Hired Gun

Now that the hardware side is pretty much wrapped up I wanted to build a simple enclosure so that I could start field testing. I’ve already spent way too much time trying to find a prebuilt enclosure that would fit the components nicely. As I get further along with my projects I’ll write more about enclosures.

Inspired by the design of the Digg Button and Wave Bubble, I decided to do a simple sandwich of acrylic sheets and spacers. I couldn’t sandwich the boards directly like was done in those projects, so I decided to mount my boards to the back and face of the enclosure. With this basic design in mind, and some notes on what size screws each board could take, I set off for the hardware store.

For the mounts I used 1/4” x 030 (6.35 mm) aluminum rod. I chose this size since it’s just small enough to be tapped for a 10-32 screw.

While SAE and metric sizes are not interchangeable, a M5 x .08 is pretty close to this.

I cut the rod to length with a twist pipe cutter and then tapped both ends with a 10-32 tap.

Tapping the aluminum is pretty easy going, just make sure to use a little oil to keep the tap moving smoothly, and back up every few turns to keep the cut smooth.

I cut and tested a small piece of the rod before moving on to the full length pieces.

Once I was happy with the test piece I measured, cut, and tapped the 4 corner pieces.

I Used a paper towel between the rod and the pliers to keep from marring the aluminum while tapping them.

Even after measuring and cutting carefully there was still a small difference in the length of each piece. I used a sander to bring them all to the same height, and chased the threads with the tap again.

I found rubber grommets to use as standoffs for the Arduino and Lithium Backpack boards, but couldn’t find any small enough to use for the Bluetooth board. For these I used short sections of macroline that I had in my paintball gear box.

Macroline is 1/4” nylon tubing that cuts easily with a razor blade. I had a bit of trouble getting it to cut straight at such short lengths, so I used the pipe cutter to score a deep ridge (the pipe cutter can’t cut the line since it is too soft). Then I used the ridge as a guide to cut the line by rolling it on the desk while pressing the razor blade into it.

I cut the Lexan (polycarbonate) sheet that I got at Lowes, and rounded the corners.

I just happened to have a small corner rounder from a prior business venture, but you could achieve the same effect with a band saw, or sander.

Once I had cut the top and bottom sheets, I taped both together and marked and drilled the outside corner holes. I also marked the holes for the Arduino and the Lithium Backpack while I was laying everything out.

Next I mounted the corners and the boards to the polycarbonate sheet. The boards are mounted with 4-40 screws (again, not exact but M3 x .05 is pretty close). I had purchased nylon washers in case the nuts were too close to a traces or component on the boards, but it turned out that I didn’t need them.

I then repeated the steps with the front. The LCD is mounted with 4-40 screws, as is the Bluetooth.

The Bluetooth only has 3 screws right now, since the hardware store only had 9 of that size in stock, and I used 6 to mount the boards. For now it does the job just fine.

The switch is mounted with 2-56 screws (approximately 2.1 mm).

Once I had the face done, I started routing wires to get everything hooked up, and bolted both halves together.

Below are a few shots of the completed enclosure.

Final thoughts:

Overall I’m happy with the results of the enclosure as a general purpose concept. For the GPS unit it’s a bit large, and not horribly strong, but I knew that this was going to be an issue going in. From the hardware standpoint I have learned quite a bit;

• Connectors and wires take up a lot of space.
• Extra boards take up a lot of space.
• The area taken up by mounting screws adds quite a bit of size to the enclosure.
• I don’t care for the membrane buttons. I prefer the tactile switches, but that’s me.

For now I’m done with the hardware side of this project, but I have quite a bit more software work to do. Once I get the software completed and field tested (and probably another project done), I’ll focus on reducing the overall size of the components, and then finding or making another enclosure for it.

After two evenings (late into the night) of soldering and de-soldering and re-soldering,  I have the hardware side of the project pretty much complete.
Here’s the entire setup; in this shot I had everything pushed in tightly to get an idea of the final size. You can see the electrical tape on the back of some of the boards to prevent shorts.

Instead of tactile switches, I chose to use a silicone button that resembles a TV remote. Here the PCB half of the buttons is mounted on a 1" (2.5 cm) square ProtoBoard along with a header that provides power, ground, and a return to the two button pins. I built the pull down resistor circuits on the board under the button PCB.

Two more of the little boards make up the breakout board for the LCD. I thought that I had ordered a larger board to build this on, but must have missed it. Luckily I had ordered a few extra of these just to have around. They are nice little boards; each group of 3 holes has a trace between them. Wiring up small breakouts like this is a breeze since everything is through hole, and there are no other connections on the back.

The trim pot for the LCD contrast is on this board as well. This is one part of the design that will probably change at some point. It is a pain to adjust since it is behind the LCD. Depending upon how easy it is to see the LCD outside, I may even have to put a full size pot (or a linear one) somewhere.

I removed the original headers from the GPS Logger Shield, and replaced them with polarized connectors. I figured that this would make maintenance and hookup easier. Ultimately I would like to have projects that are contained on shields or custom boards, but since I’m not sure what the end result of finding or building an enclosure will be, I decide to go with connectors for this one.

Since the unit can be powered from USB or battery, and since the battery will charge when the Arduino is plugged in, I built a small power distribution board to make sure that everything that needed power got it.

The Arduino’s 5v and ground are running to the LCD which in turn is forwarded to the power board. The battery plugs directly into the power board, and then feeds the Bluetooth and buttons.

Below are a few shots of the various cable runs. I really should get some more wire, in different colors, but they only had 3 colors at Sparkfun!

Connectors to the LCD, and power board.

Everything plugged in up top. Notice the new jumper for the GPS. I didn’t want to hard wire anything at this point. The white wires running to the right go to the buttons and Bluetooth.

Everything hooked up!

At this point there is still one more wire to hook up: I forgot to do the battery monitor wire.

I still have some software issues to tackle, but I think I’m going to get it bolted to something first. I’m planning to take the entire unit for a short hike on Saturday, so I’ll get to test it out.