Ultraviolet Business Card Flashlight




If you've seen some of my other business cards, I like the idea of giving away cards that are useful in some way.  Most of my others have been more proof-of-concept, but this one is not only fun but easy to make, inexpensive and has a real wow factor.

It's a simple ultraviolet torch, but the basic design can easily be made as a normal (white) flashlight instead, and the components only cost a couple of dollars.  I designed it originally for the Science Museum in London as an interactive workshop for kids, but the adults liked them as much as the children so I decided to keep the design as a business card.  It's a great way of advertising that you see things in a different light, particularly if you are involved in science, technology or a creative field.  If you're into making things, you might even want to give it away as a kit that recipient makes themselves.

The plain white version is also great and makes a very useful addition to somebody's wallet that they will probably not want to throw away.  The second image is a picture of a 'morse code' version as suggested by Johenix in the comments below (thanks for the great idea!).

There are lots of really fun things you can look at with UV light and the pictures above show a few of them - looking at invisible marks on things like bank notes, passports, and driving licenses, testing out how well you really  wash your hands, making things glow in the dark (the "Hi!" message was 'drawn' on glow in the dark material), making yellow highlighter ink fluoresce (the skeleton on the hand),  investigating certain bacteria, and writing in invisible ink - more about these on the last step of the instructable.  You can even go scorpion hunting (not recommended!) as they fluoresce under UV light.

I'm also going to publish this instructable at the same time as an educational version with a simpler build, and more details of how to use it in teaching.


Step 1: What You'll Need

I told you this one was simple ... all you need is
  1. A couple of blank PVC ID cards
  2. A 3mm ultraviolet LED
  3. Some 5mm (or 1/4") thick foam of some sort that squishes and returns to shape again 
  4. 1 x CR2032 lithium battery
  5. If you want your own design on the front, you'll also need some way of printing on it. I used some inkjet compatible white self-adhesive vinyl.
You'll also need some way of gluing it together (hot melt glue is good) - I used some double-sided tape I had on hand.

I've put together a few links below of suggested suppliers and parts - with postage costs and minimum quantities you're probably looking at $2-$3 each if you order them in 100+.   

If you do the build yourself, this is what you'll need:

Step 2: The Artwork

First of all, design your artwork - getting this looking good can make the cards look really professional.  Use whatever graphics package you're familiar with - "Inkscape" is a good open source package, or you might already have Microsoft Publisher.  You'll want to design them around standard credit card dimensions (85.6mm x 53.98m or 3.370" × 2.125") but you'll also want to make them just a bit bigger on all sides (called a bleed) - I made mine 92mm x 60mm).

You will probably want to print some sort of icon for the switch (around 20mm in diameter) towards one end of the card - mine looks like a light bulb.  This is really just a part of the card which will be pushed to turn it on (you will see later how this works - it will align with the battery which is in a circular cutout in the foam behind this).

Then print them out, and cut them roughly around the border (they should still be over-sized).  Depending on your inkjet, you might want to spray a clear protective inkjet varnish over the printout before the next step if you want them to be really long-lasting.

Step 3: Sticking It Together

Peel the backing off the self-adhesive vinyl, and stick it to one of the blank cards, keeping an even border around each edge.  Turn it upside down, and carefully trim around with a scalpel or sharp hobby knife (the second picture shows it partly trimmed down the right side and bottom right corner -  if I had three hands I would have photographed myself cutting it as well!)  The third picture shows the finished card after trimming.

Step 4: The Foam Insert

Cut out a foam rectangle a few mm shorter on each side than the credit card, and cut a 20mm diameter hole into the middle of one end as shown - exactly where isn't crucial, but it should be close to one end.  A scalpel will help with this.  I cheated and did my prototypes like this, but ran off the rest on my laser cutter.  You can also cut out a notch as shown about the same width as the LED, but this isn't crucial.  To make it easier if cutting out with a scalpel, it actually works just as well if you make the 20mm cutout a square (with sides 20mm) rather than a circle.  Have also attached a PDF of the pattern I used if that helps anyone.

Now you'll need to stick the foam onto the card - I used two strips of special high-strength double-sided tape, but for most people a glue gun will be a better choice.  It works best just putting adhesive along the two long edges I've found.  If you've designed your card with a picture of a button on it like mine, you'll want to make sure the cutout hole is behind the printed button!

Step 5: Put in the Battery

Now put in the coin cell into the cutout in the foam.  Don't worry about which way up you put it at the moment.

Step 6: Attach the LED

You will need to pull apart the legs of the LED a little, but trying to keep them roughly parallel.  They need to be about 5mm (1/4") apart, but this is not super critical.

Now push one leg underneath the cell (will go between the card and foam), and the other leg should go above the cell.

If you have done it correct, when you press down on the leg which is on top of the cell, the LED should glow purple.

If it doesn't glow at all, pull it out, rotate it 180 degrees (so the opposite leg is on the bottom), and put it back in.

If it glows all the time (without pushing), you will need to pull it out, spread the legs further apart, and put it back in again.

Step 7: Complete the Other Side

Now stick the other card down in a similar way to the first one.  Your second one will probably be blank - mine had a logo on the back as well as this was the one I did for the Science Museum.

Step 8: Have Fun!

Now it's time to give your cards away ... although you might want to have a play yourself first!

