3. FabISP In-Circuit Programmer


This week, we covered electronics production, including PCB production and assembly. The project for the week was to construct our own FabISP board, which is an in-circuit programmer for AVR microcontrollers. In other words, we’ll be able to program the microcontrollers on the other boards we make throughout the semester.

We covered two ways to make circuit boards in the shop. The first is to mill the board using the Modela machine. It’s a relatively slow method (about 10 minutes per square inch), but easy to use.

Circuit stickers are awesome.

The downside of the machine is that the endmills can wear out easily, especially when it’s being used by twenty people. The real art to a Modela is setting the proper speed and depth. When I was using the Modela, there was a tendency for one end of the board to get milled deeper than the other end. We had to compensate by increasing the z-depth from the default 0.1 mm to 0.14 mm. For me, I found that lowering the speed from the default 4 mm/s to 3.7 mm/s helped tremendously. When the speed is set too high, the endmill tends to tear up the board, making it bumpy and uneven.

Stuffing the board turned out to be a challenge all on its own. I had done an analog electronics class for two terms at Caltech, so I theoretically knew how to solder, but I was not prepared for handling such small components. Here’s the finished board, with a penny next to it for scale. I actually lost a capacitor by accidentally breathing on it.

Thankfully, I managed to program my board the first time through. I borrowed my officemate’s FabISP (thanks Akito!) from last year to program my board. At first, I kept on getting an rc=-1 connection-status-not-found error, but that was because I did not line up the six-pin cable correctly. Once correctly oriented, programming the board was straight-forward.

Later that day, Jie gave a demo for vinyl-cut circuits. Instead of milling a board, you can use the vinyl cutter to cut out the circuit design on copper tape. Instead of tens of minutes, it takes seconds to cut a board. The hard part, however, is in weeding the tape to get rid of unwanted copper. It took me about 40 minutes to weed my board.