Flickering pumpkin lantern

What they will learn

• That a series circuit (battery → switch → LED → battery) only works if the path is closed and the LED is the right way round: long leg to the red wire (+).
• That a «candle» LED has a tiny chip inside that switches it on and off very fast and at random, which is why it looks like a flame without fire.
• That a translucent material (baking paper, tracing paper) spreads the light: the whole face glows instead of showing a single point of light.
• How to build an 8-sided prism by folding a strip of cardboard: 8 folds of 45° make a full 360° turn.
• Why this LED needs no resistor with 2 AA batteries (3 V), and what changes if the LED is a different type.

Prepare beforehand

• Print the templates at 100 % («actual size») and check with the ruler that the 50 mm square measures 50 mm. Halves A and B take up one sheet each.
• Find a box with flat cardboard: halves A and B measure 235 × 128 mm and must have the flutes running vertically (parallel to the folds).
• Test the LED with the battery holder before the session (long leg to the red wire): it should light up and flicker. Read the seller's listing: if it says 2.0–2.4 V, have the 47 Ω resistor ready.
• If the battery holder's wires are only stripped 3–5 mm, strip 15 mm from each wire.
• Put the poster paint on a plate and protect the table with newspaper.
• For groups: cut out halves A and B with the face, and octagons T and P, beforehand yourself.

Materials and tools

• Corrugated cardboard — 1 sheet of ≈ 50 × 40 cm (or several pieces: 2 of 24 × 13 cm and 5 of 16 × 16 cm)
  single-wall, 3 mm, from a delivery box, flat and not crushed
• Flickering candle LED — 1 (+1 spare) buy
  5 mm, amber, yellow or warm white, «candle / flicker / candle effect», for 3 V (the listing says 3–3.4 V or 2.5–5 V, 20 mA); it has the flicker chip built in
• 2 × AA battery holder with switch and cover — 1 buy
  3 V, with an ON/OFF slide switch, red (+) and black (−) wires 10–15 cm long with stripped ends; ≈ 60 × 32 × 18 mm
• AA batteries — 2 buy
  1.5 V alkaline (3 V together)
• 47 Ω resistor (only if needed) — 1 buy
  1/4 W; only if your LED's listing says 2.0–2.4 V or «no limiter», or if you use an ordinary LED
• Baking paper — 1 piece of 12 × 8 cm
  ordinary kitchen parchment: translucent, acts as a diffuser
• Green card — 1 piece of 12 × 8 cm
  160–200 gsm, for the stem and the leaf
• White or yellow card — 1 piece of 8 × 6 cm
  160–200 gsm, for the candle that holds the LED
• Orange poster paint — ≈ 30 ml
  thick, with little water so the cardboard does not warp
• PVA glue — ≈ 15 ml
  wood or school PVA
• Glue stick — 1
  for sticking on the templates and the diffuser paper
• Sticky tape — ≈ 40 cm
  electrical tape (best) or 19 mm clear tape, for the wire joints
• Clothes pegs — 4
  to hold the tabs while the glue dries
• Double-sided tape (optional) — ≈ 6 cm
  to fix the battery holder and the candle to the base

Tools

• Craft knife with metal ruler and cutting mat — cutting the outline of the two halves and the eyes and mouth adult
• Scissors — cutting out the octagons, the card and the paper
• Empty ballpoint pen and ruler — scoring the 10 folds of the body so they fold straight
• Awl or sharp pencil — making the 14 mm hole in the lid and the 5 mm hole in the candle disc adult
• 2 cm flat brush — painting the body orange
• Wire strippers or old scissors (optional) — stripping 15 mm from the wire ends if they are too short adult

Pieces and templates

• A · Body: front half (with the face) ×1 — 3 mm corrugated cardboard
• B · Body: back half ×1 — 3 mm corrugated cardboard
• T · Lid ×1 — 3 mm corrugated cardboard
• C · Base ×1 — 3 mm corrugated cardboard
• P · Plug octagon (fits inside) ×3 — 3 mm corrugated cardboard
• S · Stem ×1 — green card 160–200 gsm
• H · Leaf ×1 — green card 160–200 gsm
• V · Candle (holds the LED) ×1 — white or yellow card 160–200 gsm
• W · Flame disc ×1 — white or yellow card 160–200 gsm
• D · Diffuser paper ×1 — baking paper (parchment)

Step by step

1. Stick on the body templates

   Cut out templates A and B leaving a 5 mm margin and stick them onto the cardboard with a glue stick, with the flutes running vertically (parallel to the fold lines). Stick them on the uglier or printed side of the cardboard: that will be the inside.

   Check: The flutes of the cardboard run from top to bottom, like the green fold lines.
   Tip: Cover the whole template with glue, edges included, so it doesn't lift while you cut.

