Nuts and Volts - May 2010

■ A near space flight can climb to over 30 km, so it can experience a significant increase in UV-B but still not receive much UV-A. This graphic is from the Geophysical Institute at the University of Alaska, Fairbanks.

a lunar or Martian rover can use a sun sensor to determine directions in lieu of a compass (this is an issue for these two locations since neither the moon or Mars have global magnetic fields). A robot can also use a sun sensor to determine when the sun has risen or set — very important to a solar powered rover. If calibrated properly, a robotic rover might also use the sun sensor to determine the intensity of the sunlight. If you plan to use the sun sensor in robotics, I recommend drilling out the large mounting holes in the PCB (see January’s issue). Bolt two-inch long #4-40 machine screws to the PCB so the sun sensor can attach to the robot. Raise the sun sensor on a mast that keeps it above any other structure on the robot (to avoid shadows). (In the near future, I plan to write an article for SERVO Magazine about an articulated robot that uses the sun sensor in this way. So keep your eyes open.)

NearSys is in the process of producing a sun sensor kit for those who do not want to make their own PCB or gather the needed components. Go to NearSys.com/catalog for more information.

ADDING UVC TO THE ENVIRONMENTAL TEST CHAMBER

68 May 2010

In my Nov ‘09 column, I discussed how I incorporated ultraviolet into the simulator. However, I’m not totally happy with this UV source (UV LEDs) because its ultraviolet light is too close to the violet part of the visible spectrum. As many Nuts & Volts readers know, scientists divide the UV spectrum into three ranges: A, B, and C. Ultraviolet A, or UV-A, spans a wavelength range from 400 to 315 nm — this is the band of UV

■ A $10 source of hazardous ultraviolet. Just the thing for the near space environmental test chamber on a budget. Do not look at the UV from these lamps.

emitted by the affordable UV LEDs in my environmental chamber. Of the remaining two bands, UV-B spans from 315 to 280 nm and UV-C spans from 280 to 100 nm.

UV-A from the sun reaches the surface and is not usually considered very dangerous. However, it can cause indirect damage to our DNA (via the creation of free radicals, or highly reactive molecules). Too much damage can lead to cancers. UV-C is the just the opposite — it’s known to be extremely hazardous.

Fortunately, it’s blocked high in the stratosphere. Ozone in the stratosphere strongly attenuates UV-B, but some still reaches the surface. The small amount that does make it to the ground promotes the healthy production of vitamin D and is responsible for the sunburns we get each summer.

To improve the quality of my near space simulator, it needs to have a source of UV-B. LEDs capable of emitting this frequency are commercially available, but are too costly at this time. UV-C, however, is used as a germicide and is readily available in items like water purifiers for backyard ponds. Since I can’t get an inexpensive source of UV-B yet, I decided I’d settle for mixing some UV-A and UV-C (am I right in thinking that mixing UV-A and UV-C averages out to UV-B?). Actually, I initially thought I had found a good UV-B source when I ran across the UV source described this month. It wasn’t until after I had installed the lamps into my environmental chamber that I discovered they were really UV-C. Therefore, here’s my experience with adding another ultraviolet source to my environmental test chamber. Readers may find other uses for this inexpensive UV-C source.

The Den Tek toothbrush sanitizer is a plastic case containing batteries, an ultraviolet lamp, drive circuit, and power switch. The drive circuit boosts the voltage of its two AAA batteries to the 470 volts peak-to-peak it takes to operate the UV lamp (which is an uncoated mercury vapor lamp). So, let’s violate another warranty by bashing this commercially available product into something more useful.