Christmas tree with LED lights

What they will learn

• What a parallel circuit is: each LED has its own path between the + track and the − track, which is why they all light up on 3 V and, if one fails, the others keep shining.
• Why two LEDs in series won't light from a 3 V battery: each needs about 2.8 V, so together they ask for more than 5.5 V.
• That an LED has polarity: the long leg goes to + and the short one to −; if you put it in the wrong way round, it simply doesn't light.
• That a coin cell has an “internal resistance” that limits the current, and when you need to add a resistor (AA batteries, red or yellow LEDs).
• How two flat pieces with 4 mm slots fit together in a cross to make a shape that stands up on its own.

Prepare beforehand

• Print the templates at 100 % (“actual size”) and check with a ruler that the 50 mm square measures 50 mm. Piece B (28 cm high) comes out on two sheets that you join at the overlap mark.
• Measure the thickness of your cardboard: if it is more than 3.5 mm, cut all the slots 1 mm wider than the template shows.
• Test the 8 LEDs with the CR2032 before the session (long leg on the + side). They should all light with a similar brightness; set aside any that are much brighter than the rest: they are usually 2 V red, yellow or yellowish-green ones.
• Keep the battery in its packaging, somewhere high up, until it is time to fit it. Keep the packaging handy so you can put the battery back if it has to come out.
• Cut the copper tape in advance: 1 length of 40 cm (+ track), 1 of 52 cm (− track), 12 patches of 10 mm, 1 strip of 45 mm and 2 of 25 mm for the battery.
• For groups: cut out pieces A, B, E and M yourself with the craft knife and pierce the LED holes before the session.

Materials and tools

• Corrugated cardboard — 2 sheets of 20 × 29 cm and 1 piece of 10 × 15 cm
  single-wall, 3 mm, from a shipping box, flat and not crushed
• 5 mm “3 V” LEDs — 6 (+2 spare) buy
  diffused or clear, forward voltage 2.8–3.2 V: white, warm white, blue, pure green (520–525 nm) and pink. Legs 25 mm or longer
• CR2032 coin cell — 1 buy
  lithium, 3 V, 20 mm diameter and 3.2 mm thick; ideally a brand with child-resistant packaging
• Self-adhesive copper tape — ≈ 1.3 m buy
  5 or 6 mm wide; ideally with “conductive adhesive” (it says so on the roll)
• Large metal paperclip — 1
  32 mm or larger, bare wire with no plastic coating: it is the switch
• White card — 1 piece of 4 × 4 cm
  200 gsm, for the battery lid
• Green and yellow poster paint — ≈ 40 ml of green and 10 ml of yellow
  thick, with little water so the cardboard doesn't warp
• Clear sticky tape — ≈ 30 cm
  19 mm, to seal the battery lid and insulate the copper under the edge of the battery
• Glue stick — 1
  to stick on the templates
• PVA glue — ≈ 10 ml
  for the battery frame and the star
• 100 Ω resistor (optional) — 1 for each red, yellow or orange LED buy
  1/4 W; only if you want to add LEDs in those colours

Tools

• Craft knife with metal ruler and cutting mat — cutting out the outline of the pieces, the slots and the opening in the battery frame adult
• Scissors — cutting out the card lid and the lengths of copper tape
• Drawing pin or fine awl — making the 12 holes of 1.5 mm for the LED legs adult
• Ballpoint pen or wooden skewer — rubbing the copper tape so it sticks down well and folding the corners
• 2 cm flat brush — painting the tree and the star
• Small pliers or nail clippers (optional) — trimming any excess LED legs adult
• Multimeter (optional) — measuring the battery voltage in the experiments

Pieces and templates

• A · Tree with the lights (front piece) ×1 — 3 mm corrugated cardboard
• B · Crossing tree (side piece) ×1 — 3 mm corrugated cardboard
• E · Star ×1 — 3 mm corrugated cardboard
• M · Battery frame ×1 — 3 mm corrugated cardboard
• T · Battery lid ×1 — card 200 gsm

Step by step

1. Stick on the templates

   Cut out templates A, B, E and M leaving a 5 mm margin and stick them onto the cardboard with a glue stick, with the corrugations running vertically (from the top to the bottom of the tree). Template A goes on the side that will be the BACK: you will stick the copper on top of it. Join the two sheets of B by lining up the overlap mark.

