Heating paper/cardboard (thermochromatic ink)

[Moriambar]

Hi,
a friend of mine has bought some thermochromatic ink (I hope the English term is correct) which goes transparent when heated to ~40 °C.
I was wondering how to design (mostly: where to learn about) a circuit that heats the paper or cardboard where the ink is used in order to control when to make it "disappear". Hopefully not becoming a fire hazard.

My main problem is that I don't know what to use to heat. Knowing the component to use it will be fun trying and designing a circuit and then coming back for further insight.

So can you point me towards some informative material? And to a heating component?

Thanks

[beanflying]

The simple thing to use is a resistor and push it toward it's rating.

Keeping the maths simple a 100  1/4W resistor running on 5V will draw 50mA. This gets you P=VI of 5x0.05 or .25W Should see the Resistor at 80-100C on the surface (a guess) well below any danger point for your paper catching fire. So any old USB power supply or power bank a 100  resistor and play.

If you want a larger flat surface area than the round resistor chop up an aluminium can and wrap a bit of it around the resistor as a crude heatsink with a flap hanging out. It should still hold over 40C if you don't make it to large.

From there you could use several Resistors and make a simple PWM (pulse width modulation) controller. This could be as simple as a 555 timer switching a FET feeding your resistors or an Arduino and a bit of code to generate the PWM to then feed a FET.

There is plenty of ways to generate heat including the likes of these eBay auction: #254795972487 go to something solid state like a Transistor or Fet as a load on a supply but these all add complexity.

Just a few ideas to start you looking/playing.

[Moriambar]

Resistors pushed to 100°C sound a bit scary, but thanks for the ideas.

[beanflying]

You haven't lived until you have let an appropriate level of smoke out 

Without checking to much Resistors are generally rated to 70-80C at full load so if you tweak the number to the next Resistor you have available higher you will drop down in Temperature. Generally the worst case with Resistors is a pop and a fizz and nothing to exciting. You are more likely to get an ouchie because you touched it and found it to be hot