July 5th, 2010
One day, while changing the batteries on a little pocket laser, I realized that it required very little power to operate, which gave me an idea: shouldn’t it possible to run it all day long on batteries charged by solar panels? I went to work and did some measurements: my pocket laser required 14 mA at 5 volts to operate. That’s 70 mW to operate a laser with an output of only a couple of mW, which is extremely inefficient, but it’s still so tiny that solar cells could easily generate lots of power. So I built a box for the laser, stuffed it full of NiCad batteries, and bought some solar cells. The solar cells output 200 mA at 6 volts in full sunlight; at that rate, one hour’s charging can run the system for 14 hours, and with the large batteries (which can hold several days’ worth of charge), I should have plenty of power. My biggest concern was making the system secure against rain, snow, and insects, so I built a solid little box with plenty of caulking and multiple coats of paint. Then I mounted it all on a big log I planted in the ground:
The box containing the laser and batteries is on top; the solar cells are facing due south just above the altitude of the sun at winter solstice. After all, there’ll be plenty of sunlight in the summer, but winter is when the system will be most strained for power because the days are shorter and the sun is lower. So I optimize the system for winter, not summer.
The next task was to aim the laser at a designated point. I chose a point on the driveway about 100 yards away from the laser. The beam at that distance is only about 6 inches in diameter, so it must be aimed with extreme precision: about six minutes of arc (the full moon is 30 minutes of arc in diameter, to give you an idea of how tricky this is). So I built a system for aiming it precisely. Here’s a closeup of the mounting system:
Note the hinge on the left underneath the box, and the bolt head in shadow underneath the box. The vertical angle of the box is controlled by turning that bolt; adjustments of just one-sixth of a turn are enough to raise or lower the beam by 6 inches. With a lot of effort, I was able to get it aimed correctly. If you stand on a painted marker on the driveway and you’re my height, then the beam will hit you at eye level. Here’s what it looks like if you’re just 6 inches off the beam:
If you’re twelve inches or more off the beam, then you see no laser light at all. Here’s what it looks like when you’re right on the beam:
Here’s a telephoto shot of the same scene:
As you can see, it’s quite stunning – but only if you’re dead on the beam. You have to be looking right at it when you’re at exactly the right position. Driving in a car, the best I ever notice is a quick flash as I drive past the target point – and that’s only because I know where to look. Here’s the shot as seen from above:
The driveway is about ten feet wide.
What’s it for? I don’t know. Think of it as landscape sculpture of a peculiar character.
Addendum November 8th, 2011: I redesigned and built a new meadow laser: Meadow Laser 2.0!