2. Cut the outline of the two halves adult

   With a craft knife and metal ruler, cut the straight sides of A and B (the 235 mm bottom edge and the edges of the tab). Cut the 4 arcs along the top freehand with the craft knife, slowly, or with strong scissors. Do not cut the green lines.

   Check: Each half has 4 segments with a curved top edge and a 15 mm tab at one end.

3. Cut out the eyes and mouth adult

   On half A, cut out the two eyes (30 × 24 mm triangles) and the two halves of the mouth with the craft knife. Cut from corner to corner and push the piece out with your finger. Leave the 8 mm middle tooth whole: the central fold runs through it and it holds the face together.

   Check: Hold half A up to the light: you see two eyes and a smiling mouth with a tooth in the middle and a tooth at the top on each side.
   Tip: First make a cut at each tip of the teeth, then the long sides: that way the corners come out clean.

4. Score the folds

   Lay the ruler on each green line and run the empty ballpoint pen along it, pressing hard, twice. There are 4 folds on each half (3 between segments and 1 at the tab). Fold each one slightly towards the template, about 45°, and open it out again.

   Check: When you fold, the cardboard bends along a straight line, without creasing at the sides.

5. Paint the pumpkin orange

   Turn the two halves over and paint the outside (the side without the template) with thick orange poster paint, brushing from top to bottom. Leave to dry for 20 minutes on newspaper, painted side up; meanwhile, do steps 6 to 11.

   Check: The cardboard is orange and flat. If it warps, put a book on top once it is dry to the touch.
   Tip: To make the segments show, paint a darker stripe (orange with a drop of brown) over each fold once the orange is dry.

6. Make the base and the lid pieces

   Cut out lid T and base C (156 mm octagons) and the 3 small P octagons (138 mm) with scissors. Glue one P in the centre of base C with PVA glue, corners lined up. The other two P are for the lid.

   Check: On the base, the small octagon leaves an even border of about 8 mm all round.

7. Make the stem and the leaf

   Cut out piece S from green card and cut the 4 fringe lines up to the fold line. Roll it into a 14 mm tube (roll it around a thick felt-tip pen), glue the 6 mm flap and bend the 5 fringes outwards. Cut out leaf H.

   Check: The stem is a 45 mm high tube with 5 fringes spread out like a flower at the bottom.

8. Assemble the lid with the stem adult

   Make the 14 mm hole marked in the centre of lid T: pierce it with the awl and widen it by twisting the scissors. Push the stem in from the top until the fringes touch the underside and glue them with PVA. Glue the 2 P octagons on top of them, one on the other, corners lined up, covering the fringes. Glue leaf H next to the stem.

   Check: Pull gently on the stem: it lifts the whole lid without coming unstuck.
   Tip: Wind a green pipe cleaner round a pencil and glue it next to the stem: it looks like the tendril of a real pumpkin.

9. Test the LED

   Put the 2 AA batteries in the holder, following the + and − marks. Touch the long leg of the LED with the stripped end of the red wire and the short leg with the black one, and set the switch to ON. The LED should light up and flicker like a flame.

   Check: If it doesn't light, check that the long leg goes to red. If it lights but doesn't flicker, it is an ordinary LED, not a candle one.
   Tip: If your LED's listing says 2.0–2.4 V, now is the time to add the 47 Ω resistor between the red wire and the long leg (see «Science»).

10. Join the wires without soldering

    With the switch at OFF, wind 15 mm of the stripped end of the red wire tightly around the long leg, and the black one around the short leg. Bend each leg back over its own wire so it can't slip off and wrap each joint separately with 3 cm of electrical tape.

    Check: Set the switch to ON: it flickers. Move the wires a little: it doesn't go out.
    Tip: First trim the long leg to 20 mm and the short one to 18 mm: that way the two joints are at different heights and don't touch.

11. Make the candle that holds the LED

    Cut out piece V from white card and roll it into a tube 20 mm in diameter and 50 mm high; stick the 7 mm flap down with tape. Pierce the centre of disc W to make a 5 mm hole. Feed the wires through the tube from top to bottom, push the head of the LED up through the hole in the disc from below and stick the disc on top of the tube with 2 strips of tape.

    Check: The head of the LED pokes out at the top like a flame and the wires come out of the bottom of the tube.

12. Glue the diffuser paper behind the face

    When the paint is dry, cut out the 110 × 74 mm piece of baking paper D. Run the glue stick over template A around the eyes and mouth (without touching the openings) and stick the paper on top, inside the marked box, pulled taut.

    Check: Look at the face from outside against a window: the eyes and mouth look white and bright, with no wrinkles.
    Tip: Baking paper sticks badly on its shiny side: if it comes loose, hold its 4 edges down with sticky tape.

