University of Arkansas - AgriScience Project

AGRISCIENCE EXERCISE

PHYSICAL SCIENCES IN AGRICULTURE

Key Concept: Electricity and Electronic Systems

Sub-Concept: Electronics

Agricultural Application: Basic electronics principles are applied in common agricultural equipment.

Exercise: Constructing a Light Intensity Metering Circuit

Applied Principle(s): Basic principles of electricity

Goals:

  1. Read an electronic schematic.
  2. Assemble a light intensity metering circuit.
  3. Explain the function of each component in the circuit.
  4. Explain the basis for the circuit's operation.
  5. Describe possible circuit applications.


Materials:



References: Refer to the attached information sheet and/or any basic electricity or high school physics textbook for more information

Procedures for Conducting the Activity:

1. Divide the class into small groups of no more than 3 students each.

2. Provide each student with an instruction/data sheet, and allow the class time to read the entire activity and attached information sheet before proceeding.

3. Provide each group with the necessary materials.

4. Allow the student to proceed through the activity, providing assistance as necessary.

5. When all groups have finished, discuss the activity questions and results as a class. Discuss the practical applications of the knowledge gained through the activity.





AGRISCIENCE EXERCISE

- Constructing a Light Intensity Metering Circuit -

Student Data/Instruction Sheet

1. Read the attached information sheet before proceeding through this exercise, to gain a better understanding of the relationships that exist in this model.

2. Secure the following equipment and supplies:



3. Assemble the light intensity metering circuit as indicated in the schematic below. (Note: Properly set the DMM to measure DC voltage before connecting the meter in the circuit.)

























4. Test the circuit as indicated:

A. Place your hand over the face of the photocell and observe the reading on the DMM. What happened to the reading?





B. Remove your hand from over the face of the photocell and observe the reading on the DMM. What happened to the reading?



C. Place a box or other object over the photocell to completely block all light from reaching the photocell. Carefully adjust the potentiometer until the DMM reads zero (or as close to zero as possible).

D. Uncover the photocell. What is the reading on the DMM?



E. Shine a flashlight directly on the face of the photocell. What is the reading on the DMM?



5. Complete the following optional activities, as directed by your instructor.

A. Determine the relative light intensity of various areas in your school. (For example, compare your classroom with the agricultural mechanics laboratory, greenhouse and/or other areas in your school.)

B. If a commercial light intensity meter is available (perhaps from the science department), develop a calibration chart that will allow you to convert your voltage readings to lumens and/or foot-candles.

C. Measure, record and graph the light intensity of the same outdoor location for a one week (or longer) period.

6. Disassemble the light intensity metering circuit and return your supplies and equipment to your instructor.





BACKGROUND INFORMATION SHEET

- Constructing a Light Intensity Metering Circuit -

The light intensity instrumentation module is also a voltage divider circuit. The circuit is built around a cadmium sulfide photocell and a 10 K potentiometer. A photocell is an electronic device with a resistance that varies in inverse proportion to light intensity (resistance increases as light intensity decreases; resistance decreases as light intensity increases). The figure below is a schematic drawing of the light intensity instrumentation circuit.

As shown in the figure, the light intensity module uses the same basic circuit as does the temperature module, except the thermistor is replaced by a photocell. The source voltage is supplied by a 6-volt battery and the DMM is connected to measure the voltage across the potentiometer.

As light intensity increases, the resistance of the photocell decreases. This decreased resistance reduces the voltage dropped across the photocell. Thus, according to Kirchoff's Voltage Law, the voltage drop; across the potentiometer must increase. Since the DMM is set to measure the voltage across the potentiometer, an increase in light intensity is displayed as an increased voltage reading on the meter.

































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