Thursday, May 1, 2014

The 6.115 Saga

I started work on this project almost a year ago. A majority of what follows happened over summer 2013, as of last week I finished the last task needed to get it to production.

This requires a bit of explanation.

There is a class taught by Prof. Leeb called Microcomputer Project Laboratory, or 6.115 in MIT speak. It runs each spring and students are taught how to work with microcontrollers and then are required to complete a sufficiently complex electronics project of their own design. To aid in this process, each student is given a briefcase size kit which holds protoboards, a microcontroller interface, power supplies, handheld tools, a bag of parts and whatever else they choose to use. It is highly encouraged that the kits be brought home so work can be done outside of class.

The 6.115 kit as used today and 10 years previous.

The current kits are based off of a Global Specialties PB-505 workstation that has been rehoused in a custom wood briefcase. The microcomputer board is stored in the lid and connects via a ribbon cable and a special connector with pins that plug into the breadboard. Some of the kits are getting a bit tired and for a variety of reasons, Leeb decided a redesign was in order. 

The requirements, in no particular order:
  • Portable
  • No exposed 120 VAC line voltage
  • DC power: +12V, -12V, +5V
  • Test circuitry: logic monitor, logic switches, speaker, voltage divider
  • Space for four regular size breadboards
  • Space for microcomputer board
  • Connections for microcomputer board
  • Space for tools and parts
  • "Dingus proof"
  • Bonus: looks good
That's pretty much where he turned the project over to me.

The first thing on the agenda was a case to hold everything. At the time I was playing with vacuum forming but quickly decided that this was not a DIY type problem. Buying the cases would mean they would be robust, have good tolerance, and be readily available if replacements were needed.  After weeks of searching and comparing the Nanuk 920 was chosen. It's very similar to the more ubiquitous Pelican cases, but what sold me was the ring of screw holes around the edges of the case. There is a waterproof panel accessory for the case which gets mounted with those holes. Pelican has similar kits, but the mounting holes aren't molded into the cases; they need to be added to the stock unit. Nanuk's intention is that you buy the plastic panel then customize it for your own use. I didn't do that.

Once a sample case was in hand I started with panel designs. From the beginning the plan was to house all the extra bits, like wires and tools, underneath a hinged panel while the work space and related boards were mounted to the topside of the panels. Here's a cardboard mock up from some point in the process.

This version has a compartment under the top left area to house power supplies and related circuitry (120 V socket, switches, fuses). The left side panel would be bent into that shape and then screwed in. The biggest issue here was sealing. Bad things would happen if small bits of wire or other conductive debris that would inevitably be kicking around in the bottom of the case got into that compartment.

More google-fu and and deliberation led me to a triple output stand alone Mean Well power supply. This unit  gives the required voltages and has protection built in. Best of all, the 120V will never be seen by the kit or the students.

This meant the enclosed compartment below the panel was no longer needed. That meant the job could be done with all flat panels. The panels have to fit in the case nicely, which normally wouldn't be a big problem, except the screws locations are not regular. It seems like Nanuk purposefully moved the mounting holes around to discourage people from copying their waterproof panel accessory.

See those holes? They aren't aligned horizontally or vertically and the distance between each is different. This meant I spent a good chunk of time taking measurements and making CAD of the case. I laser cut a plastic panel to check the fit and with a few tweaks I had a panel that fit exactly how I wanted.

After adding mounting holes for the circuit boards and hinges I made nice drawings and put them up on It's basically like a reverse ebay for manufacturing jobs. Buyers put their designs up and companies bid to make the part. I picked a company called Gauthier Industries in Minnesota. They sent a prototype panel for evaluation to ensure the fit was all correct.

It's really gratifying to send out a part and have it come back just like you asked. Even though it was exactly what I specified, the smaller hinge panel was just a bit too long which caused it to scrape on the bottom edge of the case as the panel closed. I made changes to the drawings and gave the go ahead to produce them in quantity.

In the mean time, another undergrad in the lab was working on the circuit board you see above on the left hand side. This board has the power connections and test circuitry for the kit. The right side board is the microcontroller carried over from the old version. This layout means the the boards will allways be connected by the ribbon cable, which reduces setup time for students.

Leeb wanted permanent labels for each kit with a serial number so he could keep track of them. I looked at engraved plaques and industrial stickers but nothing was very pleasing. Eventually I realized that because the final panels were black anodized, laser engraving would be an option. It took about six hours all together to get the panels done. Most of the time was waiting for machine cycles to complete so it wasn't too bad.

Test heatsink to work out laser settings, with new and improved "Dude!"

That number slowly grew to over 200. The tape was only used on the first batch of 5 to find proper alignment in the machine bed.

After a few plastic clips for wire management, the design was done. I got lucky and didn't have to drill any more holes for the clip by using one of the panel's mounting holes.

The very last steps involved sourcing and ordering all the parts needed for assembly. Proxy Manufacturing in Andover will be putting the kits together. They came in vans to get the parts on Tuesday, so now it's out of my hands.

Soon there will be brand new kits ready for 6.115.

Until next time.