The Secret PIN Number





Introduction: The Secret PIN Number

About: Innovative Projects, Diy's, Life Hacks

Have you ever forgot the PIN number even though you use the credit card every day?

It happened to me (more than once).

Last time I went to the Scotland, I could not take out any money as I forgot the PIN number.

It's interesting how you can suddenly forget the number combination that you use every day.

Luckily it came back to me the next day. The trick is to let your hands/fingers do the thinking not your brain :)

They say you should never carry a written PIN number inside of your wallet. If somebody will steal it, they will most likely find the PIN and use your card.

I personally think it's OK to write down your pin as long as you encode it and hide it.

You could always write down your PIN one way and read it the other way.

For example: The pin 1234, could be read as 4321 or 2314, 2341 etc.

Not secure enough?

Let's hide the pin number in the plain sight.

What's needed:

  • Thermochromic Pigment
  • Access to the printer
  • Paper, cardboard, glue
  • Nail Polish or in my case Gel effect Nail Polish

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Step 1:

I mixed the pigment with gel effect nail polish.

Step 2:

I printed out a fake business card and artwork with a PIN number on it.

The pin number was printed with a light blue ink - a few shades lighter than the pigment.

Step 3:

I glued the business card on top of the cardboard to make it thicker.

Step 4:

Then I painted the paw blue.

Step 5:

Thermochromic pigment will become transparent when heated.

I heated up the paw with a hair dryer and cut it out.

Step 6:

Then I glued it to the business card.

Step 7:

That's it!

If somebody will steal your wallet, thief will not know that the business card has a PIN number written on it.

If you ever forget the PIN number, just warm up the paw with your finger to see it.


There are different temperature pigments available.
I used 31C (87F) pigment, but I have also done it with 33C (91F) pigment and it worked fine.

31c, 33c - That's the temperature when the pigment will start to change the color.

There are also lower temperature pigments available.

28C, 21C and even 18C. Lower temperature pigments are great if you want to use more layers of nail polish, but I don't like the way it looks with a thick layer. Keep in mind that low temperature pigment+thin layer of nail polish might make the pin number appear even without touching it (if it's hot outside or the wallet is very warm). With 31c - 33c pigment the layer can't be too thick, otherwise it will take you ages to heat it up with your fingers. So if you use a 31c - 33c pigment, you have to find the balance between the right shade of font color, quantity of the pigment you put in the nail polish and the thickness of the layer.



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    19 Discussions

    that is very cool. hopefully no instructable users are also thieves... :)

    Why not enter your pin into your mobile (cell) phone. Simply create a name which 'refers' in your mind to your credit or debit card. Then enter a 'phone' number with the last 4 digits being your PIN number. Simples.

    4 replies

    Where do you write the pin number for your phone? :-)

    Yes, you can. There are many ways to do it.

    This is just one of the ways.



    2 years ago

    Very high tech - love it.

    Low tech version -

    Something I have done before is write out a 10 by 10 grid of numbers, which is then kept in my wallet and hide the PIN numbers inside that, you can use real phone numbers, or numbers of houses you know, or years etc. To any thief the numbers would seem random, plus you can write them from bottom up, diagonally etc - as long as it triggers something for you in your mind.

    1 reply

    That is very cool. I think it will work because the thief wont know to warm up the card.

    Cool music too.

    1 reply

    What's even easier: take the business card's phone number and have it include your pin at positions 1, 3, 5 and 7 (or where ever you like). So instead of 637-872-010 you'd print 132-832-410 (not bold-facing them, it's just you see that 1234 is hidden in the phone number). Or simpler the first 4 positions: 123-472-010.


    2 years ago

    hm theif has warm thumbs too! lol... i dont think it good for security purposes. i see alot of potentials in pranks though!

    1 reply

    Yes, but you have to press it in a specific spot for a while.

    I don't think the thief will be checking out a pet sitting service business card - but you never know :)

    Interesting idea. How long does the nail polish take to wear off? I know I always seem to wear the ink off of business cards if I leave them in my wallet.

    Have s great day! :-)

    2 replies

    I have no idea.

    I don't think I'll be using it too often to wear off the nail polish :)



    2 years ago

    I like this. You should come up with more hidden in plain sight DIYs


    2 years ago

    A forghotten grocery list in your wallet can do the trick as well (if your code is 1234):

    -1 lbs of tomatoes

    -2 roll of toilet paper

    -3 chocolate plate

    -4 pairs of socks


    Most phones have obsolete contacts, not yet deleted. A password or code can have a strong reminder in the contact list., or a grocery list, or even an image.