1
\$\begingroup\$

I’m an elementary / primary school teacher and I would like my students to be able to create Christmas / holiday cards with LEDs, using either copper tape or graphite pens (like circuit scribe) … BUT I can NOT use coin / cell batteries due to there being children involved (rules in my country prevent these being sold without serious safety warnings etc and the school just can’t touch anything so restricted).

I want to find a replacement! I looked up 2 AA batteries and that I’d need resistors … was hoping for a one step solution (mostly due to cost) … a CR123 battery? How could they be connected in a circuit?

What alternatives could I try that would be

  • Cheap (it would be for 100+ students)
  • Few components
  • Easily assembled by children
  • Less bulky (it will be attached to a greeting card)

Likely to be maximum 3 LEDs per card (or only 1 if that’s easier), haven’t bought the LEDs yet so not sure if colour is important. (Could I buy a string of Christmas lights and disconnect them to get cheaper LEDs?!)

Would really appreciate any help as my electrical knowledge is very limited and I don’t wish to electrocute children!!! Thank you!

\$\endgroup\$
12
  • 2
    \$\begingroup\$ Gloves? Fitting the battery into the card is or is not a requirement? Is swallowing the cell the safety concern? Perhaps a small/thin Li-po (e.g. 200 mAh) would work. However, almost nothing will be less expensive than coin cells. \$\endgroup\$
    – JYelton
    Commented Aug 22, 2023 at 5:34
  • \$\begingroup\$ Thanks, Coin cell doesn’t pass the risk assessment as it can be detached and swallowed! Can spend a bit more than coin batteries if I can find a solution. \$\endgroup\$
    – TeacherLC
    Commented Aug 22, 2023 at 8:05
  • \$\begingroup\$ Is the size of the cell the only issue? Take the cell, put it in a battery holder, put some shrinking tube or tape around. This should make it bulky enough to not count as a hazard any more. \$\endgroup\$
    – asdfex
    Commented Aug 22, 2023 at 10:17
  • \$\begingroup\$ What country are you in? That may help us answer. Do you want the lights to turn on when card is opened. Or to flash? Or ... ? White/ Red / ... LEDs wanted and/or OK \$\endgroup\$
    – Russell McMahon
    Commented Aug 22, 2023 at 13:04
  • \$\begingroup\$ CR123 is a good choice for some LEDs (Red Orange Yellow, some green, some blue). Some white but not as long. A long long long time on the 1st 3 colours and maybe green. || DANGER while it is harder to swallow a CR123 battery, they can do as much OR MORE damage than a coin cell if swallowed. \$\endgroup\$
    – Russell McMahon
    Commented Aug 22, 2023 at 13:16

2 Answers 2

0
\$\begingroup\$

As a base plate I would recommend using tiny prototype pcbs, which are cheap to buy.

For the LEDs I would recommend 3mm or 5mm Colour LEDs, they are easily obtainable in a 100/500/1000 package.

Maybe use a button in series with the LEDs, so they light up when you press it. Tactile switches are easy to get in bulk.

You can use a USB type A connector for power delievery. Your students can plug it into their devices which have a USB Port. Be aware that this solution requires a solid electrical design and a lot of thoughts should be spent on safety! The design should also be combined with a fuse of some kind, PPTC Fuse or else. 5 Volts can be touched without any harm, nevertheless using hotglue to isolate the connections is never bad for safety!

You can drive each LED with a current limiting resistor, this would be the simple solution. If you are using 5 Volts, you can combine 2 LEDs in series so that their voltage drop is slightly greater than 5 Volts. Typical values for green LEDs range from 2.7 to 3.5 Volts, red ones lies between 1.8 and 2.2 Volts. Be aware that the voltage drop depends on the current flowing through the LED, use the datasheets to calculate a current which is a good margin under the maximum current. It should be noted that cheap LEDs differ slightly, the tolerances are not great!

\$\endgroup\$
2
  • 1
    \$\begingroup\$ I think this sounds a bit too complex for the age group (5-12 year olds)! Thanks for your great information. \$\endgroup\$
    – TeacherLC
    Commented Aug 22, 2023 at 13:30
  • \$\begingroup\$ Thinking out of the box! \$\endgroup\$
    – greybeard
    Commented Aug 22, 2023 at 14:43
0
\$\begingroup\$

For a switch you can use a sandwich of foil - paper with hole in - foil. Tape foil square around edges to give tension. Pressing over hole makes circuit.

Instead of foil you can use wire mesh sold for eg screen doors (must be conductive.
You can also make mesh slight into domes to get click action.

To adjust switch force use thicker separator (several sheets of paper or light card) , and/or vary hole size.

Battery:

You MAY be able to use a version of a potato or lemon battery !!! :-).
See links below.

Summary:

  • A "real" battery can be made by separating two different metals by a spacer soaked in an electrolyte.

  • Common electrolytes are lemon juice (citric acid) and potato (phosphoric acid)t.

  • Common metals are zinc and copper.
    Better is magnesium and copper.

  • Zinc and copper can be coins or pieces of wire or nails or ...

To light a red LED you need at last two "batteries" in series. Three is better.

If you were using eg coins (call the two types Ca and Cb) then a two cell battery would be:

  • Ca-blotter-Cb-Ca-blotter-Cb.

  • Connect wires to end coins

Lemon or potato juice can be soaked into the blotters.
Coins are also able to be swallowed BUT are nowhere as hazardous as batteries.
The whole assembly can be covered with thick tape over eg card or plastic sheet.
Wire may be able to be used VERY safely.

See the many references for ideas.
Web search on: lemon battery - or - potato battery

A potato battery will make MORE voltage.

  • Ca_potato slice-Cb_Ca_potato slice-Cb :-)

You can stick wires into eg lemons or potatoes, or use slices, or use juice soaked blotter or paper towel.
A dried out blotter battery can be rejuvenated with a small drop of water.

lemon battery Image search - every image leads to a web page.

Lemon battery video

Wikipedia lemon battery

and more
and more
and more

Green LED - 4 lemons OR slices OR blotter

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.