Light-up Christmas card

What they will learn

• That the light only comes on when the electricity's path is closed (battery → copper → LED → copper → battery), and that a switch is nothing more than a gap that opens and closes.
• How a "normally open" push switch works: a flap with copper that sits less than 1 mm from the battery and only touches it while you press it.
• What polarity is: the battery has a + side (the big one, with writing) and a − side, and the LED only lets current through if its long leg goes to the + side.
• That copper tape conducts through its metal side, and through the glue only if it has "conductive adhesive"; that is why the corners are folded instead of cut and joined.
• What a parallel circuit is: two LEDs between the same two tracks, each with its own path, lighting up at the same time.

Prepare beforehand

• Print the templates at 100% ("actual size") on A4 sheets and check with the ruler that the 50 mm square measures 50 mm. Templates P and C take one sheet each; the tree, the present, the flap and the rings go together on another.
• Read the description of your copper tape: if it says "conductive adhesive" you can join pieces by overlapping them; if not, each track is made from ONE single strip, folding the corners (step 6).
• Test each LED with the battery before the session: pinch the battery between the two legs, the long one on the + side (the one with writing). It should light up. Set aside any that don't.
• Keep the battery in its blister pack or in a closed jar until step 13: an adult fits it.
• For groups: cut out the stars on the fronts with a craft knife and the cardboard rings E yourself beforehand.

Materials and tools

• Coloured card for the greeting card — 1 A4 sheet (297 × 210 mm)
  200–250 gsm, red, dark green or midnight blue; opaque (when you hold it up to your phone's light, no light should come through)
• Green card — 1 piece, 9 × 11 cm
  160–200 gsm, for the tree
• Gold, yellow or red card — 1 piece, 4 × 3 cm
  for the present that acts as the button (a different colour from the card)
• White card — 1 piece, 8 × 8 cm
  200 gsm, for the switch flap and the adjustment ring: it must be stiff and spring back when you let go
• Corrugated cardboard — 1 piece, 6 × 4 cm
  single-wall, 3–4 mm, flat and not crushed (from a delivery box): for the ring that holds the battery and for the button
• White A4 sheets — 2
  80 gsm, to print templates P (front) and C (copper route) at 100%
• Tracing paper — 2 pieces: 40 × 40 mm and 24 × 24 mm
  drawing type, 90 gsm: translucent, it acts as a diffuser behind the star and the bauble
• Copper tape with conductive adhesive — ≈ 70 cm (you use ≈ 56 cm) buy
  5 mm wide, 0.05 mm thick, with "conductive adhesive" in its description (if it says "single conductive" or "non-conductive", the glue insulates: see step 6)
• 3 mm warm white LED — 2 (+1 spare) buy
  diffused (frosted lens) or clear, 2.8–3.2 V / 20 mA, both the same type and colour; 3 mm because a 5 mm one is 8.6 mm tall and won't fit with the card closed
• CR2032 coin cell — 1 buy
  lithium, 3 V, ≈ 220 mAh, Ø 20 × 3.2 mm, new
• Clear sticky tape — ≈ 30 cm
  19 mm: holds the LED legs and the flap, and shuts the battery in
• Glue stick — 1
  to stick the templates, the tree, the present and the tracing paper
• PVA glue — ≈ 5 ml
  school or wood glue, for the ring and the cardboard button
• Masking tape — ≈ 15 cm
  to hold template P on the front while cutting
• Red felt-tip pen — 1
  to mark the + leg of each LED before trimming it

Tools

• Craft knife with metal ruler and cutting mat — cut out the star on the front adult
• Scissors — cut out the tree, the present, the flap, the rings and the copper tape
• Small scissors or side cutters — trim the LED legs (they fly off: cover them with a finger) adult
• Awl or sharp pencil — start the 12 mm bauble hole and the hole in the rings adult
• Empty ballpoint pen and ruler — score the two spine folds
• Lolly stick or your fingernail — rub the copper tape so it sticks well and fold the corners
• Multimeter (optional) — check that the two tracks don't touch and measure the battery

Pieces and templates

• A · Card (A4 sheet with spine) ×1 — card 200–250 gsm
• P · Front template (for cutting out the star) ×1 — paper
• C · Copper route template ×1 — paper
• B · Tree ×1 — green card 160–200 gsm
• R · Present (button) ×1 — gold or red card
• D · Star diffuser ×1 — tracing paper
• K · Bauble diffuser ×1 — tracing paper
• E · Ring that holds the battery ×1 — corrugated cardboard 3 mm
• G · Adjustment ring (only if needed) ×1 — card 200 gsm
• F · Switch flap ×1 — card 200 gsm
• H · Button ×1 — corrugated cardboard 3 mm

Step by step

1. Score and fold the spine

   Lay the A4 card landscape with the outside facing you. Mark two vertical lines 144.5 mm and 152.5 mm from the left edge and go over them with the empty ballpoint pen and the ruler, pressing hard. Fold inwards along both lines: you get a 144.5 × 210 mm card with an 8 mm spine.

