Hand Crank Generator Experiment: Use an Old Motor

The thing to listen for is the moment your kid says “wait, it got heavier.” Crank the motor with nothing attached and it spins freely. Touch the two wires to an LED and the handle suddenly resists. Nothing was added. No extra mass, no brake, no friction. The resistance appeared because the LED started taking energy out, and the only place that energy can come from is your arm. That is the entire first law of thermodynamics delivered through a wrist, and it is why this hand crank generator experiment is worth 40 minutes. Everything here generates its own electricity from muscle. Nothing touches a wall socket.

Key Takeaways

• You’ll turn a small DC motor into a generator with a rubber-band step-up drive, light an LED, and measure voltage against cranking speed.
• It teaches Faraday’s law, that a motor and a generator are the same machine, and Lenz’s law as a force you can feel.
• Best fit is ages 10 to 15. Below about 10, the build is fiddly, though the “it got heavier” moment still lands.
• The usual failure is cranking the motor shaft directly. Small motors need thousands of revolutions per minute to make useful voltage, so you need a speed-up drive.
• A snapping rubber band is the real hazard here. Safety glasses, and keep faces out of the plane of the band.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Small brushed DC motor, 3–6 V	1	Dead toy car, broken printer, hobby shop	A gearmotor from a toy gearbox is even better; crank its output shaft
Multimeter	1	Hardware shop, ~$12	You lose the measurement half of the experiment
Red LED, 5 mm	2	Electronics shop, pennies	A low-voltage buzzer, which is louder and more obvious
Old CD or DVD	1	A drawer	Any stiff disc 10–12 cm across
Wooden dowel or pencil, 15 cm	1	School supplies	A chopstick
Thick rubber bands	3–4	Kitchen drawer	A loop of elastic from a sewing kit
Stiff cardboard, 20 × 15 cm	2 pieces	Any box	Thin plywood if you have a saw and an adult
Tape, strong	1 roll	Anywhere	Hot glue, with adult hands only
Safety glasses	1 pair each	Hardware shop	Required, not optional

Time: prep 30 min for the build, experiment 25 min, cleanup 5 min Cost: under $5 if the motor comes from a dead toy Mess level: low, plus cardboard offcuts

Do the Experiment

Step 1 — Prove the motor works backwards before you build anything

Clip the multimeter across the motor’s two terminals, set to DC volts on its lowest range. Spin the shaft hard with your fingers, both directions.

What you should see: a reading that flickers up to perhaps 0.2 to 0.8 V and changes sign when you reverse direction. It is not much, and that is the point: finger speed is nowhere near enough. Now you know why the rest of the build exists.

Step 2 — Build the step-up drive

Tape the motor to one cardboard panel with its shaft hanging over the edge. Push the dowel through a hole in the second panel so it turns freely, and tape the CD to the dowel as a crank wheel. Stand the two panels so the CD’s rim sits about 3 cm from the motor shaft. Loop a rubber band around the CD’s outer edge and around the bare motor shaft.

What you should see: turning the CD once makes the motor shaft spin many times. Count it: a 12 cm CD driving a 2 mm shaft is roughly a 60:1 step-up in theory, and maybe 20:1 after slip. One lazy turn per second becomes thousands of motor revolutions per minute.

Step 3 — Measure voltage against crank speed

Glasses on. With the meter still connected, crank slowly and read the voltage. Then crank at a comfortable pace. Then fast. Have a second person count crank turns over ten seconds for each pace and write the pairs down.

What you should see: voltage rising roughly in proportion to speed. Double the turns per second and you roughly double the volts. A reasonable setup reaches 3 to 6 V at a brisk crank.

Step 4 — Light the LED, and notice the handle

Disconnect the meter. Twist the LED’s legs onto the motor wires and crank. If it does not light, reverse the LED. Once it lights, crank at a steady pace, then stop and pull one LED leg off and crank at the same pace again.

What you should see: the LED glows, brightening with speed. And with the LED connected, the crank is noticeably harder to turn at the same speed. Have your kid close their eyes while you connect and disconnect the LED; most can tell you which state they’re in from the handle alone.

Step 5 — Add a second LED facing the other way

Twist a second LED across the same two wires but with its legs reversed, so the two point in opposite directions. Crank one way, then the other.

What you should see: only one LED lights at a time, and which one depends on crank direction. That is because a brushed motor’s commutator produces output whose polarity flips with rotation direction. Two LEDs make an electrical direction indicator out of a toy.

What’s Actually Going On

Faraday’s law of induction says the voltage generated in a loop of wire equals the rate at which magnetic flux through that loop changes: EMF = -dΦ/dt. Inside your motor there are permanent magnets fixed to the case and coils of wire wound on a rotating armature. Turn the armature and the flux through each coil swings from one direction to the other and back, once per revolution. Faster rotation means a faster rate of change, which means more voltage. That proportionality is exactly what Step 3 measured, and the Georgia State University treatment of Faraday’s law gives the formal version.