13. Fold and join the two halves

    Fold the creases of A and B inwards. Put PVA glue on A's tab and stick it inside the free edge of B, up to the line marked at 15 mm. Do the same with B's tab on the free edge of A. Hold each joint with 2 clothes pegs for 15 minutes.

    Check: Seen from above, the body is a regular octagon of about 14 cm: 8 equal segments, with no gaps at the joins.
    Tip: With hot glue (used by an adult) it sticks in 1 minute, but you have to get it right first time.

14. Put the light inside

    Stick the battery holder to the base with double-sided tape, off to one side, with the switch facing up. Stick the candle in the centre of the base with another piece of tape. Place the body on the base, fitted around the small octagon, with the face pointing away from the battery holder.

    Check: The LED flame sits in the centre, about 6 cm above the base, halfway between the eyes and the mouth.
    Tip: If you don't want the body to come away from the base when you move it, hold it with 2 strips of masking tape on the inside: that way you can unstick it to change the batteries.

15. Switch on and test in the dark

    Lift the lid by the stem, reach in and set the battery holder switch to ON. Close the lid, fitting the small octagons inside the body, and switch off the room light.

    Check: The eyes and mouth glow with a warm, flickering light; the lid doesn't move if you blow on it.
    Tip: Put it in a window or by the door on Halloween night: from outside you see the face glow and flicker as if it had a candle inside, with no danger at all.

Does it work? The test

In a dark room, switch the pumpkin on, close it and look at it from the front at 2 m. Then look at the sides and the back.

What should happen: The eyes and mouth look like even patches of light (you can't make out the point of the LED) that change brightness irregularly several times a second. A little light escapes through the gaps between the segments and the lid, outlining the crown of the pumpkin. At the back and sides the cardboard lets no light through.

If something goes wrong

• The LED doesn't light — LED the wrong way round, batteries put in wrongly or a loose wire joint. Solution: Check that the long leg goes to the red wire and the batteries follow the + and − marks on the holder. Redo the joints with more turns, squeezing them with your fingers before taping.
• It lights but doesn't flicker — It is an ordinary LED, not a candle LED with a chip. Solution: Swap it for a «candle / flicker» one. If you use an ordinary LED, put the 47 Ω resistor in series: it will glow steadily, safely.
• The LED gets hot, is extremely bright or burns out within minutes — Your LED is a 2.0–2.4 V type with no internal limiter and is getting the 3.2 V of new batteries without a resistor. Solution: Put a 47 Ω resistor between the red wire and the long leg (wound on in the same way as the wire) and use a new LED.
• The light goes off and on when you move the pumpkin — Poor contact at a joint or at the springs of the battery holder. Solution: Tighten the joints and wrap them again. Turn the batteries slightly in the holder to clean the contact.
• The face shows a strong point of light instead of glowing all over — The diffuser paper has come unstuck or the LED is too close to the face. Solution: Stick the baking paper down well and keep the candle in the centre of the base, about 6 cm from the face. With 2 layers of paper the light spreads even more.
• The lid won't go in or falls inside — The small P octagons are too big or too small for your body (cardboard compresses differently in every box). Solution: If it won't go in, trim 1 mm all round the P pieces. If it wobbles, wrap a turn of tape round the edge of the P pieces or glue on a third P.
• My battery holder has no switch — Some cheap models don't come with one. Solution: Make a homemade switch: cut the black wire, strip 15 mm from each end, wind one round a metal paperclip and leave the other loose; to switch on, hook the loose end onto the paperclip.

Safety

• Never put a real candle or any flame inside: cardboard and baking paper burn. This pumpkin is designed only for the LED, which barely gets warm (it uses about 0.06 W).
• The craft knife is used by an adult, with a metal ruler and cutting mat, always cutting away from the hand that is holding.
• Never connect the poles of AA batteries with a paperclip or aluminium foil: the short circuit makes them hot. Set the switch to OFF before touching the joints.
• With children under 3 around, put away the spare LED, the batteries and the scraps of card: they are small parts that can be swallowed.
• Used batteries go in the battery recycling bin, not in the rubbish.

The science behind it

For children

A real Halloween pumpkin has a candle inside, but candles burn. We use a candle LED: inside its plastic head there is a chip smaller than a breadcrumb that switches the light on and off very fast, sometimes more and sometimes less, as if the wind were moving a flame. The electricity leaves the batteries through the red wire, goes through the LED and comes back through the black wire: it is a path in a circle, and if it is broken anywhere the light goes out. Baking paper is like fog: the light goes in at one point and comes out spread around, which is why the whole eyes glow.

