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Building the meter was the most challenging part for me. I had very little experience fabricating anything when I set out to do this in 2013, and unlike writing code, there's no easy way to undo changes! I didn't have a step-by-step guide to follow and I don't have one to offer, but on this page I'll cover the parts I used and offer some information on how I put things together and some of the suggestions I have based on my experience.
The circuit is very simple:
The most important thing to note is that the resistors for the meters aren't correct. The meters reach 100% at 1mA of current, so at 5V the resistance should be:
5 / .001 = 5,000
5kΩ resistors aren't very common, but 4.7kΩ resistors are. That will give a little too much current:
5 / 4,700 = .00106
But that's okay, because it can be compensated for in the software, and that's exactly what I did. See the Arduino Code page for more information on that.
A simpler solution would be to use 0-5VDC voltmeters instead of ammeters, then no resistor is required.
This was one of my earliest attempts at making something like this, so it was very much a learning experience. I used a Dremel with the Circle Cutter & Straight Edge Guide accessory to cut the large holes needed for the meters. Despite double-checking my measurements, the holes came out a little too small, so I had to enlarge them freehand. If I knew what I do now, I would have backed the holes with some scrap wood and used that to guide the circle cutter for a second cut, but I didn't know to do that back then. Actually, knowing what I do now, I would have tested the cut on a piece of scrap before cutting the enclosure at all! Lesson learned.
I had a little trouble figuring out how to determine the mounting location of the two bolts that hold the meter in place, so I made a template from some scrap wood to use as a guide. That worked fairly well, though it wasn't quite perfect. But it was close enough, and like I said, I was pretty green when I did this. The good news is once it was all assembled, it looks good!
The meters ship with metal faces that indicate a reading from 0 to 1ma. Disassembling the meters is very easy using a small screwdriver, allowing for the faces to be replaced with custom ones. I scanned one of the original meters faces into my computer, imported it into Inkscape, and then drew new faces for CPU% and MEM% based on the scanned image. The designs I came up with are pretty basic as I don’t have much experience with Inkscape but I’m pretty happy with them. I printed the faces on card stock and used a hobby knife to cut them out and trim them to fit inside the meters.
The Arduino Leonardo and USB cable are mounted to the bottom portion of the Serpac enclosure, and the meters and LEDs are mounted to the top portion. The position of the USB cable was carefully chosen so the cable runs in between the two terminals attached to the meter that’s mounted above it. It's somewhat of a tight fit, but it fits.
The circuit is simple, so I skipped breadboarding the four resistors and just soldering them inline with the wiring. The resistors for the meters are soldered directly to the ring terminals which made them easy to implement. The wiring going the Arduino is soldered as well to ensure it doesn't come loose.
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