A motor and a generator are not two devices that resemble each other. They are one device, read in two directions. Put voltage in and you get torque out; put torque in and you get voltage out. Every large power station on the grid does the second thing, which the US Energy Information Administration’s page on how electricity is generated describes at utility scale. Your CD-and-rubber-band rig is the same physics, several million times smaller.

The heavy handle in Step 4 is Lenz’s law. When current flows in the armature coils, those current-carrying coils sit in the magnets’ field, and a current in a magnetic field experiences a force. The direction always works out to oppose the motion that created it, so the generator develops a braking torque proportional to the current it is delivering. Connect a bigger load and the braking gets stronger. This is not a quirk. If it were not true, you could extract electrical energy from a freely spinning wheel forever, and energy would not be conserved.

Two points where popular explanations go soft. First, the minus sign in Faraday’s law is not decoration; it is the whole reason a generator takes work to turn. Second, this motor does not produce clean DC. The commutator switches connections mechanically twice per revolution, so the output is a bumpy, pulsing voltage that averages out to something a meter calls DC. Put the LED on and crank slowly and you may see it flicker. Real generators solve this with many coils and many commutator segments, or by generating AC deliberately and rectifying it with diodes.

How to Explain It to Your Kid

Ages 5–8: Pushing light out of your arm

Let them crank and see the light. Then ask: where did the light come from? There’s no battery anywhere. Trace it back with them, hand to handle to wheel to band to motor to wire to light. Food made the arm work, the arm made the wheel turn, the wheel made the light.

Then do the heaviness test with eyes closed. Kids at this age are very good at noticing this, and it gives them a felt, physical version of “nothing is free.”

Ages 9–12: Faster means more volts

Introduce the word generator, and the rule that more speed makes more voltage. Use their own table from Step 3 and ask them to predict the voltage at a crank speed they haven’t tried, then try it. Getting a prediction roughly right from their own data is a different experience from being told a formula.

Add the honest caveat: it isn’t perfectly proportional, because friction and the motor’s own resistance eat into it at high speed. Our build of a motor from scratch runs the same machine in the other direction.

Ages 13+: Measure the power, then reality-check a crank charger

Give them EMF = -dΦ/dt and Lenz’s law properly, then set the real problem. Measure voltage and current simultaneously while powering the LED, multiply for watts, and you’ll likely find somewhere between 0.02 and 0.2 W. Now ask what it would take to charge a phone. A typical phone battery holds roughly 15 watt-hours. Divide 15 by their measured power and the answer comes out in days, which explains why emergency crank radios power a radio and not a laptop. If they want to chase it, the step-up ratio and the motor’s winding are the two things to change, and both are measurable. For a non-mechanical route to the same induction idea, see shaking a magnet through a coil.

The question to ask: “If we attach two LEDs instead of one, will the crank get harder or easier, and will each LED be as bright?”

Why Teaching This Early Pays Off

Energy conservation is the most reused idea in physics and the hardest one to teach from a page, because on paper it is a bookkeeping rule and nothing more. Felt in a wrist, it becomes a constraint on how the world can behave. Students who hold a “energy gets used up and disappears” model carry it a long way, and children’s intuitive models of invisible systems tend to be coherent but wrong, patched rather than replaced, as Vosniadou and Brewer showed for models of the Earth. A demonstration that physically contradicts the model is a better intervention than another explanation.

This specific build also pre-loads electromagnetic induction, which turns up in secondary physics as a block of abstract formulae, and it demystifies the entire electricity grid. Roughly everything in a national grid comes from spinning a conductor in a magnetic field. A kid who has built the two-dollar version is in a different relationship to that fact.

Be careful not to oversell it. One afternoon does not change a trajectory. What the evidence supports is that guided activity, with an adult explaining alongside, beats both lecturing and unsupervised exploration: Alfieri and colleagues’ 2011 meta-analysis found unassisted discovery produced no measurable benefit while assisted discovery did. The prediction step matters too. Asking for a guess before the LED goes on is the Predict-Observe-Explain structure that White and Gunstone set out in Probing Understanding, and the National Research Council’s K-12 science framework names planning investigations as a core practice, and the prediction only counts if it comes first.

Before You Start: What Parents Should Know

Best age	10 and up for the build. Younger kids enjoy cranking a generator an adult has already built, and the heaviness test works from about 6.
Supervision	Adult builds the drive and supervises the first cranking. Adult hands only on any hot glue or cutting tool.
Real hazards	A rubber band under tension can snap and strike an eye: safety glasses throughout, and nobody’s face in the plane of the band. Spinning shafts catch long hair, so tie it back. A motor shaft spun at speed can nip fingers. Nothing here produces a dangerous voltage.
Don’t	Don’t connect this to a mains outlet in either direction, and don’t try to “plug the generator into” anything. Don’t substitute a drill for the crank with a child holding the motor: if you use a drill, clamp the motor first and let an adult run it.