   Check: When closed, the front sits parallel to the back, with an 8 mm gap all across the card.
   Tip: Make the marks at the top and bottom of the sheet and join the dots with the ruler: that way the lines come out parallel.

2. Cut out the star and the bauble on the front adult

   Open the card and lay the front on the cutting mat. Place template P on top with its left edge exactly on the spine fold and hold it with 2 strips of masking tape. Cut out the grey star with the craft knife (10 straight cuts, from point to notch) through the paper and the card. Pierce the centre of the bauble and cut out a 12 mm circle. Remove the template.

   Check: Holding the front up to the light, you see a 30 mm star at the top, 42 mm from the edge, and a round 12 mm hole lower down, on the left.
   Tip: Always cut from each point towards the centre of the star: the points won't tear.

3. Stick the tracing paper inside

   On the INSIDE of the front, run the glue stick around the star (a 5 mm frame, without touching the opening) and stick the 40 × 40 mm square of tracing paper D on top, pulled smooth. Do the same with the 24 × 24 mm square K over the bauble hole.

   Check: From the outside, the star and the bauble look white and smooth, with no wrinkles or glue in the opening.
   Tip: If you want the star to shine more evenly, stick a second square of tracing paper on top of the first.

4. Glue the tree and the present on the front

   Cut out tree B from green card, along with its 12 mm hole. Glue it on the front with a glue stick, with the bauble hole right over the one in the front and the tip 2 mm below the star. Cut out the 34 × 26 mm present R and glue it centred at the foot of the tree, with its centre 72 mm from the spine and 180 mm from the top edge.

   Check: Measure with the ruler: the centre of the present is 72 mm from the spine fold and 180 mm from the top edge (right above where the battery will go).
   Tip: Position the tree first without glue and hold it up to the light: when you see light through the bauble, mark its corners in pencil.

5. Stick on the circuit template

   Open the card. Cut out template C along its 144.5 × 210 mm outline, cover it all over with glue stick and stick it on the RIGHT-hand cover (the inside of the back), with its left edge on the spine fold and the words "← SPINE" next to it. Smooth it with your hand from the centre outwards.

   Check: Close the card and hold the star up to the light: through the tracing paper you can see the template's LED 1 mark underneath.

6. Stick down the negative track (−)

   Cut 23 cm of copper tape without removing the backing paper yet. Peel back 3 cm and stick the tip on "− NEGATIVE: start here". Carry on, peeling a little at a time and rubbing with your fingernail along the copper-coloured band. At each brown dot (corner), fold the tape back over itself and turn it 90°: don't cut it. Finish at the centre of the ring circle and cut off what's left.

   Check: The − track is one single 5 mm strip, with no cuts or wrinkles, with 2 folded corners, and it ends at the centre of the small circle.
   Tip: If your tape does NOT have conductive adhesive, a track made from two pieces won't work: the glue insulates. Use a single strip from start to finish and fold the corners.

7. Stick down the positive track (+)

   Cut 26 cm of copper tape and stick it the same way, from "+ POSITIVE: start here" to the end of the band under flap F, folding the 4 marked corners. Rub the whole track with the lolly stick. Check that there is always 7 mm of paper between the + and − tracks: if a sliver of copper joins them, trim it off with the scissors.

   Check: With a multimeter on continuity (beep), touching one track and the other makes no sound; touching both ends of the same track does beep.
   Tip: Folding a corner: stick up to the dot, fold the tape back over the stuck part (copper side against copper), crease the fold with your fingernail and turn the strip in the new direction.

8. Prepare the two LEDs adult

   Mark the long leg (+) of each LED with the red felt-tip pen. Bend both legs to 90° right next to the LED, one to each side, like open arms: the LED stands up. Trim each leg 8 mm from the body of the LED, covering them with a finger so they don't fly off.

