Near Space
CdS
Lay out the components of the light
sensor on a table. Be attentive to the fact
that you may want to locate the light sensor
some distance from the HOBO. This
allows you to store the HOBO well inside
the near spacecraft (NS craft), where it
will remain warmer and still be able to
expose the CdS cell to the elements. In
my light sensor, I made the cable two feet long.
I’ll refer to red wire for 2.5 volts, white for signal, and
green for ground. Adjust my directions for any color
changes in your light sensor. Cut the wires to length and
strip about 1/4” of insulation from one end of each wire.
From the remaining ends of the wires, you can strip 1/2”
of insulation. Twist the strands of the wires and tin them.
The short ends of the wires are soldered to the 3/32”
stereo jack. The red wire goes to the tip, the white wire to
the ring, and the green wire to the base. There’s not a lot
of room to work on the stereo jack, so work slowly and
avoid shorting it out. After soldering the wires to the stereo
jack, use a DMM to ensure there are no shorts.
Cut the leads of the fixed resistor and CdS cell to about
1/2” and tin the leads. Slide a length of heat shrink tubing
over the red wire. Hold the red wire against one lead of the
fixed resistor and heat both wires with a soldering iron.
Solder will flow from the tinned lead and wire, soldering
them together. Let the solder cool and cover the
connection in heat shrink. Repeat this process with the
green wire and one lead of the CdS cell.
Determine where the fixed resistor will solder to the
signal wire. At that point in the white wire, use wire strippers
and cut a 1/2” band of insulation. You’ll need to use a
sharp Exacto knife to remove the band of insulation from
the wire. Do this carefully or your light sensor will suffer
from nicks. Slide a larger diameter heat shrink over the
resistor and the area where it solders to the signal wire.
Slide a short length of thin diameter heat shrink over
the white wire and solder the remaining end to the free lead
of the CdS cell. At this point, your light sensor is complete.
However, there is a problem with the current design. The
CdS is sensitive to its pointing direction. This may not be
a problem in some cases, but, when you want to measure
the brightness of the sky, it becomes a problem when the
NS craft rotates the CdS cell into and out of the sun. Here’s
my solution to this problem.
A photographer’s light meter records the average light
background by using a diffuser. The diffuser is a hemisphere
of white plastic (glass?) covering the light-sensitive element
of the light meter. After giving it some thought, I concluded
that a ping pong ball can make a great diffuser. So, I used
an Exacto knife to drill a small hole in the surface of a ping
pong ball. I made sure to drill the hole through the portion
of the ball that was stamped with lettering.
This left the rest of the unmarked ping pong ball to
diffuse sunlight. The hole I drilled was made just large
enough to admit the CdS cell. After placing the CdS cell
JULY 2004
1,000
1,200
1,400
1,600
1,800
Fixed
100
= 2.5*[$A$3/($A$3+B2)]
20,000
= 2.5*[$A$4/($A$4+B2)]
Range
= +B4-B3
Table 1
just inside the ping pong ball, I glued it into place with hot
glue. The final product reminds me of a large eyeball with
a copper-based optic nerve. In one of my NS missions this
year, I plan to dangle the eyeball from beneath the bottom
module of the NS craft and record the voltage across the
CdS cell.
Calibration
I have yet to find a data sheet explaining how the resistance
of a typical CdS cell varies according to light intensity.
Eventually, I’ll experiment with the light sensor to find out. I’ll
record the voltage drop across the CdS cell in a dark room
as I bring a light source closer to the eyeball. Remember that
light intensity drops off as 1/r 2. So, when the light source is
brought to one half the distance away from the sensor, the
light intensity increases by a factor of four.
Circle #134 on the Reader Service Card.
21
