

Thankfully, I got started early this week, so after my milling adventure I was able to take a break and come back to this later with fresh eyes. After the soldering practice from week 2, stuffing the board this week went pretty smoothly. Most importantly, when I hooked up the board via the FTDI header to the USB port, the power light came on! Success!!!. Next, I'll have to see if I can program the board, but that can wait for the time being. On to Molding/Casting.....