   Check: The cardboard corrugations run from top to bottom, just like the slots in the centre.
   Tip: Spread glue over the whole template, including the edges and the track lines: the copper will be stuck on top of the paper.

2. Cut out tree A and tree B adult

   With a craft knife and metal ruler, cut the outline of A and B along the solid line. Cut their slots too: on A, the top one (4 mm wide and 95 mm long); on B, the bottom one, which has 2 sections: 105 mm at 10 mm wide from the base and a further 35 mm at 4 mm wide.

   Check: A's slot reaches halfway down the tree (95 mm from its top edge) and B's reaches halfway up (140 mm from the base).
   Tip: For the slots, cut the two long sides with the ruler first and then the end: push the strip out with your finger.

3. Cut out the star, the frame and the lid adult

   With the craft knife, cut out star E (80 mm, with a 4 × 25 mm slot in the bottom point) and frame M (a 30 mm square with a 21 mm round opening in the centre). With scissors, cut the 30 × 30 mm lid T and two 12 × 12 mm squares for the shim out of the card.

   Check: A CR2032 battery (20 mm) fits into the frame opening without forcing and sits almost flush with the cardboard.

4. Test the cross joint

   Stand A upright. Lower B from above, crosswise, sliding its bottom slot into A's top slot, until both bases touch the table. Check that the tree stands up straight, then take it apart by pulling B upwards: you will assemble it again at the end.

   Check: Seen from above, the two pieces form a cross with 4 equal arms and the bases are level.
   Tip: If it is very stiff, widen the narrow slot by 1 mm; if it wobbles, stick a strip of masking tape inside A's slot.

5. Paint the tree and the star

   Paint the front of A (the side WITHOUT the template) and both sides of B green. Paint star E yellow on both sides. Leave to dry for 20 minutes on newspaper; in the meantime, do step 6. Don't paint the back of A.

   Check: Template A is still clean and you can read the track lines.
   Tip: With a finger dipped in white or red poster paint you can paint baubles and a garland linking the spots where the LEDs will go.

6. Test the LEDs with the battery adult

   The adult holds the CR2032 between the two legs of each LED: the long leg touches the + side (the one with the writing) and the short leg the − side. Test the 6 LEDs and the 2 spares, one by one. They should all light with a similar brightness. Put the battery back in its packaging.

   Check: You have picked out 6 LEDs that shine equally well. When you turn the battery over, none of them lights: polarity matters.
   Tip: Set aside any LED that is much brighter than the others: it is a 2 V one (red, yellow, yellowish green) and in parallel it would leave the others in the dark.

7. Stick on the + copper track

   With template A facing you, stick a ≈ 35 cm length of copper tape along the line marked “+”, without cutting it: down the left + track, round the corner, across the tree along the lower line (137 mm from its top edge) and up the right + track. Peel the backing paper off a little at a time.

   Check: The + track is a single U-shaped piece and doesn't touch any “−” line. Rub it with the pen so it sticks down well.
   Tip: At each corner, without cutting: fold the tape back on itself and turn it in the new direction; the fold ends up diagonal and the copper stays in one piece.

8. Stick on the − copper track

   Stick another ≈ 47 cm length of copper along the “−” line: down the left − track (the outer one), round the corner, across along the very bottom line (8 mm from the lower edge of the branches) and up the right − track. The + and − tracks run parallel, 7 mm apart.

   Check: There are two copper Us, one inside the other, that don't touch anywhere. Look carefully at the bottom corners.

9. Pierce the LED holes adult

   Rest A on a piece of old cardboard, template side up. With the drawing pin, pierce the 12 marked holes (6 pairs, one per LED, with the dots 3 mm apart, between the two tracks) until it comes out on the green side. Twist the pin to make holes of about 1.5 mm.

   Check: From the green side you can see 6 pairs of little holes. None of them has cut the copper tape.

10. Push in the LEDs and bend the legs

    From the green side, push the two legs of an LED into a pair of holes until its base touches the cardboard. Turn the tree over: the long leg must come out of the “+” hole (the one next to the + track). Bend the long leg flat onto the + track and the short one onto the − track. Repeat with all 6 LEDs.