If you didn't see some of the fun pictures in the Introduction to the instructable have another look now for ideas ... here are a few things to try:
  • Look at foreign bank notes, passports, drivers licenses - even a 2nd class (UK) stamp has a hidden mark on it.  Tonic water fluoresces (only slightly), credit cards are great as well and usually have a couple of secret marks on.
  • Draw on something with a highlighter pen (yellow and orange works best I think) and then look at it under the UV torch.  I like to draw a picture of a skeleton on the back of my hand with a yellow highlighter when I'm doing workshops - the kids love this.
  • If you've got anything that glows in the dark, you'll be able to energise it with the torch (make it glow).  If you hold it really close (with the LED touching it, you will probably be able to 'draw' with light!
  • Get yourself a UV pen to security mark your valuable at home or just to write messages in invisible ink.  If you do want to use this for secret messages, it works great on the back of one of the blank PVC cards.  Standard paper has whiteners added that make it hard to see the message - you might have more luck with brown paper.
  • When it is really dark, go hunting in your bathroom to see how well it is being cleaned ... certain bodily fluids (like urine) fluoresce under UV light.
  • One activity which is really fun is to see how well you wash your hands.  You can get a cream that you rub onto your hands before washing them (http://www.glitterbug.com).  Anything left afterwards (and there will probably be a surprising amount) will glow strongly.
  • Some types of bacteria glow under UV light, although you will probably need to culture them properly in a petri dish as small quantities on surfaces, etc will not fluoresce.
  • Theoretically you could also find many types of scorpions with this light, as they fluoresce as well!
Have fun!



    • Gardening Contest

      Gardening Contest
    • Woodworking Contest

      Woodworking Contest
    • Arduino Contest 2019

      Arduino Contest 2019

    28 Discussions


    6 years ago on Step 7

    How does the lead of the LED not touch the battery when you attach the second ID card?

    1 reply

    Reply 6 years ago on Step 7

    The foam is a little thicker than the battery - this, and the previous step (spreading the legs of the LED apart) mean that they don't both touch at the same time unless you squeeze the card.


    7 years ago on Introduction

    Dear Ultra Violet Light Business card thanks for your help from your idea & synchronized with a few other Canadians, a sauna guy & his Dad, I came up with a heated Trumpeter Swan Nest, just really a giant version of your uv biz card with a 12Volt battery portable that comes with a flashlight from Amazon, your black foam but so thick it hides the whole battery & casing, with a trip switch instead of a pressure point, & let the Trumpeter Swans figure out how to switch it on & off themselves at night, through the waterproof ink printed on white viny sandwich top...I will post the picture on the bottom of my How to make a Trumpeter swan nest Instructable now...give me a 5 to do please...


    7 years ago on Step 6

    Shouldn't there be a current limiting resistor in the circuit?

    1 reply

    Reply 7 years ago on Step 6

    Well spotted! Actually we are bending the rules here but it is quite justified for a couple of reasons:

    1) Typical lithium cells of this size can't source enough current for long enough to kill the LEDs (usually quite a bit over 25mA)

    2) Minimal current flows through an LED anyway until you overcome their 'forward voltage drop' which is usually a little over 3V for white, blue or UV LEDs. As this is less than the cell voltage (around 3V) then we get only a small amount of current (but then again, less light than we could otherwise)


    7 years ago on Introduction

    I think i will do this for our makerspace, except I am thinking of replacing one of the cards with an rfid card since our front door is rfid controlled. I think this would add some function at a small price increase.

    1 reply

    Nice momentary switch. I'd bend the tips of the legs on the LED, so that they anchor into the foam. Angled outwards to not short against each other, of course. What about adding some EL wire to the outer edge of the card? Is there a driver small enough to insert into the foam?

    1 reply

    7 years ago on Introduction

    Spray adhesive - available in art shops - might be tidier (more professional?) than hot-melt or double-sided. Just make sure you get the kind that works for your materials, there are many to choose from and not all stick to everything (that's why there are many to choose from, doh!)

    1 reply

    Reply 7 years ago on Introduction

    Good idea - you will find though that you don't see the tape or glue when assembled , particularly as the foam rectangle is slightly smaller than the cards on either side.


    7 years ago on Introduction

    I was only advising to hire a professional GRAPHIC designer for that element as the benefits are well worth it. I apologise that I was not more clear in my wording.

    Have a nice day all!

    Shadetree Engineer

    7 years ago on Step 2

    Lasertag cards? Nice! That would be an awesome giftcard to a lasertag business. The giftcard should have a small laser. I've seen some 3mW red laser modules on ebay that had the lens cast in place around the diode, so it's a really small package. No link, sorry.


    7 years ago on Introduction

    I think your idea is great! It would be nice to have a unit that put out all of the band widths of light. Why didn't you use higher wattage LEDs? With the cost of the Chinese one, five and ten watt units at good priced I have a 1 watt UV unit and a one watt IR unit with a 5 watt white. It does a good job for not much more money.



    7 years ago on Step 2

    If you are piggy backing off someone else's cool contribution to the Maker community it would be a 'Nice' thing to refer to your own helpful 'Instructable' which shares additional experience and fun. Many curious Makers will find that these cool things are fun but, when doing for our own businesses they will distract from our bread and butter. What does this mean? People will hire a professional but, likely not one taking from someone else's contributions.....