Motorised Christmas carousel

What they will learn

• That a small pulley driving a large one with a rubber band reduces the speed: the large one makes one turn while the small one makes as many turns as its diameter fits into the large one's.
• That two reductions in a row multiply: 15 × 25 ≈ 390, so a motor turning at about 7000 revolutions per minute drives a carousel at about 15.
• That what you lose in speed you gain in torque (turning force), and that the band slips if you hold the carousel back: it acts as a safety clutch.
• That friction is reduced by resting the axle on a point (point bearing) and guiding it through a smooth straw.
• Why a carousel has to be balanced: weight on one side tilts the axle, it rubs more in the bearing and the carousel wobbles.
• How a real Christmas pyramid works: hot air from the candles rises and pushes the blades (convection), and why here we use a motor and no fire at all.

Prepare beforehand

• Print the templates at 100 % (“actual size”) and check with a ruler that the 50 mm square measures 50 mm. The base and the lid (190 × 190 mm) and disc D1 (Ø 180 mm) take up one sheet each.
• Find flat cardboard: the base, the lid and the two tiers must be flat so the carousel doesn't wobble.
• Test the motor with the battery holder before the session: touch its two tabs with the stripped ends of the wires and set the switch to ON. It should hum and spin.
• Check that your skewers are at least 25 cm long and straight (roll them on the table). Cut one to 250 mm and from the other a 72 mm piece with the point (an adult does this).
• Find two matching bottle caps and two thin office rubber bands (one ≈ 100 mm and the other ≈ 120 mm long, laid flat).
• For groups: cut out the box, the window, the discs and the motor mount yourself beforehand, and make the dimples in the bottle caps in advance.

Materials and tools

• Corrugated cardboard — ≈ 0.25 m² (2 sheets of 50 × 40 cm or one large shipping box)
  single-wall, 3 mm, flat and not crushed; the base, the lid and the Ø 180 mm disc come from 19 × 19 cm pieces
• 3 V DC motor — 1 buy
  type 130 (RF-130 or similar): body 25 mm long, Ø 20 mm with two flat sides 15 mm apart, Ø 2 mm shaft 8–10 mm long; runs at about 6000–9000 rpm with no load at 3 V
• 2 × AA battery holder with switch and cover — 1 buy
  3 V, ON/OFF slide switch, red (+) and black (−) wires 10–15 cm long with stripped ends
• AA batteries — 2 buy
  1.5 V alkaline (together, 3 V)
• Wooden skewers — 2
  wooden, Ø 3 mm and 25 cm long (if they are 30 cm, trim one); straight: roll them on the table and discard any that wobble
• Plastic or stiff paper straw — 1
  Ø 6 mm, straight; 2 pieces of 15 mm are used
• Matching bottle caps — 2
  from water or soft-drink bottles, Ø 28–32 mm and 10–13 mm tall, both the same model
• Thin rubber bands — 2 (+2 spare)
  office type, ≈ 1 mm thick and 1.5–3 mm wide: one ≈ 100 mm long (laid flat) for the motor and another ≈ 120 mm for the carousel axle
• Coloured card — 4 A5 pieces (brown, green, red and yellow or gold)
  160–200 gsm; brown for 3 reindeer, green for 3 trees, red for 2 sleighs, yellow for the star and the propeller
• A4 sheet — 1
  for the axle marking templates
• PVA glue — ≈ 30 ml
  wood glue or school glue
• Glue stick — 1
  to stick the templates onto the cardboard
• Electrical tape — ≈ 60 cm
  19 mm; cut into narrow strips for the motor pulley, the sleeves and the band guides, and for the wire joints
• Masking tape — ≈ 1 m
  to reinforce the corners of the box and hold the lid on (it can be peeled off)
• Double-sided tape — ≈ 15 cm
  to stick down the bottle caps and the battery holder
• 1- or 2-cent coins (for balancing) — 2–4
  2–3 g counterweights taped under the tier that wobbles

Tools

• Craft knife with metal ruler and cutting mat — cutting the box, the window, the discs and the motor-mount slot adult
• Scissors — cutting out the figures, the propeller, the star and the tape strips
• Compass (optional) — drawing the outline of the discs if you don't print the templates
• Awl or sharp pencil — making the 3 mm holes in the discs and the 6 mm holes in the lid adult
• Side cutters or pruning shears — cutting the skewers to 250 and 72 mm adult
• Set square or a book with straight covers — checking that the axles are vertical and the tiers horizontal
• Ruler and fine felt-tip pen — marking the heights on the skewers
• Stopwatch (on a mobile phone) — counting how many turns per minute the carousel makes