   Check: Resting on the table, the LED stands up on its two open legs, one with a red dot and one without.
   Tip: If the red mark rubs off: the side of the LED's rim with a straight (flat) edge is the short leg's side (−).

9. Fix the star LED

   Stand LED 1 on its mark, between the two top tracks: the short leg (−) pointing up, on the − track, and the long red leg (+) pointing down, on the + track. Tape each leg to its track with a 15 mm piece of clear tape and rub with your fingernail to press it flat against the copper. Don't cover the head of the LED.

   Check: Both legs touch metal (not paper) and the LED stands upright, pointing up.
   Tip: With conductive-adhesive tape you can hold each leg down with a small 10 mm square of copper instead of clear tape: the contact is even better.

10. Fix the bauble LED

    Stand LED 2 on its mark, between the two vertical tracks: the short leg (−) pointing left, on the − track, and the long red leg (+) pointing right, on the + track. Tape each leg down with 15 mm of clear tape and rub with your fingernail.

    Check: Both LEDs have the red leg on the + track (the inner one, which runs towards the flap).

11. Glue down the battery ring

    Cut out ring E from corrugated cardboard (36 mm outside, 22 mm inside): pierce the centre with the awl and cut out the hole with scissors. Put PVA glue on one side and stick it over the dashed circle on the template, on top of the − track. Leave it to dry for 10 minutes with a book on top.

    Check: You can see the end of the − track in the centre of the ring; a 20 mm battery fits in the hole without forcing and without wobbling.
    Tip: Don't let any glue drip into the hole: the end of the − track must stay clean and shiny.

12. Prepare the switch flap

    Cut out the 68 × 30 mm flap F from white card. On the side WITHOUT the drawing, stick a 68 mm strip of copper tape along the middle, from end to end. Gently fold the two marked folds (at 36 and 42 mm) without flattening them: the flap forms a step of about 4 mm.

    Check: With the copper facing the table, the 26 mm part lies flat and the 36 mm part is in the air, about 4 mm above.

13. Fit the battery and close the flap adult

    The adult puts the CR2032 battery in the ring with the + side (the big one, with writing) facing up. Place flap F with the copper facing down: the 26 mm part over the end of the + track (copper to copper) and the 36 mm part over the ring. Hold the stuck-down part with 2 strips of clear tape and tape the free end to the ring with another strip: the battery is shut in.

    Check: Without touching, the LEDs are off. Press the centre of the flap with your finger: both light up. Let go: they go off.
    Tip: If the LEDs light up without pressing, the ring is too low: the adult peels back the tip, takes out the battery, glues adjustment ring G on top of E and closes it again.

14. Glue on the button and test with the card closed

    Cut out the 14 mm disc H from corrugated cardboard and glue it with a drop of PVA glue to the centre of the flap, over the battery. Close the card and press with your finger on the present on the front, with the card resting on the table.

    Check: When you press the present, the star and the bauble light up; when you let go, they go off. With the card closed and not pressed, they stay off.

15. Write your greeting and test it in the dark

    Write your message on the inside of the front (the left-hand cover), around the tracing paper squares. Turn off the light, close the card and press the present: the star on top and the bauble on the tree shine through the tracing paper.

    Check: In the dark, from 1 m away, you can see the whole star lit up (brighter in the centre) and the round bauble.
    Tip: Write "Press the present" on the outside of the back. Send it in a C5 envelope (162 × 229 mm): it fits with the 8 mm spine.

Does it work? The test

With the card closed on the table, in a dark room, press the present 5 times in a row and then hold it down for 10 seconds. Then open it, stand it up and press the flap directly.

What should happen: Every press lights the star and the bauble at the same time, and when you let go they go off instantly. Without pressing, they never light up, even with the card closed. You see a lit 30 mm star, brighter in the centre and softer at the points, and a 12 mm circle of light. No light comes through the rest of the front.

