AGRISCIENCE EXERCISE

PHYSICAL SCIENCES IN AGRICULTURE

Key Concept: Electricity and Electronic Systems

Sub-Concept: Electronics Applications

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

Exercise: Constructing and Using an Electronic Polarity Indicator

Applied Principle(s): Basic principles of electricity

Goal:

  1. Read an electronic circuit schematic
  2. Calculate the resistor value necessary to limit current flow through an LED to a specified value.
  3. Assemble an electronic polarity indicator.
  4. Explain the basis for the indicator's operation.
  5. Use the indicator to determine the polarity of low voltage (<6V) DC power sources.


Preparation Time:

Materials:(per group)



References: Attached information sheet

Teacher Preparation Notes:

*The "mystery box" is a sealed box containing four, 6 volt batteries. A pair of wires (one black and one red) are taped together and extend from each battery to the outside of the box. Once students have constructed their polarity indicators, they will use them to determine the polarity of each pair of wires in the "mystery box."

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 students 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 and Using an Electronic Polarity Indicator -

Student Data/Instruction Sheet

1. Read the attached information sheet before proceeding through this exercise, to gain a better understanding of the principles being illustrated.

2. Secure the following equipment and supplies:



3. Calculate the required resistor value using the formula below:

Rvalue = Input voltage - LED voltage

LED forward current rating

Note: Specifications can be found in the LED data sheet.



4. Assemble the polarity indicator circuit as indicated in the schematic below.































5. Determine the operating characteristics of your polarity indicator using the 6 volt battery. Attach the tester's ground wire to one terminal of the battery; touch the probe to the other battery terminal. Observe the LEDs. Reverse the probe connections and observe the LEDs. Use your observations to complete the reference table below.



LED Condition Probe to POSITIVE Probe to NEGATIVE
Red LED ON OFF ON OFF
Green LED ON OFF ON OFF




6. Based on the reference table you just developed, write a "decision-rule" that you can use with your polarity indicator to identify the wires attached to the positive and negative terminals of the batteries in the "mystery box."

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7. Use your polarity indicator and the decision-rule just developed to determine the polarity of each pair of wires extending from the "mystery box." Record your results in the table below.

Battery Red Wire Black Wire
A Positive OR Negative Positive OR Negative
B Positive ONegative Positive OR Negative
C Positive OR Negative Positive OR Negative
D Positive OR Negative Positive OR Negative




8. Check the accuracy of your results by opening the "mystery box" and comparing the actual polarities to the ones in your table. Were your results correct? If so, good! If not, can you identify the cause(s) of any errors?

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9. Disassemble the circuit and return all of your supplies and equipment to your instructor.



10. Answer the following discussion questions. Be prepared to share your answers in a later class discussion.

a. What is the purpose of the series resistor in the polarity indicator circuit?

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b. What do the terms "forward-biased" and "reverse-biased" mean?

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c. Which LED in the circuit schematic is forward biased? Which LED is reverse-biased?

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d. A light emitting diode (LED) is a special type of diode. Briefly explain the function of a diode.

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e. Describe the basis of operation of the polarity indicator you constructed.

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This activity is provided as a service of the AgriScience Project at the University of Arkansas- Fayetteville.

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