Pieces and templates

• BA · Box base ×1 — corrugated cardboard 3 mm
• PF · Front wall (with window) ×1 — corrugated cardboard 3 mm
• PT · Back wall ×1 — corrugated cardboard 3 mm
• PL · Left wall ×1 — corrugated cardboard 3 mm
• PD · Right wall (with cable hole) ×1 — corrugated cardboard 3 mm
• TA · Lid ×1 — corrugated cardboard 3 mm
• R · Lid bearing reinforcement ×4 — corrugated cardboard 3 mm
• M · Motor collar ×4 — corrugated cardboard 3 mm
• N · Collar legs ×6 — corrugated cardboard 3 mm
• G1 · Middle pulley: core ×1 — corrugated cardboard 3 mm
• F1 · Middle pulley: flanges ×2 — corrugated cardboard 3 mm
• G2 · Carousel pulley: core ×1 — corrugated cardboard 3 mm
• F2 · Carousel pulley: flanges ×2 — corrugated cardboard 3 mm
• D1 · Lower tier ×1 — corrugated cardboard 3 mm
• H1 · Lower tier hub ×2 — corrugated cardboard 3 mm
• D2 · Upper tier ×1 — corrugated cardboard 3 mm
• H2 · Upper tier hub ×2 — corrugated cardboard 3 mm
• HH · Propeller hub ×1 — corrugated cardboard 3 mm
• HE · Propeller ×1 — yellow card 160–200 gsm
• RE · Reindeer ×3 — brown card 160–200 gsm
• AR · Tree ×3 — green card 160–200 gsm
• TR · Sleigh ×2 — red card 160–200 gsm
• E · Star ×2 — yellow or gold card 160–200 gsm
• Q · Main axle marking template ×1 — paper
• QB · Middle axle marking template ×1 — paper
• S · Straw bearings ×2 — straw Ø 6 mm
• EA · Main axle ×1 — wooden skewer Ø 3 mm
• EB · Middle axle ×1 — wooden skewer Ø 3 mm

Step by step

1. Cut out the box adult

   Stick templates BA, TA, PF, PT, PL and PD onto the cardboard with a glue stick, with the flutes vertical on the 4 walls. With a craft knife and metal ruler, cut the base and the lid (190 × 190 mm), the front and back walls (190 × 74 mm) and the two side walls (184 × 74 mm).

   Check: Put the lid on top of the base: they match on all 4 sides.
   Tip: Cut each line with 2 or 3 gentle passes instead of one hard one: the cut comes out straight and the cardboard doesn't get crushed.

2. Open the window and the holes adult

   With the craft knife, cut the 130 × 50 mm window in front wall PF. With the awl, make the 6 mm hole in right wall PD, the 2 in lid TA and the one in the centre of each of the 4 reinforcements R (40 × 40 mm): pierce the cross and widen it by twisting until the straw just fits.

   Check: The straw goes into each hole in the lid and the reinforcements with no play.
   Tip: Widen the holes in the lid little by little, testing with the straw: they have to hold it without squashing it.

3. Assemble the box without the lid

   Put glue on the bottom edge of back wall PT and stick it upright on the base, outside the 3 mm line. Then glue the left wall PL and the right wall PD (with the hole) and, finally, the front wall PF, with the window on the side marked FRONT. Reinforce the 4 corners on the outside with masking tape and leave to dry for 20 minutes.

   Check: The box measures 190 × 190 mm on the outside and the lid sits on top without rocking.
   Tip: The crosses and lines on the base must be visible on the inside: stick the template face up.

4. Fit the lower and upper bearings adult

   With the awl, make a dimple 1 mm deep, without going through, in the centre of the top face of each bottle cap. Stick the 2 caps upside down with double-sided tape on the circles on the base, centred on their crosses. On the lid, glue 2 reinforcements R under each hole and push a 15 mm piece of straw S into each one, so that it sticks out 6 mm above.

   Check: The 2 straws are perpendicular to the lid: check with the set square.
   Tip: The dimple is the point bearing: the skewer tip turns in it touching only a tiny spot of plastic, just like the glass axle bearing of a real Christmas pyramid.

