Hands On Physics

Core Project
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.
  1. Make sure the toggle switch handle is at the CENTER or LEFT position.

  2. 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 case.

  3. Snap a 9-volt battery to the second connector. Then insert the battery into the battery clip closest to the bottom of the case.

  4. Connect the probes of a voltmeter to the output terminals. Don't over tighten the terminals or they might come loose.

  5. Set the voltmeter switch to read DC VOLTS. (CAUTION: At other settings the voltmeter may be damaged.)

  6. 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.

  7. 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 case.

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:
  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.
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.

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