    Check: Each leg rests on its track and the legs of the same LED don't touch each other.
    Tip: If the long leg comes out of the “−” hole, pull the LED out and turn it half a turn. Trim any legs that stick out more than 10 mm beyond their track (an adult does this with pliers).

11. Secure each leg with a copper patch

    Stick a 10 mm patch of copper tape over each leg, on the track, and rub it hard with the pen so the leg is pressed tight against the copper underneath. That's 12 patches, one per leg.

    Check: Gently pull each LED from the green side: it doesn't move.
    Tip: If your tape doesn't have conductive adhesive, this patch is what makes the contact: press firmly on both sides of the leg.

12. Make the battery cradle

    Stick a 45 mm copper strip from the centre of the square marked “BATTERY” to the left and then downwards, stopping 3 mm short of the − track (“CLIP” area). Cover the copper that will be under the edge of the battery with clear tape, leaving 5 mm bare in the centre. Glue frame M onto the square and, finally, stick a copper strip from the + track up over the top side of the frame.

    Check: Only the copper in the centre shows through the frame opening; the strip in the “CLIP” area doesn't touch the − track.
    Tip: To join copper to copper, fold 5 mm of the end of the strip back (copper facing out) before laying it on the + track, and cover it with a small piece of tape.

13. Fit the battery and close the lid adult

    In the centre of the underside of lid T, stick a 12 × 12 mm card shim (two if your cardboard is 4 mm) and, on top of it, a 22 mm copper strip from the top edge of the lid. The adult puts the battery in the opening with the + side (writing) facing out, places the lid on top with the copper towards the battery and tapes it down on all 4 sides.

    Check: The battery can't be seen or taken out without cutting the tape; the lid doesn't move.
    Tip: Before taping, press the lid with your finger and touch the “CLIP” area with the paperclip: if the LEDs light up, the contact is good.

14. Switch on with the paperclip

    Slide the metal paperclip upwards onto the bottom edge of the branches, right in the “CLIP” area, so that its back part presses on both the end of the battery strip and the − track at the same time. All 6 LEDs light up at once. To switch off, take the paperclip off.

    Check: With the paperclip on, all 6 LEDs shine; without it, none. If one doesn't light, check which way round it is and its patches.
    Tip: Keep the paperclip hooked on the edge, but 2 cm away from the “CLIP” area, so you don't lose it.

15. Assemble the tree in a cross

    With the paperclip off, stand A upright with the lights facing you. Lower B from above, crosswise, sliding its bottom slot into A's top slot, as in step 4, until both bases touch the table. The wide part of B's slot passes over the copper tracks without rubbing them.

    Check: The tree stands up on its own, straight, and the copper tape at the back hasn't come unstuck.

16. Add the star and switch it on

    Put a drop of glue on the tip of B and fit the slot of star E onto it, with the star facing forwards (like A), until the tip touches the end of the slot. Leave to dry for 10 minutes, put the paperclip on and turn off the light.

    Check: The star sits straight on top of the tree, without touching A's branches, and the 6 LEDs shine in the dark.
    Tip: Put the tree in a corner or against the wall: that way the back, with the copper and the battery, stays hidden.

Does it work? The test

In a dark room, put the paperclip on and look at the tree from the front from 2 m away. Then take one of the LEDs out of its place (peel off its two patches) and look again. Finally, take the paperclip off.

What should happen: With the paperclip on, all 6 LEDs light up at once with a similar brightness and can be seen clearly from 2 m in the dark. When you remove one LED, the other 5 stay on (and shine a little brighter): that proves they are in parallel. Without the paperclip, they all go out. The battery doesn't get warm.