5. Mark and prepare the axles

   Lay the 250 mm skewer (EA) on template Q and the 72 mm piece with the point (EB) on QB, with the point at the bottom edge, and copy all the marks with a felt-tip pen. On each axle, between the “sleeve” marks, wind a 14 × 60 mm strip of electrical tape. On EB, also wind 2 rings with 2.5 × 100 mm strips: one between 41 and 43.5 mm and another between 47.5 and 50 mm.

   Check: Each sleeve goes into a straw and turns freely, without wobbling sideways.
   Tip: Between the two rings on EB the wood is left bare: it is the small pulley for the second band (Ø 3 mm). The rings stop the band moving up or down.

6. Glue the pulleys onto the axles

   Cut out discs G1 (Ø 76), two F1 (Ø 84), G2 (Ø 100) and two F2 (Ø 110) and pierce their centres with a skewer. Thread onto EB, from the point end, one F1, G1 and the other F1 with glue between them, until the bottom one reaches the 23 mm mark. Do the same on EA with F2, G2 and F2 from the 41 mm mark. Leave to dry for 20 minutes with the axles standing upright.

   Check: Turn each axle between your fingers: the pulley doesn't wobble and is at right angles to the skewer.
   Tip: Put a drop of glue around the skewer above and below each pulley: that way it doesn't slip on the wood.

7. Build the motor mount adult

   Cut the 4 collars M (40 × 40 mm, with the 20.5 × 15.5 mm slot) and the 6 legs N (40 × 10 mm). Glue the collars one on top of another with the slots lined up, and the legs in 2 stacks of 3. Glue the legs on their lines on the base and the collar on top. Push the motor in from above, shaft up and tabs down, between the legs.

   Check: The motor stands vertical and tight, and its tabs don't touch the base.
   Tip: If the slot is loose, wrap one turn of masking tape around the motor body before pushing it in.

8. Make the motor pulley and connect it

   Wind a 5 × 60 mm strip of electrical tape tightly around the motor shaft, right against its body: it makes a spool of ≈ 4 mm. Pass the battery holder wires through the hole in the right wall, twist each stripped end through the little hole in one of the motor tabs and cover each joint with tape. Stick the battery holder to the outside of the right wall with double-sided tape.

   Check: Put the batteries in and set the switch to ON: the motor hums and the spool turns without shaking. Switch it off.
   Tip: On top of the spool, wind 4 turns of a 1.5 mm strip: it acts as a rim and the band can't escape over the top.

9. Fit the middle axle and the first band

   Put the point of EB in the dimple of the right-hand bottle cap. Pass the ≈ 100 mm band around the groove of pulley G1 and around the motor spool. Move the motor up or down inside the collar until the band is horizontal, at the same height on both sides.

   Check: Hold EB upright at the top and switch on: the pulley turns about 7 times per second, much more slowly than the motor.
   Tip: The band should be slightly stretched (≈ 1 cm beyond its length), not tight: if it pulls hard, it slows the motor and tilts the axle.

10. Fit the main axle and the second band

    Put the point of EA in the dimple of the centre bottle cap. Pass the ≈ 120 mm band around the groove of pulley G2 and around the bare wood of EB, between the two tape rings.

    Check: Hold both axles upright and switch on: EA turns slowly, roughly one turn every 3–5 seconds.
    Tip: Now the band goes around a 100 mm pulley from a 3 mm stick: this is the big reduction, about 25 times.

11. Close the lid

    Lower the lid slowly, guiding the end of EA through the centre straw and that of EB through the other, until it rests on the 4 walls with FRONT over the window. Hold it in place with 4 strips of masking tape. Switch on and watch the pulleys turn through the window.

    Check: EA comes out of the lid vertically (set square) and turns without rubbing; the bands don't slip off.
    Tip: If an axle won't go in, don't push: look through the window to see where it catches and straighten the axle with your finger.

12. Cut out the figures

    Cut out of card 3 reindeer RE (brown), 3 trees AR (green), 2 sleighs TR (red), 2 stars E and the propeller HE (yellow). Cut the 10 mm slit that splits the tab of the trees and sleighs. On each figure, fold one half of the tab forwards and the other backwards.

    Check: Each figure stands up on its own on the table.
    Tip: Paint the reindeer's noses red and the decorations on the trees before folding the tabs.

13. Assemble the lower tier

    Glue the 2 H1 hubs under the centre of D1 (Ø 180) with the holes lined up. Thread D1 onto EA from the top until the hub reaches the 76 mm mark and add a drop of glue. Glue the figures' tabs onto the 6 lines: reindeer and trees alternating, all facing the same way.