When It Doesn’t Work

• The LED never lights. Three causes, in order of likelihood. The LED is backwards: reverse it. The crank is too slow: check voltage with the meter and confirm you exceed about 1.8 V for a red LED. Or the band is slipping, which you can hear as a squeal.
• The rubber band keeps jumping off the motor shaft. The bare shaft is too smooth and too narrow. Wrap two turns of masking tape around it to make a slightly larger, grippier pulley, or slide a short piece of silicone tubing on.
• Voltage reads high but the LED stays dim. Open-circuit voltage is not the same as what the LED gets. Under load the generator’s internal resistance drops most of it. This is informative rather than broken: measure voltage with the LED connected and the number will be much lower.
• The crank is impossibly hard with nothing connected. Your bearings are misaligned or the band is far too tight. Loosen the panel spacing until the wheel coasts for a second or two when you let go.
• A gearmotor from a toy makes almost nothing. Some toy gearboxes include a clutch or a ratchet that disengages when driven backwards. Try the other shaft, or switch to a plain motor and the band drive.

Take It Further With the Hand Crank Generator Experiment

Change one ratio and measure. Swap the CD for a smaller disc, or wrap more tape on the motor shaft, and re-measure voltage at the same crank speed. Plot voltage against step-up ratio. The relationship should be close to linear until the band starts slipping, and finding the slipping point is itself a result.

Store it, then release it. Add a 470 µF electrolytic capacitor across the output, minding its polarity, and crank for ten seconds. Stop. The LED keeps glowing for a moment on stored charge. Time how long for different capacitor values and plot it. That is the same idea as a battery, built from a part that holds no chemistry at all.

Account for the energy. Hang a known mass on a string wound round the crank dowel and let it fall a measured distance, with the LED connected and then disconnected. Time the fall both times. The difference in fall time is the energy that went into the LED, and getting a number out of this is a genuinely hard, genuinely satisfying measurement for a 15-year-old.

What to Watch For Over the Next 3 Months

• Week 4: they ask about the dynamo on a bicycle light, or why a bike with a dynamo is harder to pedal.
• Month 2: “where does the energy come from” appears unprompted in a conversation about something else, often electric cars or regenerative braking.
• Month 3: they can state, without prompting, that a motor and a generator are the same thing, and they use the word induction correctly.

Frequently Asked Questions

Which motors work best as generators?

Small brushed DC motors with permanent magnets, the kind in toy cars and old CD drives. Avoid stepper motors from printers, which have many windings and behave oddly, and avoid brushless drone motors, which generate three-phase AC and will not light a single LED cleanly.

Can we really charge a phone with this?

No, and the arithmetic is the lesson. Measure your output power, divide a phone battery’s 15 watt-hours by it, and the answer comes out in days of continuous cranking. Commercial crank chargers manage minutes of talk time, not a full charge, and they use far better generators than a toy motor.

Why does my kid insist the light “makes” the heaviness?

Because that is what it feels like, and the feeling is almost right. The load does cause the extra force, through Lenz’s law. The correction is only that the energy comes from the arm and passes through the magnetic field, rather than the LED pulling on anything directly.

Is any of this dangerous electrically?

No. A hand-cranked toy motor produces a few volts at a few tens of milliamps, which you cannot feel through dry skin. The hazards here are mechanical: the band, the shaft and long hair. Mains electricity is a different category of danger entirely and has no place in this build.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. Georgia State University. “Faraday’s Law.” HyperPhysics. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html
2. U.S. Energy Information Administration. “How electricity is generated.” Electricity Explained. https://www.eia.gov/energyexplained/electricity/how-electricity-is-generated.php
3. U.S. Energy Information Administration. “Electricity explained.” https://www.eia.gov/energyexplained/electricity/
4. Alfieri, L., Brooks, P. J., Aldrich, N. J., & Tenenbaum, H. R. (2011). “Does discovery-based instruction enhance learning?” Journal of Educational Psychology, 103(1), 1–18. doi:10.1037/a0021017
5. Vosniadou, S., & Brewer, W. F. (1992). “Mental models of the earth: A study of conceptual change in childhood.” Cognitive Psychology, 24(4), 535–585. doi:10.1016/0010-0285(92)90018-W
6. White, R., & Gunstone, R. (1992). Probing Understanding. Falmer Press.
7. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. doi:10.17226/13165
8. Occupational Safety and Health Administration. (2002, revised). Controlling Electrical Hazards, OSHA Publication 3075. https://www.osha.gov/sites/default/files/publications/osha3075.pdf