EE3300/EE5300 Electronics Applications
Week 4 Practical

Written by Luke McCarthy and A/Prof Bronson Philippa
College of Science and Engineering, James Cook University
Last updated 22 January 2026

Equipment

  • 2x LM358 dual op-amps.
  • 1x Motorised potentiometer (either pre-built 50 kΩ, or custom built 10 kΩ).
  • 1x 10 kΩ potentiometer.
  • 2x Breadboards (Likely you will need the extra space as today we build one large circuit).
  • 1x 2N3904 NPN BJT.
  • 1x 2N3906 PNP BJT.
  • 1x MJE2955 PNP Power BJT.
  • 1x MJE3055 NPN Power BJT.
  • Various capacitors.
  • Various resistors.

Instructions to students

  • Work individually on these activities.
  • Focus on neat circuit breadboarding.
  • Include power supply decoupling capacitors.

Exercise 1: Constructing a Motor Driver Circuit

Motivation

This week we explored different classes of amplifier and discussed some of their advantages and disadvantages. In Exercise 1 you will use a Class B amplifier (push-pull stage) to drive a motorised potentiometer.

Your task

Your task is to characterise a motorised potentiometer load, then construct and interpret a typical motor-driver circuit arrangement.

Procedure

  1. Familiarise yourself with your motorised potentiometer. Figure 1 illustrates the pinout configurations of two motorised potentiometer designs. (You will be given one of these).

Figure 1
Figure 1:

Pinout diagram for two motorised potentiometer designs.

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  1. Measure the resistance across the outer terminals of the potentiometer section and determine which pin will correspond to the wiper.

    Use your multi-meter to measure the overall resistance and from the wiper to one end of the potentiometer, as you rotate the motor arm manually, to verify the range of the outer legs over which the potentiometer can sweep.

  2. If not already present, attach a small piece of tape to the arm of the motorised potentiometer. This will act as a flag and make it easier for you to see the motor rotating and in what direction.

  3. Next, rotate the arm of the motorised potentiometer to the approximate middle of its range of motion. You can do this by rotating all the way clockwise/anticlockwise and then moving back the other way slightly.

    Begin with a small DC voltage across the motor pins. Slowly increase the voltage magnitude until the motor starts turning. How much current is being drawn from the power supply? What happens when you reverse the input polarity (swap the DC connections around)?

    👉 Write down the required voltage and current and your comments about the requirements to drive this motor in your portfolio.

  4. Construct the circuit shown in Figure 2. Note: The motorised potentiometer draws the most current of any load we’ve covered thus far in this subject. This means you may need to increase the current limit dial on your DC power supply to allow more current to flow from the supply. We will discuss this current in more detail in the next section, but for now you should choose a value that will protect your equipment and allow your motor to turn.

Figure 2
Figure 2:

Motor driver circuit.

If you power on the circuit without Vin connected, the output signal may swing to one of the rail voltages (or slightly less e.g. +/- 13V). Why might the output swing high (or low) with the non-inverting input floated?

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  1. Connect a 1 Vpp, 0.5 Hz square wave to the circuit (as the signal labelled ) and observe the motorised potentiometer.

    👉 What happens to the arm of the motorised potentiometer? What happens as you increase the signal amplitude? What about if you increase the frequency? Based on your experiments, answer the questions in the portfolio.

Exercise 2: Designing a Current Limiting Protection Arrangement

Motivation

Another big part of robust circuit design is using hardware designs to enforce circuit protection. In this activity, we will design circuitry to limit the possible output current, for example, to protect the output transistors in the event of a short circuit at the output.

Your task

Modify your circuit design to implement current limiting protection.

Procedure

  1. Study the current limiter circuit design in Figure 3.

    👉 Answer the questions in the portfolio.

Figure 3
Figure 3:

A push-pull output stage with additional functionality for current limiting.

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  1. Build the circuit onto the output stage of your motor driver circuit but do not turn it on yet.

  2. BE VERY CAREFUL WITH THIS STEP AS WE WILL BE CREATING A FAULT CONDITION. As a secondary line of protection, set a current limit on your bench power supply (slightly higher than your own circuit’s limit).

    With the power turned off, deliberately create a fault condition by shorting the motor terminals together. Turn the power on (briefly) and observe the current reading on the power supply. If the power supply entered “constant current” (CC) mode, then your own limiter did not work. Turn the power off and check your circuit.

    You should see the current limiting being implemented by your circuit, meaning that the power supply will stay in “constant voltage” (CV) mode, and the current will match the limit that you chose when you designed the circuit.

    👉 Save a photograph of your current limiting circuit in action, showing the power supply in constant voltage mode with the current matching your chosen limit.

Exercise 3: Constructing a Proportional Position Controller

Motivation

Now that we have a robust, protected motor driver output stage, we will build a simple position controller circuit. Based on your past practicals and Exercise 1, you should have noticed that an applied voltage across the DC motor turns the arm with a speed proportional to the applied voltage magnitude. In this exercise, we will use a control arrangement to synchronise a position input with the position of the motorised potentiometer output.