    Check: Switch on and look at the tier from the side, at eye level: the edge doesn't rise or fall as it turns.
    Tip: Make all the figures face the direction the carousel turns: it looks like a procession.

14. Assemble the upper tier, the propeller and the star

    Glue the 2 H2 hubs under D2 (Ø 130) and thread it on down to the 160 mm mark. Glue the 2 sleighs on their lines. Thread on hub HH down to 220 mm and glue the propeller on top; fold its 8 blades about 30° to the same side. Glue the 2 stars back to back with the tip of EA pushed 20 mm in between them.

    Check: The sleighs are 9 mm or more below the propeller and nothing rubs as it turns.
    Tip: The propeller is the part that, in a real Christmas pyramid, is moved by the hot air from the candles. Here the motor moves it, so you can put the blades the wrong way round and nothing happens.

15. Balance the carousel adult

    Through the window, take the second band off EB (leave it hanging on G2). An adult holds the box on its side, with the axle almost horizontal and the star a little higher than the base (so the point doesn't come out of its dimple). Spin the tiers by hand and let go: if they always stop with the same side at the bottom, that side is heavier. Tape a 1-cent coin under the tier on the opposite side, and repeat until they stop in any position.

    Check: When you let go in 4 different positions, it stays still where you leave it.
    Tip: With 3 reindeer and 3 trees alternating and 2 sleighs opposite each other, the carousel already comes out almost balanced: usually 1 coin or none is enough.

16. Switch on and count the turns

    Stand the box upright, put the band back on EB between the rings and set the switch to ON. Watch one reindeer and count how many turns it makes in 1 minute with the stopwatch on a mobile phone. Write the number in your inventor's notebook.

    Check: The carousel turns smoothly, between 12 and 20 turns per minute (estimate), and the star on top doesn't draw circles.
    Tip: Turn off the lights and shine a torch on the carousel from one side: the reindeer's shadows go round the wall.

Does it work? The test

Put the carousel on a flat table, switch on and let it turn for 1 minute so the bands settle. Then mark one reindeer with a small piece of tape, start the stopwatch and count the turns for 1 minute. Also look through the window: count how many turns pulley G1 makes while the carousel makes 1.

What should happen: The carousel turns in one direction, smoothly and without jerks, at about 12–20 turns per minute (estimate: it depends on the motor and how fresh the batteries are), that is, one turn every 3–5 seconds. Pulley G1 makes about 25 turns for each turn of the carousel, and the motor about 390. If you hold back the lower tier with a finger, the carousel stops and the band slips without the motor stopping: nothing breaks.

If something goes wrong

• The motor hums and spins, but the carousel doesn't move — A band is loose or has come off: the motor band from the spool, or the second one from between the rings on EB. Solution: Look through the window to see which one isn't pulling. Replace it with a slightly shorter band (one that stretches ≈ 1 cm beyond its length) or give it an extra twist in a figure of 8 around the small pulley.
• The motor band slips off the top or bottom of the spool — The motor isn't vertical or the spool is at a different height from the G1 groove. Solution: Move the motor up or down inside the collar until the band is horizontal. Add the rim of 4 turns of 1.5 mm tape above the spool and check with the set square that the motor is straight.
• The carousel jerks or stops at the same point on every turn — Something rubs at one point: a figure against the upper tier, the hub against the straw in the lid, or pulley G2 against the reinforcements. Solution: Turn the carousel slowly by hand and find where it catches. Check the heights with template Q: between the straw (71 mm) and hub H1 (76 mm) there must be a 5 mm gap.
• The star draws circles and the carousel wobbles — The axle is bent or tilted, or a tier is glued on crooked or is unbalanced. Solution: Roll the skewer on the table: if it wobbles, replace it. Check that the straws in the lid are straight and that each tier is at right angles to the axle. Balance with coins (step 15).
• It turns too fast — Your motor is one of the fast ones (9000 rpm or more) or the motor spool has ended up thicker than 4 mm. Solution: Remove turns of tape from the spool (at 3 mm the speed drops by 1/4) or use 1.2 V rechargeable batteries (2.4 V in total). You can also try with just 1 AA battery, bridging the other slot with a wire: it turns at less than half speed.
• The motor doesn't turn at all — A loose wire joint, the batteries in the wrong way round or the switch set to OFF. Solution: Check the batteries against the + and − symbols in the battery holder, redo the joints by twisting the wire more tightly through the tab and test the motor on its own, taking it out of the collar.
• The main axle comes out of its dimple and falls sideways — The dimple is too shallow or the lid isn't sitting properly on top of the base. Solution: Make the dimple a little deeper (1–1.5 mm, without going through) and check that the hole in the lid is exactly above the cross on the base: the lid goes on with FRONT over the window.

