- Difficulty
- Beginner
- Build time
- 45-70 min
- Estimated cost
- $0-$12
- Age range
- 11-16
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The guarded fan starts from a switch, runs for one minute without warming, and moves a tissue strip at least 20 cm away.
Learning goals
- Identify how switch signal and 3-volt battery current produces motor rotation and directed airflow.
- Construct and explain a electrical energy-to-rotary blade motion system.
- Measure how blade pitch changes performance.
- Diagnose losses caused by bearing friction and blade drag.
Before you build
Materials, tools, and safety
Reuse-material cost: Usually under $5 with an existing kit. Supervision: Adult guidance recommended for wiring and cutting.
Tools
- Small screwdriver
- Wire stripper
- Multimeter
- Low-temperature glue gun or tape
Low-cost swaps
- Use alligator-clip leads for a no-solder version.
- Build and test the mechanism manually before adding electronics.
- Use a commercial guarded 3 V motor-and-propeller module rather than making a blade.
Wiring table
| From | To | Purpose |
|---|---|---|
| Battery +3 V | Motor + | Supply motor voltage |
| Motor - | MOSFET drain | Switch motor return current |
| MOSFET source | Battery negative | Complete common return |
| Diode | Across motor, stripe to +3 V | Clamp inductive voltage |
| Button and 10 kΩ resistor | MOSFET gate and ground | Create a defined on/off command |
Project-specific safety
- Use only the listed low-voltage battery supply; never use mains electricity.
- Disconnect power before changing wires and stop if a motor, wire, or battery becomes warm.
- Never run an exposed blade; disconnect power before adjustments, secure the guard, and stop if the motor, transistor, wire, or battery becomes warm.
Orient the build
Place the build so switch signal and 3-volt battery current is on your left and motor rotation and directed airflow is on your right. Call the side facing you the front, the far side the back, the tabletop the bottom, and the opposite face the top.
Build it
Step-by-step instructions
Step 1
Build the motor stand
Clamp the motor body in a low cardboard cradle without covering ventilation holes.
Point the shaft horizontally.
Step 2
Install the guard
Fix a rigid mesh cylinder around the full blade path with 15 mm clearance.
Make the guard removable only with tools.
Step 3
Wire the power stage
Connect battery positive to motor positive and motor negative to MOSFET drain.
Leave the battery pack switched off.
Builder checkpoint: After wire the power stage, the first subassembly should stay aligned when handled gently.
Step 4
Add protection
Place the diode across motor terminals with its stripe at positive.
Connect source to battery negative.
Watch for: If this stage binds or drifts, inspect imbalance vibration before adding more parts.
Step 5
Build the button input
Connect the button from battery positive to gate and 10 kΩ from gate to ground.
Label the common ground.
Step 6
Check before blades
Use a multimeter for shorts, then pulse the bare motor for one second.
Confirm correct direction and no heating.
Builder checkpoint: After check before blades, operate the build slowly and confirm that motor rotation and directed airflow begins without binding.
Step 7
Fit and enclose the propeller
Press the balanced propeller onto the shaft and close the guard completely.
Rotate by hand to confirm clearance.
Step 8
Run airflow trials
Operate for 10, 30, then 60 seconds while checking temperature.
Measure the farthest tissue movement.
Builder checkpoint: At the final checkpoint, The guarded fan starts from a switch, runs for one minute without warming, and moves a tissue strip at least 20 cm away.
See the engineering
Why it works
- Input
- switch signal and 3-volt battery current
- Output
- motor rotation and directed airflow
- Motion
- electrical energy-to-rotary blade motion
- Energy losses
- bearing friction, blade drag, electrical resistance, imbalance vibration
Why this works
Motor-driven airflow
Current in the motor windings creates magnetic forces that turn the shaft. Angled blades push air backward and produce a forward airflow through the guard.
Look for: Hold a tissue strip at measured distances and compare movement with shallow and steeper blade pitch.
Where the energy goes
Efficiency and losses
The ideal model leaves out bearing friction, blade drag, electrical resistance, imbalance vibration. These effects turn some input energy into heat, sound, vibration, or unwanted motion, so measured performance will be lower than an ideal calculation.
Look for: Run the build slowly and locate the first place where bearing friction becomes visible or audible.
Math bite
Estimate motor electrical power
Formula: power = voltage × current
- Voltage = 3.0 V
- Running current = 0.20 A
Substitute: power = 3.0 × 0.20 = 0.60 W
Result: The motor draws about 0.60 watts electrically.
Only part becomes useful airflow.
Current changes with blade load and battery condition.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Pulse the bare motor for one second before fitting the guarded propeller.
Success looks like: The fan runs one minute without warming and moves tissue at 20 cm.
Measure: Current, temperature by careful touch after power-off, noise, vibration, and airflow distance.
Change: blade pitch
Keep constant: motor, voltage, guard, tissue, room, and run time
- shallow pitch
- medium pitch
- manufacturer propeller pitch
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The motor does not start | Gate is low or ground is missing | Measure gate-to-source voltage while pressed | Reconnect button and common ground |
| The transistor warms | MOSFET is unsuitable or motor is stalled | Measure current with blade removed | Use a rated logic MOSFET and clear obstruction |
| The fan vibrates | Propeller is bent or off-center | Rotate slowly by hand | Replace and reseat the balanced propeller |
| Airflow is weak | Rotation is backward or pitch is low | Check tissue on both guard sides | Reverse motor leads with power off or use correct propeller |
Choose your tradeoff
Reduce vibration and guard rubbing before increasing pitch. Steeper blades can move more air but demand more current and may stall a small motor.
Keep experimenting
Try another version
Commercial module
Measure airflow from a pre-guarded fan.
Two-speed control
Add a rated series resistor or PWM controller.
Efficiency proxy
Compare airflow distance with measured electrical power.
Build together
Classroom and access options
Classroom version
Teams can compare blade pitch while keeping motor, voltage, guard, tissue, room, and run time. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Color-code and label every wire at both ends.
- Use clip leads, larger controls, and pre-crimped connectors when fine motor work is difficult.
- Use a large pushbutton, pre-crimped leads, and a tactile mark on the guarded airflow side.
Reflect on the design
- How did blade pitch change the measured result?
- Where did bearing friction affect the build most strongly?
- What evidence shows that motor-driven airflow explains the motion?
- Which change would improve motor rotation and directed airflow without creating a new problem?
Glossary
- Motor-driven airflow
- Current in the motor windings creates magnetic forces that turn the shaft.
- Input
- The action or energy supplied to a system; here it is switch signal and 3-volt battery current.
- Output
- The useful response produced by a system; here it is motor rotation and directed airflow.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
One build can start the next.
Share what you learned, change one variable, and help another builder understand what worked.
Explore more guidesSources and build notes
A platform-agnostic low-voltage robotics or electronics project with original assembly guidance.
- Low-voltage design review: Battery voltage, polarity, component roles, current paths, and motor or LED protection were editorially checked.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.