The meter is a relatively old piece of hardware, and I knew its accuracy might not be perfect. In particular, I couldn’t count on voltage and needle position to map to one another linearly. In other words, if the needle could move through a 90 degree range of motion, it would be a mistake to assume that a 50% voltage would leave the needle pointing at the 45 degree position. So I wrote some calibration code that slowly swings the needle through its full range. The user hits a key when the needle passes the markers on the meter’s gauge. By remembering what the PWM setting was at the time of each keypress, a calibration file can be generated. In the future, you can just decide which number on the gauge you want to display, and the support library does some simple arithmetic to interpolate between recorded calibration points, putting the needle just where it ought to be.
So I had flexible meter. What did I want to measure? Well, a big part of Sunlight’s mission is measuring political influence, of course, and this seemed like a nice way to highlight the capabilities and limits of the data that government provides. So, with help from Jacob, I adapted one of the techniques used by his Lobbyist Registration Tracker. Every few minutes a Python script running on the Pi fills out a form on the Senate Office of Public Records website, and counts how many new registrations have shown up this week. Unfortunately, the SOPR site only seems to post new registrations once per day (though there are some strange exceptions to the usual schedule — we’re still waiting to hear back from them about why this is). Still, over the course of a week the numbers add up and the delays become less important. We sure would prefer real-time disclosure, though…
At any rate, the needle slowly ticks up from zero starting on Sunday morning. If it hits 90, an amber LED that I added to the meter turns on. And if the meter maxes out at 100, it turns red. With the software working, all that was left was to change the labeling on the meter to reflect its new capabilities. I scanned the original label, and in seemingly no time Amy had vectorized and modified it into the lovely variation you can see below:
Each Friday is PiDay here at Adafruit, be sure to check out our posts, tutorials and new Raspberry Pi related products. Have you tried the new “Adafruit Raspberry Pi Educational Linux Distro” ? It’s our tweaked distribution for teaching electronics using the Raspberry Pi. But wait, there’s more! Try our new Raspberry Pi WebIDE! The easiest way to learn programming on a Raspberry Pi.
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