Hands On Physics
Testing the Sun Photometer
Now it is time to test the instrument, and align the sun target. When these
steps are successfully complete, your sun photometer is ready to collect
information about HAZE.
See tools and materials for a complete
list of the parts required.
Testing the TERC VHS-1
Before going on, be sure you have checked each step in the previous list.
Correct any errors before going further.
- Make sure the toggle switch handle is at the CENTER or LEFT position.
- Snap a 9-volt battery to the connector whose red lead is at hole F23.
Then insert the battery into the battery clip closest to the top of the
- Snap a 9-volt battery to the second connector. Then insert the battery
into the battery clip closest to the bottom of the case.
- Connect the probes of a voltmeter to the output terminals. Don't over
tighten the terminals or they might come loose.
- Set the voltmeter switch to read DC VOLTS. (CAUTION: At other settings
the voltmeter may be damaged.)
- Now apply power to the VHS-1 by pushing the toggle switch handle to
the RIGHT. The voltmeter will indicate a small voltage. If the voltage exceeds
several volts, immediately push the toggle switch to the CENTER or LEFT
position and remove both battery connectors. Then carefully review all the
wiring to find the source of the problem.
- When the circuit is working, shine a bright lamp or flashlight on
the LED. If the voltage displayed on the voltmeter increases when light
strikes the LED, your VHS-1 is ready to be aligned and tested outdoors.
Aligning the Sun Target
The two angle brackets on the side of theVHS-1 allow you to point the instrument
directly at the Sun. When you point the front end of the VHS-1 toward the
Sun, sunlight will enter the case through the sunlight port. Sunlight will
also shine through the hole in the angle bracket closest to the end of the
Your goal: Make sure that when the beam of sunlight from the sunlight
port is centered directly over the detector, the beam passing through the
front angle bracket always strikes a specific position on the lower bracket
which we will call the Sun Target.
Begin by placing a self-adhesive white label about the size of a postage
stamp on the rear angle bracket (the one closest to the 2 terminals). This
label is the Sun Target. Then take the VHS-1 outdoors or to a clean window
through which the Sun is visible and follow these steps:
NOTE: You may wonder why the alignment of the Sun Target may change
slightly when the VHS case is closed. The VHS case is more flexible when
open than when closed. Therefore, the alignment may change slightly when
you close the case.
- Open the case and point the sunlight port toward the Sun. As you move
the case in various directions, the beam of sunlight passing through the
sunlight port will form a bright circle of light on whatever it strikes
inside the case.
- Carefully adjust the position of the case until the circle of sunlight
falls directly over the detector. You should then see a bright circle of
sunlight on the paper label stuck to the rear angle bracket.
- Use a pencil to mark a small dot on the label at the center of the
circle of sunlight. Be sure to hold the VHS-1 steady while marking the label.
It's best if a friend holds the case and makes sure the circle of sunlight
is centered over the detector while you mark the Sun Target.
- Close the VHS case, connect a volt meter to the output terminals,
and turn the power switches on the voltmeter and the VHS-1 to the ON position.
- Point the VHS-1 toward the Sun until the Sun Target is centered in
a circle of sunlight. Now watch the volt meter readout while slowly moving
the circle of sunlight around the Sun Target. The voltage will jump around
as you move the instrument. If the sky is clear, you will notice that there
is a point at which the voltage is relatively stable and higher than at
other points. If the first mark you made on the Sun Target is not centered
in the spot of sunlight, make a second, darker mark in the center of the
circle of sunlight when the voltage reaches its highest point. Again, it
helps to have a friend hold the instrument while you make the mark.
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