Monday, April 08, 2013

Trailer Lights Physical Stuff

It's past time to get these lights actually built. The previous several posts have outlined the design and testing of an LED lighting system for the interior of a boy scout equipment trailer. Four steps to that.
1) PCB design and manufacture.
2) Circuit assembly.
3) Case design and manufacture.
4) Installation.
Let's take these each in turn.

PCB Design and Manufacture

The PCB layout for this circuit is straight-forward. A two layer circuit board suffices, with the bottom layer being a solid ground plane. I stuck with through-hole components for everything except the inductor. I kept the surface mount inductor that the manufacturer recommended; I am cautious about messing with the inductor. I used as wide traces as would fit.

I used Eagle PCB layout software. The software is straight-forward to use. The price is good. The instructions are great. I failed to install the Linux version; apparently the software is 32-bit and a large batch of libraries would have had to be installed on my 64-bit system. Instead, I used the Macintosh version; it installed easily. I had to make my own symbols and footprints for almost every component, which slowed things down a bit.

I used Silver Circuits to get the boards made. The price was good, the service was easy to use, the delivery was quick, the boards were of good quality. I would use their services again. 

Figure 1. Top and bottom of PCB.

Circuit Assembly

There isn't much too say about the assembly. Push the leads through the PCB then solder and trim the leads. The inductor took a bit more care. I melted some solder on to the inductor pads and the corresponding PCB pads then used a big chisel tip placed in between the two to melt the solder again. That worked well.

Figure 2. Top side of finished board.
Figure 3. Bottom side of finished board. Ground Plane.

Case Design and Manufacture

Let's leave the electronics and make this thing usable. I designed and printed a case in four parts. The criteria is a case that can hold a a UPS battery (I chose a CSB GP1272F2 battery because I had two different UPSes need a new battery, and both used this battery, so I figure this battery will be easily available). Holes are arranged so that if any water gets inside the case, it will have a clear path to continue out the bottom. The four bolts are longer than are easily available, but they can be ordered. The bolt holes are setup for 1/4" bolts. On the underside of the lid is the list of spare parts. I uploaded a model to Google's 3d warehouse

Battery: GP1272F2
Fuse: 3A
Bolts: 4-40x2"

The base takes about 28 hours to print on a MakerGear M2.

Figure 4. Prototype of 3d printed case.

I have enjoyed working with the M2. The learning curve was steeper than I expected. It turns out large, flat surfaces are hard to 3D print. I found a 1/4" lip drawn as part of the bottom layer and using the Hilberton curve for the bottom layer kept the corners down pretty well.


Installation must have been easy. I got a call that the troop's adult quartermaster wanted to install it in the trailer but I wasn't available to do anything more than drop it off and give a brief hookup explanation. He said it worked. I didn't even manage to get any pictures of the finished case before installation.

The troop will be at Camporee this weekend. That will be a good test of the charging system and well it holds up.

This finally finishes the trailer lights saga. When I get some pictures of the final box installed in the trailer, I will edit those pictures into this page. I'm not sure what comes next. I am considering doing a quick bias cell design.