If something goes wrong

• No LED lights up when pressed — The battery is upside down, the flap doesn't touch the + track, or a track is broken (a piece joined with non-conductive adhesive tape). Solution: Check that the side of the battery with writing faces up. Press the stuck-down part of the flap with your fingernail and add another strip of tape on top. If a track is in two pieces, stick a copper bridge across with its tip folded back (metal side against metal) and hold it with clear tape.
• One LED lights up and the other doesn't — The unlit LED is the wrong way round or one of its legs is touching paper instead of copper. Solution: Remove its tape and check that the red (long) leg is on the + track. Press the legs flat against the copper and tape them down again with new tape.
• The LEDs are always on — The flap touches the battery without pressing: the ring is too low, the cardboard is crushed or the battery is thicker. Solution: The adult takes out the battery, glues adjustment ring G on top of E (0.3 mm more) and closes it again. If it still touches, glue on another ring G.
• You have to press very hard or it lights up on and off — The ring is too tall (4–5 mm cardboard) or button H doesn't reach the front. Solution: Replace ring E with 3–4 card rings G glued together (≈ 1 mm above the battery). If it's the button, glue a second disc H on top.
• The LEDs flicker when moved or shine weakly — Loose contact between a leg and the copper, or a flat battery. Solution: Rub the strips of tape on the legs again with your fingernail or replace them with small copper squares. Measure the battery: below 2.7 V, replace it with a new one.
• The battery gets warm or runs down in a day — Short circuit: a sliver of copper joins the + track to the −, or the edge of the battery (which is also +) touches the − copper. Solution: Look with a magnifying glass for a spot where the tracks touch and cut it. Cover the part of the − track under the edge of the battery with clear tape, leaving only the centre exposed.
• I'm using red or yellow LEDs and they shine very brightly or one doesn't light — 2 V LEDs get too much current from the 3 V battery, and if you mix colours in parallel the one with the lower voltage takes all the current. Solution: Use 2 identical LEDs. With red or yellow LEDs, put a 100 Ω resistor in the + track just before the flap: cut a 10 mm gap in the track and tape each leg of the resistor to one end with clear tape.

Safety

• Coin cell: if a child swallows it, it can burn their oesophagus in less than 2 hours. An adult keeps it and fits it, and the card is closed with the flap taped down so the battery can't come out. Don't give this card to children under 6 or leave it where younger siblings can reach it.
• If you suspect someone has swallowed a battery or put it in their nose or ear, go to A&E immediately or call 112, even if there are no symptoms. Toxicology Information Service: 91 562 04 20.
• Don't connect the two sides of the battery with copper, aluminium or any metal: the short circuit heats it up and drains it. Check that the + and − tracks never touch.
• Only an adult uses the craft knife, with the cutting mat underneath, always cutting away from the hand that is holding.
• The edges of copper tape are slightly sharp: peel it slowly and don't run your finger along the edge. Trim the LED legs while covering them with your finger, away from your eyes.
• No candles: this card gives light only with the LEDs, which barely get warm. When the battery runs out, cover it with tape and take it to a battery recycling bin.

The science behind it

For children

Electricity is like a train that can only run if the rails make a complete circle: it leaves the + side of the battery, travels along the copper track, crosses the LEDs and comes back along the other track to the − side of the battery. While the flap isn't touching the battery, a piece of the rails is missing and the train stops: it's an open circuit. When you press the present, the flap goes down, touches the battery and completes the circle: the circuit is closed and the LEDs light up. LEDs are like a door that only opens one way, which is why the long leg has to go to the + track. The tracing paper is like a fog that spreads the light across the whole star.

