Wilteq WILTEQArduino Uno Course
Week 6 · Drivers & Motors

Session 26: Driving loads with transistors and MOSFETs

Teach & build120 minLesson 16: Driving Loads: Transistors, MOSFETs & Relays

Learning outcomes

  • Explain why an Uno pin cannot drive a motor, relay or fan
  • Wire a logic-level MOSFET low-side switch with gate resistors and a flyback diode
  • Measure VDS and capture the turn-off spike on the scope
  • Choose between an NPN transistor and a MOSFET for a load

Kit and components

  • Uno, logic-level N-MOSFET (for example IRLZ44N), 220 Ω and 10 kΩ resistors, 1N4007 diode
  • Small DC motor or fan and a separate supply
  • Optional: BC337 NPN transistor and 1 kΩ base resistor
  • Multimeter and two-channel oscilloscope

Pre-reading about 25 min

Students read these on their phones before class. They are listed on the student Before class page.

  1. Lesson 16: Why loads need a driver
  2. Lesson 16: Logic-level N-channel MOSFET
  3. Lesson 16: Flyback diode
  4. Theory Reference: High-current output

Session plan

  1. 0:0010 min
    Recap

    Task A reflections

  2. 0:1020 min
    Demo

    From transistor theory to an Uno pin

    Build on what students already know about transistors: 5 V gate drive, logic-level parts, common GND, and why inductive loads need a diode.

  3. 0:3040 min
    Build

    MOSFET fan driver

    Students build the lesson circuit and run the worked sketch's PWM ramp, checking common GND and diode direction before power.

  4. 1:1035 min
    Measure

    Verify the switch

    Students measure VDS at full power, then capture gate and drain on the scope with the flyback diode fitted. Facilitator demo only: briefly remove the diode on your own bench so the class sees the turn-off spike - students never remove theirs.

  5. 1:4515 min
    Wrap-up

    NPN or MOSFET?

Checkpoint: ready to move on?

Each student's fan ramps up under PWM through a MOSFET with a correctly oriented diode, and they have recorded VDS and a turn-off capture.

Extension for fast finishers

Drive a relay coil with a BC337 and compare the base current with the MOSFET gate current. Or, in Code Along, give the fan a soft start and a thermostat with hysteresis.

Catch-up path

Rebuild from the lesson's wiring list step by step; a partner checks the diode and GND before power.

Homework and practice

Pre-reading for S27.

Facilitator notes

  • Students already know the transistor theory - keep the demo short and spend the time measuring.
  • The no-diode capture is a facilitator demo: remove the diode only briefly, only with a small motor, with the scope on 10x probes. Students keep their diodes fitted.
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