For the adult

Circuit: 2 alkaline AA batteries in series give a nominal 3.0 V (≈ 3.2 V when new, 2.0–2.4 V at the end of their life) with an internal resistance of ≈ 0.1–0.3 Ω each. The 5 mm candle LED has a CMOS chip in its package with a pseudo-random generator that modulates the diode's brightness (switching several times a second at different levels); the chip can usually be seen as a black dot next to the diode. Models sold «for 3 V» (listing of 3–3.4 V or 2.5–5 V, 20 mA) have current limiting in the chip itself or an InGaN diode whose forward voltage (≈ 2.8–3.2 V) is already close to that of the batteries, so with 2 AA they work without a resistor, just like commercial electric candles (estimated average current: 10–20 mA). If the listing says 2.0–2.4 V with no limiter (some amber ones), a resistor is needed: R = (V_batteries − V_LED)/I = (3.0 − 2.2)/0.02 ≈ 40 Ω → 47 Ω, the next standard value; with new batteries (3.2 V) this gives ≈ 21 mA and with batteries at 2.6 V ≈ 9 mA, always below the typical maximum of 30 mA. A 47 Ω resistor with a 3 V LED would only make it dimmer, which is why it isn't fitted by default. Battery life: ≈ 2000 mAh / 15 mA ≈ 130 h (estimate: 100–150 h, with decreasing brightness). Power: 3 V × 0.02 A = 0.06 W, which doesn't noticeably heat the cardboard; a wax candle, by contrast, gives off ≈ 80 W of heat with a flame of over 1000 °C, and paper ignites at around 230 °C. Optics: baking paper (silicone-coated parchment) is translucent: its fibres scatter light in all directions, so each opening looks like an even emitting surface instead of revealing the point source. Corrugated cardboard is opaque and light only comes out through the face and the gaps around the lid. Geometry: a regular octagon with side s = 55 mm has a distance across the flats of s / tan 22.5° = 55 / 0.4142 ≈ 132.8 mm; as the template goes on the inside, that is the internal measurement. The P plug octagon, 138 mm across the corners, measures 138 × cos 22.5° ≈ 127.5 mm across the flats: it leaves ≈ 2.6 mm clearance per side to absorb inaccuracies in the folding. On the outside, with 3 mm cardboard, the body measures ≈ 138.8 mm across the flats and ≈ 150 mm across the corners, and the 156 mm lid T overhangs by ≈ 3 mm. Each fold turns 45° (360° / 8) and the flutes of the cardboard run parallel to the folds so that they fold cleanly.

• candle LED: LED with a chip inside that changes its brightness at random to imitate a flame.
• series circuit: a single, closed path: battery, switch and LED one after another.
• polarity: the + side and the − side of a battery or an LED; if they are swapped, the LED doesn't light.
• translucent: a material that lets light through but doesn't let you see what is behind it, like baking paper.
• octagon: a shape with 8 equal sides; our pumpkin is an octagonal prism.
• diffuser: a material that spreads light so that a single point doesn't dazzle you.

By age

• 5–7 years: The adult cuts out the halves and the face, joins the wires and makes the hole in the lid. The child sticks on the templates, scores the folds with the pen, paints, rolls the stem and the candle, and tests the LED by touching the wires to the legs. Focus on «long leg to red» and «closed path = light».
• 8–10 years: They do all the steps except the craft-knife cuts. Ask them to explain why the diffuser paper is needed and to do the paper experiment.
• 11–12 years: Ask them to draw the circuit diagram with symbols, calculate the resistor needed for a 2.2 V LED and estimate how many days the batteries last. Challenge: design a 6-sided pumpkin, calculating the side length so it is the same width.

In a group or in class

One pumpkin per pair (≈ €2.50 of LED, battery holder and batteries per pumpkin). Before the session: cut out halves A and B with the face and the octagons, test all the LEDs with a battery holder and strip the wires. 90-minute session: 10 min demonstration (real candle vs candle LED and why fire isn't used), 25 min folds and painting, 25 min lid, stem and circuit while it dries, 20 min assembly and 10 min with the lights off to see all the pumpkins together. Hand out the batteries at the end, once the circuit is assembled. Keep 3 spare LEDs and 1 spare battery holder.

Challenges

• Cut a bat or a small moon (20–30 mm) into half B with its own diffuser paper: the pumpkin will have a face at the front and a Halloween picture at the back.
• Put 2 candle LEDs in parallel (long legs together to red, short to black), one behind each eye: each eye flickers to its own rhythm.
• Add an outside switch: bring the wires out through a 6 mm hole at the back and make a paperclip switch to turn it on without opening the lid.
• Design a family of pumpkins: a small one with 40 mm segments and a large one with 70 mm. Work out how big their lid octagons must be with the formula in «Science».
• Make a darkness sensor: ask an adult about a light-dependent resistor (LDR) and a transistor so the pumpkin switches itself on at dusk.

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