Near Space
by L. Paul Verhage
Approaching the Final Frontier
Near Space
Making a Light Sensor for the HOBO Data Logger
By using a photocell (CdS) as
one element in a voltage
divider, you can construct a
simple light sensor. Because it uses
a photocell, the light sensor’s
spectral sensitivity is very similar to
that of the human eye. After you
finish reading about how to build this
light sensor for your HOBO data
logger, I’ll explain a very interesting
finding about designing voltage
divider-based sensors.
Any two resistors wired in series
with a voltage source (battery) form
a voltage divider. In the voltage
divider, the voltage dropped across
one resistor element is proportional
to its resistance in sum with the
second resistor element. The voltage
drop across the resistor of interest is
given by the following formula:
condition. Now, by measuring the
voltage drop across the variable
resistor, you can measure the
environmental variable of interest.
The cadmium sulfide (CdS)
photocell is a light sensitive resistor.
Its resistance decreases when
exposed to bright light and
increases when exposed to dim
light. The photocell responds very
quickly to changing light conditions,
but not as fast as a phototransistor
or photodiode.
You will need the following
components to make a CdS light
sensor for your HOBO data logger:
Note: The value of the fixed
resistor depends on the resistance
range of the CdS cell. For my light
sensor, I used a 1.5K resistor. At the
end of this article, I’ll explain how
you can determine the best resistor
value for your particular CdS cell.
• Cadmium sulfide cell (My particular
cell has a resistance that ranges
from 100 Ω in bright light to 20K in
the dark.)
Vdrop = Vapplied X (Ri / Ri + Ro)
• Fixed resistor (A 1/4 watt resistor
is sufficient.)
In this formula, Ri is the resistor of
interest and Ro is the other resistor.
By itself, the voltage divider
circuit is not very interesting.
However, things do get interesting
when one element becomes variable
and changes its resistance due to
changes in some environmental
• 3/32” stereo jack kit
• Thin heat shrink tubing
Each input to the HOBO is
through a 3/32” stereo jack. The
stereo jack has three contacts: tip,
ring, and base. Voltage to operate
the sensor comes from the tip. The
signal to be digitized is connected to
the ring of the stereo jack and the
ground is connected to the base. The
diagram in Figure 1 illustrates the
connections.
I placed the fixed resistor close
to the stereo jack and ran a long
extension out to the CdS cell. I
decided to keep the fixed resistor
close to the HOBO to minimize the
amount of wire my sensor needed.
Note that the HOBO is digitizing the
voltage drop across the CdS cell
because the ground is connected to
one end of the CdS cell and the
signal is connected to the other end.
• #24 AWG stranded wire (preferably
three colors, to keep the wires
differentiated)
Figure 2. The completed light sensor
— minus the ping pong ball.
Figure 1. CdS light sensor diagram.
F
o
r
E
l
e
c
t
r
o
n
i
c
s
NUTS & VOLTS
E
v
e
r
y
t
h
i
n
g
20
JULY 2004