Safety

• No candles or flames: real Christmas pyramids work with candles, but this carousel is made of cardboard and card and would burn. Here the motor drives it; don't put it near candles, heaters or fireplaces.
• Only an adult uses the craft knife, with a metal ruler and cutting mat, always cutting away from the hand holding the work. An adult cuts the skewers with side cutters or pruning shears.
• The skewer point is sharp: it points downwards, inside the box. Take care when threading the discs on from the point end; do it with the disc resting on the table, never against your hand.
• Don't put fingers or long hair through the window while the motor is on: the motor shaft turns at thousands of revolutions and can catch a lock of hair. Switch off before touching the bands.
• AA batteries: don't connect their terminals with a paperclip or aluminium foil (they get hot). Take used ones to a battery recycling bin.
• With children under 3 nearby, put away the coins, the bottle caps and the small card offcuts: they can be swallowed.

The science behind it

For children

The motor spins incredibly fast, about 100 turns every second: if we stuck it straight onto the carousel, the reindeer would go flying. That's why we use pulleys. A rubber band joins the motor's tiny shaft, which is very thin, to a big cardboard wheel: every time the big wheel makes one turn, the tiny shaft has made about 15. Then another band does the same thing again, about 25 times more. In the end the carousel goes nice and slowly, but with lots of force: it's like riding a bike up a hill in a low gear, you pedal a lot and go forward a little but without getting tired. The tip of the skewer rests on a bottle cap and touches only one tiny spot, so there is hardly any rubbing. And if you put all the reindeer on one side, the carousel tilts and wobbles: that's why we spread them out evenly, like on a seesaw.

For the adult

Belt drive: the linear speed of the band is the same on both pulleys (if it doesn't slip), v = ω₁·r₁ = ω₂·r₂, so the speed ratio is i = d₂/d₁, measured on the pitch diameter (the one at the centre of the band: pulley diameter + band thickness, ≈ 1 mm). Stage 1: motor spool 4 + 1 = 5 mm → G1 76 + 1 = 77 mm, i₁ ≈ 15.4. Stage 2: skewer 3 + 1 = 4 mm → G2 100 + 1 = 101 mm, i₂ ≈ 25.3. Stages in series multiply: i ≈ 389. A type 130 motor at 3 V runs at about 6000–9000 rpm with no load (depending on the model); with such a light load it works close to no-load speed (80–90 %) and the bands slip by 3–8 %, hence the estimate of 12–20 rpm at the carousel. With a band, its thickness matters a lot on the small pulley: without counting it, stage 2 would be 100/3 ≈ 33 and the estimate would come out 30 % slower than reality. Torque: with no losses, power is conserved (P = τ·ω), so torque is multiplied by the same ratio: τ₂ = τ₁·i·η. With a motor stall torque of ≈ 2–3 mN·m (≈ 25 g·cm), the carousel could in theory receive almost 1 N·m. In practice it is limited by the grip of the band (Euler–Eytelwein equation, T₁/T₂ ≤ e^(μθ)): on the skewer the band wraps only ≈ 72° (θ = 180° − 2·arcsin((101 − 4)/(2·60)) with 60 mm between axles); with μ ≈ 0.5 and a tension of ≈ 1 N it transmits at most ≈ 0.6 N of tension difference, about 0.03 N·m at G2. That is more than enough: the carousel only needs to overcome friction (≈ 1 mN·m) and, if it is held back, the band slips and acts as a safety clutch. Friction: the weight of the rotor (≈ 55 g, 0.54 N) is carried by a point bearing: the skewer touches the cap over a circle of ≈ 0.2 mm radius, and the friction torque is ≈ (2/3)·μ·F·r ≈ 2·10⁻⁵ N·m, negligible. The significant friction is in the straw in the lid, which takes most of the sideways pull of the band (≈ 1.4 N with the band 45 mm from the point and the straw at 63 mm): μ ≈ 0.3 × 1.4 N × 2.25 mm ≈ 1 mN·m. That is why the tape sleeve (Ø ≈ 4.5 mm) is as thin as possible and the band must not be tight. It is the same arrangement as a real Christmas pyramid: a pointed steel shaft resting in a glass cup and guided at the top by a sleeve. Balance: with the axle perfectly vertical, weight on one side produces no torque about the axle. But (1) the axle has play in the straw and leans towards the heavy side; as that side rotates, the tilt rotates with it and the star draws a circle, and the sleeve always rubs at the same point on the heavy side; (2) if the table is off level by an angle α, an imbalance m·r gives a torque m·g·r·sin α that changes sign every half turn: the carousel jerks; (3) in fast machines centrifugal force F = m·r·ω² dominates: 3 g at 8 cm gives ≈ 1 mN at 15 rpm, but ≈ 2.6 N at 1000 rpm (which is why car wheels and washing-machine drums are balanced). The test with the axle almost horizontal turns the imbalance into a visible gravity torque: the heavy side always ends up at the bottom. The Christmas pyramid (Weihnachtspyramide, from the Ore Mountains in Saxony) turns by convection: each candle gives ≈ 40–90 W of heat; air at 60 °C is ≈ 12 % less dense than at 20 °C (ρ ∝ 1/T: 293/333 ≈ 0.88), so buoyancy lifts it at ≈ 0.3–0.5 m/s. As it passes through the angled blades (20–45°), the flow is deflected and the reaction has a tangential component that turns the wheel. Because the force is a matter of millinewtons, it only works with very little friction, hence the point bearing on glass. Here nothing is built with fire: the motor replaces the candles and the propeller is decorative (although you can try convection without a flame, see “Experiments”).