For the adult

Circuit: a CR2032 cell (lithium–manganese dioxide, 3.0 V nominal, ≈ 3.2 V when new, ≈ 220 mAh) powers two 3 mm warm white LEDs in parallel through a normally open push switch. The large face with writing and the rim of the cell are the + terminal; the small face, surrounded by a plastic seal, is the −. The − track ends under the centre of the cell and the + track reaches the flap, which has copper underneath. The corrugated cardboard ring (3.5–4 mm) is slightly taller than the cell (3.2 mm): the flap rests on it 0.3–0.8 mm from the cell (open circuit, zero consumption, so the card can be stored for years) and pressing the present makes button H bend the flap in the middle until it touches the + face (closed circuit). The 8 mm spine keeps the front parallel to the back: there is room for the cell with its flap and button (≈ 8 mm) and for the 3 mm LEDs (5.3 mm tall, with 2.7 mm of air up to the tracing paper); a 5 mm LED is 8.6 mm tall and would not fit. Current: the white LED (blue InGaN chip with phosphor) needs ≈ 2.8–3.1 V to conduct; the internal resistance of the CR2032 (≈ 10–20 Ω when new, more as it runs down) and the very steep curve of the LED limit the current to about 5–15 mA per LED (estimate), below the nominal 20 mA, so no resistor is needed. With red or yellow LEDs (≈ 2.0 V) the excess voltage would be ≈ 1 V and the current would rise to 40–60 mA, hence the 100 Ω resistor ((3.0 − 2.0) V / 100 Ω ≈ 10 mA). In parallel, both LEDs get the same voltage: if they are the same type they share the current; if one is red and the other white, the red one starts conducting at 2 V and the white one barely lights. Estimated battery life: with 15–25 mA in total, 220 mAh gives about 8–15 hours of light (the real capacity of a CR2032 drops at currents above 1–2 mA and the light gradually dims). Copper tape: copper foil of ≈ 0.035–0.05 mm; a 5 mm × 23 cm track has ≈ 0.02 Ω, negligible. The metal side always conducts; the adhesive can be ordinary acrylic (insulating) or "conductive", with metal particles (nickel or silver-plated copper) that link the two sides through the glue with a resistance of tenths of an ohm per join. With insulating adhesive, an overlap join doesn't conduct (metal–glue–metal); that is why each track is made from a single strip and the corners are folded, so the copper stays continuous; if a join is needed, the tip of the top piece is folded back so that metal touches metal, or it is soldered, or held with tape pressed down hard. The contact of the LED legs and of the flap is by metal-to-metal pressure, which is why you rub with your fingernail: copper is soft and moulds itself to the leg. Optics: tracing paper is paper without mineral filler, with very refined, pressed fibres; it lets through ≈ 60–70% of the light but scatters it in all directions, so the star looks like a glowing surface rather than a dot. Because the LED is only 2–3 mm from the paper, the centre is brighter: with 2 layers or a diffused LED the light spreads better. Safety: a coin cell lodged in the oesophagus, in contact with the moist lining, causes electrolysis that produces hydroxide next to the − terminal and causes severe burns within 2 hours; that is why the cell is shut in between ring, flap and tape, and an adult fits it.

• closed circuit: a complete path, with no gaps, along which electricity leaves the battery and returns to it; the light comes on.
• open circuit: a path with a gap (for example, the flap not touching the battery); electricity doesn't flow and the light goes off.
• push switch: a switch that closes the circuit only while you press it, like a doorbell.
• polarity: the + side and the − side of the battery and of the LED; the LED only lights if its long leg goes to the +.
• in parallel: two LEDs connected to the same two tracks, each with its own path.
• conductive adhesive: glue with metal particles that lets electricity pass from one piece of copper to another.

By age

• 5–7 years: The adult folds the spine, cuts out the star, prepares the LEDs and fits the battery and the flap. The child sticks on the tracing paper, the tree and the present, and sticks down the copper tracks following the drawing with help (the adult folds the corners). Key idea: "the path has to be closed" and "long leg to the +".
• 8–10 years: They do all the steps except the craft-knife cuts, the legs and the battery. Ask them to explain in their own words why the light goes off when they let go of the present, and to do the experiment on materials that conduct.
• 11–12 years: Ask them to draw the circuit diagram with symbols (battery, switch, two LEDs in parallel), check with a multimeter that the tracks don't touch and do the conductive adhesive experiment. Challenge: design their own copper route with 3 LEDs for another greeting card.

In a group or in class

One card per child, working in pairs to help each other with the copper (≈ €1.50 per card: 2 LEDs, battery and 55 cm of tape). Before the session: print the templates, fold the spines, cut out the stars with a craft knife, cut the rings E and prepare the LEDs with their legs bent and trimmed on a tray. 75-min session: 10 min of demonstration (open and closed circuit with a battery and an LED on the board), 15 min for the front, 25 min for the copper tracks and LEDs, 15 min for the ring and flap and 10 min of testing in the dark. The adult hands out and fits the batteries, one at a time, at the end, and each flap is taped shut straight away. Keep 5 LEDs, 2 batteries and spare tape to hand.

Challenges

• Add a third LED in another bauble on the tree: it just has to stand between the two tracks, with the long leg on the +. Do all three shine the same?
• Make the version that lights up when you open the card: put the flap at the edge of the cover with a card tongue from the other cover that pushes it away while the card is closed.
• Design your own greeting card: a house with lit windows or a reindeer with a red nose (with a 100 Ω resistor for the red LED). Draw the copper route on squared paper first.
• Replace one LED with a 3 mm colour-changing LED (slow RGB): does it need the same voltage? Look up its datasheet and compare.
• Measure the current the card uses with 1 and with 2 LEDs using a multimeter and work out how many hours the battery will last.

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