If something goes wrong

• None of the LEDs light — The battery is the wrong way round, the lid isn't pressing on the battery, the paperclip isn't touching both copper areas or a corner of the tracks has been cut. Solution: Check (the adult) that the + side of the battery faces the lid and that the shim is under the lid's copper. Slide the paperclip further up. If a track is cut, put a copper patch over the cut using the trick of folding the end (copper against copper).
• One LED doesn't light but the others do — That LED is the wrong way round (long leg on the − track) or one of its legs isn't making contact. Solution: Peel off its patches, take it out, turn it half a turn and put it back with the long leg on the + track. Press the patches down firmly with the pen.
• Some LEDs are very bright and others barely glow — You are mixing 2 V LEDs (red, yellow, orange or yellowish green) with 3 V LEDs: the 2 V one takes almost all the current. Solution: Swap the 2 V LEDs for 3 V ones (white, blue, pure green, pink) or put a 100 Ω resistor between the long leg of the 2 V LED and the + track.
• The light flickers or goes out when you move the tree — A loose contact: the paperclip, a patch or the battery lid. Solution: Rub all the patches again. If the paperclip is the problem, put a piece of aluminium foil folded in 4 (10 × 30 mm) under it. If the battery is the problem, the adult adds another strip of tape over the lid, pressing down.
• The battery gets warm or runs down in a few minutes — Short circuit: the + track touches the − track, a leg touches the wrong track or the copper at the bottom of the frame touches the edge of the battery. Solution: Take the paperclip off straight away. Find the spot where the two tracks touch (especially at the corners and under the edge of the battery) and separate them or insulate them with clear tape.
• The pieces don't fit or the tree wobbles — Your cardboard is thicker or thinner than 3 mm. Solution: If it won't go in, widen B's narrow slot and A's slot by 1 mm. If it wobbles, stick a strip of masking tape inside A's slot or put a drop of glue where they cross.
• The lights look very dim from day one — The battery is old or it is a thinner CR2025/CR2016 that doesn't reach the lid. Solution: Use a new CR2032. With a thinner battery, add another card shim under the lid's copper.

Safety

• Coin cells are very dangerous if a child swallows one: within 2 hours they can burn the oesophagus from the inside. An adult always fits and removes them, the battery holder lid is taped down on all 4 sides and new and used batteries are kept in their packaging, out of reach. If you suspect a child has swallowed one, go to A&E straight away (112), even if they have no symptoms.
• Never use candles or mains-powered lights on a cardboard tree: this project works only with LEDs and a 3 V battery, which barely get warm.
• The craft knife is used by an adult, with a metal ruler and cutting mat, always cutting away from the hand holding the work. The drawing pin or awl is also used by an adult (or an older child under supervision), on a piece of old cardboard.
• Never connect the + track and the − track directly with the paperclip, aluminium foil or copper: it is a short circuit that heats up the battery and drains it. The paperclip only goes in the “CLIP” area.
• The edges of the copper tape and the tips of the LED legs are a little sharp: bend them flat and wash your hands when you finish.
• With children under 3 nearby, put away the paperclip, the spare LEDs and the offcuts: they are small parts. Used batteries go in the battery recycling bin.

The science behind it

For children

Electricity leaves the battery through the + copper track, goes into each LED through its long leg, comes out through the short one and goes back to the battery through the − track. It's like a ladder: the two tracks are the sides and each LED is a rung. Each LED has its own rung, which is why they all light up and, if you take one away, the others keep shining: that's called “in parallel”. If instead you put two LEDs one after the other (“in series”), the battery has to push the electricity through both of them, and it isn't strong enough: none of them lights. The paperclip is a bridge: when you put it on, the path is closed and the light comes on; when you take it off, the path is broken.