• pulley: wheel with a groove that a rubber band or belt runs through to drive another wheel.
• reduction: drive that makes something turn more slowly but with more force (more torque).
• torque: turning force: how hard an axle “pushes” as it turns (force × distance from the centre).
• point bearing: support in which a pointed axle turns touching a single tiny spot: there is hardly any rubbing.
• balanced: with the weight spread evenly around the axle, so it turns without tilting or wobbling.
• convection: movement of air (or water) because the warm part weighs less and rises.

By age

• 5–7 years: The adult cuts the box, the discs and the skewers, and fits the motor and the bands (steps 1–11). The child copies the marks onto the axles, cuts out the figures with scissors, folds the tabs, glues them on the tiers and counts the turns. Focus on “small wheel fast, big wheel slow” and on spreading the reindeer and trees out evenly.
• 8–10 years: They do all the steps except the craft-knife cuts, the awl holes and cutting the skewers. Have them do the experiment of counting the turns of G1 per turn of the carousel and explain why two bands are needed.
• 11–12 years: Have them work out the ratio of each stage using the pitch diameter (diameter + band thickness), predict the turns per minute from the motor speed on its data sheet and compare with what they measure. Challenge: redesign the first stage with a different G1 pulley so the carousel makes exactly 10 turns per minute.

In a group or in class

One carousel per team of 3 (≈ €4 for motor, battery holder and batteries per team). Before the session: cut the boxes with the window, the discs, the motor collars and the skewers, make the dimples in the bottle caps and test all the motors. Two 75-minute sessions. Session 1: 10 min demonstration (a motor with no reduction can't turn the carousel smoothly; a Christmas pyramid in a photo or video and why we don't use candles), 20 min assembling the box, 20 min marking axles and gluing pulleys, 25 min with the motor and the first band. Session 2: 25 min with the second band and the lid, 25 min on figures and tiers, 15 min balancing and counting turns, 10 min comparing all the teams' results on the board. Have 2 motors, 4 bands and 2 skewers spare.

Challenges

• Geared motor: swap the 130 motor for a yellow TT 1:48 geared motor (≈ 100 rpm at 3 V). Work out what pulley you need in a single stage for the carousel to make 12 turns per minute (hint: 100/12 ≈ 8; with its ≈ 5 mm shaft and the band you would need a pulley of ≈ 50 mm).
• Lights: put 3 yellow or red 5 mm LEDs in parallel, each with its own 100 Ω resistor, inside the box, pointing up through 3 holes of 5 mm in the lid: they light the figures from below without fire. They run from the same battery holder.
• Third tier: add a Ø 90 mm disc with an angel or a nativity scene. Work out whether you have enough skewer left: you need 80 mm more axle, and you must check the balance again.
• Variable speed: make a second motor spool 6 mm in diameter and another of 3 mm. Predict how much the carousel's speed changes and check it with the stopwatch.
• Music box: glue a toothed strip of card onto the lower tier so that it strikes a paper tongue fixed to the lid on every turn: you hear a “tick” for each turn and can count without looking.

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