University of Arkansas - AgriScience Project

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 a Temperature Measuring Circuit

Applied Principle(s): Basic principles of electricity

Goals:

  1. Read an electronic schematic
  2. Assemble a temperature measuring 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: (per group)

References: Attached information sheet

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.




AGRISCIENCE EXERCISE

- Constructing a Temperature Measuring Circuit -

Student Data/Instruction Sheet

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

2. Secure the following equipment and supplies:

3. Assemble the temperature measuring circuit as indicated in the schematic below. (Note: Make sure the DMM is properly set to measure DC voltage before connecting the meter in the circuit.)



































4. Test the circuit as indicated:

A. Grip the thermistor between your forefinger and thumb and observe the DMM. What happens to the reading?



B. Remove your hand from the thermistor and observe the DMM. What happens to the reading?



C. Place the plastic bag of ice on the thermistor and observe the DMM. What happens to the reading?



D. Remove the bag of ice from the thermistor. Carefully place the heat source near the thermistor and observe the DMM. What happens to the reading?



E. Carefully remove the heat source. Observe the DMM. What happens to the reading?



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

A. Determine the relative temperature 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. Place the temperature measuring circuit near the thermostat in your classroom (or other school location). Determine the DMM reading when the heat (or air conditioning) comes on and goes off. See how close (in seconds) you can come to predicting the beginning and end of the next heating (or air conditioning) cycle.

C. Develop a calibration chart that allows you to convert your voltage readings to temperature readings.

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

6. Disassemble the temperature measuring circuit and return your supplies and equipment to your instructor.















TEACHER BACKGROUND SHEET

- Constructing a Temperature Measuring Circuit -

The temperature instrumentation module is a voltage divider circuit build around a thermistor and a 10,000 ohm (10K) potentiometer. A thermistor is an electronic device

having an electrical resistance that varies inversely with temperature (resistance increases with a decrease in temperature; Resistance decreases with an increase in temperature). A potentiometer is a variable resistor that can be adjusted manually to a specified resistance value within its operating range.

As shown in the figure below, the two circuit loads (thermistor and potentiometer) are connected in series. The circuit (source) voltage is supplied by a 6-volt battery. The DMM is set to read DC voltage and the meter leads are connected across the terminals of the potentiometer.

According to Kirchoff's Voltage Law, the sum of the voltage drops in a series circuit will be equal to the source voltage. Furthermore, the voltage drop across each load in a series circuit will be directly proportional to the load's resistance.

Kirchoff's Voltage Law explains how the temperature instrumentation circuit works. As temperature increases, the resistance of the thermistor decreases. Thus, the voltage dropped across the thermistor also decreases (since voltage drop across a load is proportional to the load's resistance). Since the sum of the voltage drops in the circuit must be equal to the source voltage, the voltage drop across the potentiometer must increase as the voltage drop across the thermistor decreases. Since the DMM is set to measure the voltage across the potentiometer, an increase in temperature is digitally displayed as an increased voltage reading on the meter.

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