For the adult

Circuit: a CR2032 lithium cell (3.0 V nominal, ≈ 3.1–3.2 V when new, 225 mAh at low current) powers 6 × 5 mm LEDs connected in parallel by two copper tape tracks (negligible resistance, a few mΩ per length). All the LEDs are InGaN (white, warm white, blue, 520–525 nm green, pink), with a forward voltage Vf ≈ 2.7–3.0 V at a few mA; that is why they share the current well: with the same voltage across their terminals, each draws a similar current. If an AlGaInP LED (red, orange, yellow, Vf ≈ 1.8–2.1 V) is mixed in, it pins the track voltage at around 2 V and the InGaN ones, which barely conduct at 2 V, stay almost off: this is the “current hogging” of diodes in parallel. Solution: a resistor in series with each low-Vf LED; with 100 Ω, I = (2.8 − 1.9) / 100 ≈ 9 mA. Series versus parallel: in series the forward voltages add up (2 × 2.8 ≈ 5.6 V; even 2 red ones need ≈ 3.7 V), so with 3 V no appreciable current flows and none lights; in parallel each LED sees the full 3 V. Internal resistance: the CR2032 has ≈ 10–20 Ω when new (it rises as it discharges and in the cold), so its short-circuit current is limited to about 150–300 mA and, under LED load, the terminal voltage drops: with the tree on you measure ≈ 2.7–2.9 V and a total of ≈ 10–20 mA flows (estimate: 2–3 mA per LED, enough to be clearly visible indoors). That internal resistance acts as the current-limiting resistor, which is why a 3 V LED connected directly to a CR2032 doesn't burn out. The same is not true of 2 AA batteries (internal resistance ≈ 0.1–0.3 Ω in total): there you do need one resistor per LED, R = (V − Vf) / I, for example 100 Ω for a red LED at 3 V and ≈ 15–22 Ω for a white one. Battery life: at a continuous 15–20 mA the usable capacity of a CR2032 drops to ≈ 100–150 mAh, so an estimated 6–12 hours of light, with decreasing brightness; that is why it has a switch. Power: 2.8 V × 0.015 A ≈ 0.04 W, which doesn't heat the cardboard. Polarity: the LED is a diode; it conducts from the anode (long leg, +) to the cathode (short leg, −, flat side of the package). The other way round it doesn't conduct, and at 3 V it isn't damaged (the typical maximum reverse voltage is 5 V). Switch: the steel paperclip joins the end of the battery strip to the − track; its contact resistance (tenths of an ohm) is negligible compared with the battery's internal resistance. Coin cell safety: 3 V doesn't give a shock, but if one is swallowed and lodges in the oesophagus, the current flowing through the mucosa produces hydroxide at the negative pole and a serious chemical burn within 2 hours; that is why the battery holder is taped shut. Structure: the two pieces are joined with a cross-halving joint; A's slot (95 mm) and the unslotted part of B overlap along the axis, and the sum of the slots (95 + 140 mm) is the height of the crossing (235 mm). The 10 mm wide section of B's slot leaves a 3.5 mm gap on each side of A so the tracks and patches pass through without rubbing.

• parallel circuit: several paths at once between the + track and the −: each LED has its own.
• series circuit: a single path that goes through one thing after another; if one fails, everything stops.
• polarity: the + side and the − side of a battery or an LED; the LED only lights one way round.
• internal resistance: the “brake” every battery has inside: it doesn't let out more current than it can give.
• short circuit: joining + and − with nothing in between: the battery gets hot and runs down very quickly.
• track: a copper path stuck to the cardboard that acts as a wire.

By age

• 5–7 years: The adult cuts out the pieces, pierces the holes, sticks on the tracks and fits the battery. The child sticks on the templates, paints, tests the LEDs with the battery held by the adult, pushes the LEDs in from the green side and presses the patches down with the pen. Focus on “long leg to +” and “the paperclip closes the path”.
• 8–10 years: They do everything except the craft-knife cuts, the holes and the battery. Get them to do the series versus parallel experiment and explain, using the ladder picture, why 5 LEDs keep shining if they take one away.
• 11–12 years: Get them to draw the circuit diagram with symbols, measure the battery with and without load and work out the internal resistance. Challenge: calculate the resistor that would be needed to power the tree from 2 AA batteries (one per LED) and compare how long it would last.

In a group or in class

One tree per pair (≈ €1.50 per tree for LEDs, battery and copper). Before the session: cut out A, B, E and M with the craft knife, pierce the holes, sort the 3 V LEDs by colour and cut the copper tape into lengths in envelopes, one per pair. The adult keeps the batteries and fits them at the end, one by one. 2-hour session: 15 min demonstration (series versus parallel with 2 LEDs and why we don't use candles), 20 min painting, 30 min copper tracks while it dries, 25 min LEDs and patches, 15 min battery (the adult goes round the tables) and 15 min assembling and switching on all together in the dark. Have 6 LEDs, 2 batteries and a roll of copper spare. At the end, count the batteries: as many fitted and taped shut as there are trees.

Challenges

• Add a 7th LED: find a spot between the two tracks, pierce two holes and connect it the same way. How much dimmer do the others get?
• Add a red LED with its 100 Ω resistor and check that it no longer steals the light from the others.
• Make the star shine: stick a warm white LED behind it with its own CR2032 in a card cradle just like the tree's, taped shut (an adult assembles it).
• Replace the paperclip with a push switch: a card flap with aluminium foil that joins the two copper areas when you press it.
• Design a bigger tree with two more tiers: how many LEDs could a single CR2032 power before they look too dim?

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