Reading in Tenths

Lately, I've been putting some finishing touches on the solar power system. In spite of having many more important things I should be getting done, yesterday I spent pretty much the whole day building an upgrade to the kilowatt hour meter that monitors the alternating current output of my system.

The meter that has been installed in the system since the beginning is a standard utility-type kilowatt hours meter, the type with the rotating disc and little numeric dials with counter-rotating pointers to indicate the consumption (or production, in my case). The convention in reading these meter is that you ignore any readings in between the whole numbers, rounding down to the next whole number. Obviously, my system doesn't produce electricity in whole kilowatt units, I wanted a way to track production totals more accurately.

Standard kilowatt hour meters count the rotations of the disc. Each time the disc makes a full revolution, 7.2 watt-hours have been consumed/produced. Seven-point-two is a pretty odd number, so getting anything useful out of it was going to be tricky. For some time, I used an optical sensor to count the number of times the black paint spot on the bottom of the disc went by, incrementing a digital counter each time. At the end of the day, I'd use a calculator to multiply the counter reading by 7.2 to arrive at the total - accurate to the next-lowest 7.2 watts.

Not wanting to use a calculator any more, and mostly because on the new system, I didn't have a place to mount the digital counter that I was happy with, I decided to integrate the kilowatt-hour meter and the counter, and add some enhancements.

I started by applying 71 small strips of black vinyl tape to the underside of the disc. These, with the existing black paint spot added up to 72, which means that each time the optical sensor path was interrupted by black tape or the existing spot, the counter would increment by one, indicating that 1/10 of a watt-hour had been produced.

The counter fit nicely under the disc, and a small circuit board was mounted on the back side to hold the voltage regulator and current limiting resistors used to illuminate the optical reader, the "power" LED and run the counter itself.

The finishing touch was to personalize the meter by creating my own power company name:

The counter is reading today's production total, four thousand, four hundred, seventy-three and one-half watt hours, or 4.47 KwH, about 42 cents worth of power from my $10k renewable energy system! The dials on the original meter are reading the production total from the last six years, 5196 KwH, or 5.1 megawatt hours!

Since I needed to be able to reset the counter after it was read at the end of the day, a switch would need to be mounted where I could push it. I really didn't want to have to bring more wires out the back of the meter (it already has been modified to accept a wire carrying 24 volts DC to power the counter). Then I hit on an idea. Digging around in the junk box of parts, I found an old reed-type relay. Inside this relay was an evacuated glass reed switch that responds to a magnetic field. I placed the reed switch just under the digital display inside the meter dome. When I want to reset the counter, I hold a magnet up to the glass, and the counter resets to zero.

Nothing I do is ever easy.

Now that it's finished I think about doing something similar using the guts from an optical computer mouse, it could sense rotation direction -and- speed, which could allow me to make a calculation of instantaneous power output and display that as a number. I also wouldn't have to apply all those little bits of black tape, which drove me nuts and had to be done twice to come out right...

No, I'm not going to work on that tomorrow, interesting idea, though.