If you’ve had some exposure to control systems, you may be familiar with the concept of a PID (Proportional, Integral, Derivative) controller. We will build just the proportional section today.

Your task

Your task is to construct a proportional motor controller with feedback from the motorised potentiometer.

Part 1: Understanding the Proportional Controller

  1. Study the circuit in Figure 4.

    👉 Answer the questions in the portfolio.

    Build this circuit on your breadboard. When choosing your breadboard layout, consider that you will soon need to connect the output of your controller stage to the input of your previously built motor driver circuit. Note: Because we are using the LM358 dual op-amps, you can build this circuit using only one IC.

Figure 4
Figure 4:

Proportional controller circuit.

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Part 2: Combining with the Motor Driver

  1. Build the open-loop configuration shown in Figure 5. In this configuration, we will use a 10k potentiometer at our input to set the desired speed of the motorised potentiometer.

    Note: You will need to vary the values of the output series resistors (that go from the motorised potentiometer to the rails) based on the resistance of your motorised potentiometer. As depicted in Figure 5, a 50k potentiometer should use ~22k resistors, whereas a 10k potentiometer should use 4.7k resistors.

    Confirm that the circuit works correctly. Use two channels of your oscilloscope to observe the input/output behaviours as you move the input potentiometer. You may gain further insight from measuring the output of the motorised potentiometer wiper and then the voltage across the motor.

Figure 5
Figure 5:

Open-loop configuration with proportional controller connected to motor driver.

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  1. Finally, modify your circuit to use the closed-loop configuration shown in Figure 6. You will need to change the feedback resistor in the gain stage from 220k to the much smaller 22k resistor, as depicted in the red box in Figure 6. Disconnect the inverting input 100k resistor from ground and instead connect to a feedback path from the wiper of the motorised potentiometer, again depicted in Figure 6.

    Vary the input potentiometer. You should see that the motorised potentiometer arm moves to match the position of the input potentiometer, i.e., you have built a control system that synchronises the position of the controlled motor with the configured input position.

    👉 Show photographs of your circuit and its behaviour in the portfolio.

Figure 6
Figure 6:

Proportional controller with feedback from motorised potentiometer.

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Portfolio template

Instructions: copy and paste the following template into a document. Answer each question as you work through the practical. Use that document when you ask to be marked off for this practical. You also need to include these results as part of your final portfolio submission at the end of the study period.

Exercise 1: Motor drive requirements

Make a note of the required drive voltage and current to make the motor just start to turn. Also comment on the behaviour when you drive a square wave into the driver circuit. Answer the following questions:

  1. Based on your findings, choose the magnitude of input signal that is suitable for driving the motorised potentiometer.
  2. Why do we use power transistors for our push-pull stage, rather than the 2N2904/6 models that we’ve used in previous practicals?
  3. What is the role of the resistor and capacitor (10 Ω + 100 nF) that is connected across the motor? (Hint: think about the behaviour of that branch in the limit of DC frequencies and in the limit of high frequencies.)
Exercise 2: Current limiting protection circuit

With reference to Figure 3, answer the following questions:

  1. What are the roles of the 2N3904/6 transistors?
  2. The 2N3904/6 are small signal transistors. Why is it appropriate to use small signal transistors for this circuit, rather than the power transistors of the push-pull stage?
  3. Determine a current limit you would like to set for your protection circuit, based upon the measurements you made in Exercise 1. Your chosen limit must permit enough current for your motor to turn but remain below the rated limits for your circuit components.
  4. Calculate the size of resistors R1 and R2 in order to set the desired current limit (i.e., choose a resistor size so that the 2N3904/6 transistors will turn on when the current limit is reached).

Show evidence of your current limiting circuit working correctly.

Exercise 3: Proportional position controller

With reference to Figures 4, answer the following questions:

  1. What is the role of the left-side op-amp with the 100 kΩ resistor configuration?
  2. What is the role of the right-side op-amp with the 220 kΩ feedback?

Show photographs of your final circuit breadboard, and evidence that it works as a control system to track the desired position. A nice demonstration might be to use two oscilloscope channels to show the voltages from the input and sense potentiometers and show how one tracks changes in the other.

Finally, answer the following interpretation questions:

  1. What do you notice about the movement of the motorised potentiometer arm before and after connecting your feedback path?
  2. Why is it important that we reduce the effective gain of the proportional controller before connecting our feedback path?

Conclusion

Your tutor will mark you off for completing this activity and being able to discuss the results. If you do not finish on time, bring your completed portfolio to a subsequent lab session for marking.

When you leave, make sure that the lab is just as neat or even neater than when you arrived.

Acknowledgements

This activity was adapted from Lab 10 in Learning the Art of Electronics by Hayes and Horowitz, with current limiting design based on Chapter 9 of The Art of Electronics by Horowitz and Hill.