Heating Circuit with Thermochromic Ink

Thermochromic pigments and paints change color or go clear (more like a translucent white) when they reach a certain temperature. When combined with fabric and a heating element they can be used to design dynamic textiles. Color-changing can be used as an indication, to express an emotion or to reveal something that wasn’t seen before. 

Heating elements can be designed using different materials. Conductive threads and fabrics are a great place to start experimenting. To control when the heating element goes on and off in an interactive piece a microcontroller can be used. The heating element will draw too much power for the microcontroller to handle so an external battery is used along with a MOSFET transistor. The MOSFET is used as a switch that opens when it receives current outputted by a pin on the microcontroller. Once open, the transistor lets the current flow to the heating element from the external power source. 

There are a few questions that need to be answered by a couple equations before you start. 

  1. How much current with my heating element draw? 
  2. How much power will the heating element draw? 

These two will lead to choices of heating element material and power source. 

1) The first question can be answered with Ohm’s law. Choose a power source that you have available to you. Let’s say a 3.7-volt LiPo battery. Then cut a piece of conductive thread or fabric and measure the resistance of that. For example, I have a strip of iron-on conductive fabric that measures 5.6 Ohms

Use the known voltage of the battery and the resistance of your heating element and plug those two values into Ohm’s Law to get the current it will draw from the battery. 

I = V/R

.66 Amps = 3.7V/5.6 Ohms

We now know that if that battery is connected to the heating element it will draw .66 amps from it. 

2) The power draw you also want to know so you know whether the power source you use will be enough. To get that all you do is multiply the amount of current that will be drawn by the voltage of the battery. 

.66 x 3.7 = 2.47 Watts

This equation is really useful when you get to testing heating elements that have a very high resistance or no resistance at all. Let’s run a 1,ooo ohms element through the equations. 

.0037 Amps = 3.7V /1000 Ohms

.0037 x 3.7 = 0.01369 Watts

0.01369 Watts is not a lot of energy, this will not produce any heat. We are looking for something at least around 1 watt. 

Now let’s do a piece of wire which will have nominal resistance of about .02 ohms. 

185 Amps = 3.7V / .02 Ohms

Well, we can pretty much stop there. You don’t want to be anywhere near a battery that produces 185 if you can find one. What this means for your battery, however, is that it will zap it for all it’s worth. It will empty the battery out creating a short and perhaps start a small fire. 

The heating element that read 5.6 with a 3.7 volt battery with a capacity of 2000 mAh (milliamps per hour) is a good combo. It will last us a couple hours and produce enough energy to provide heat that will change any thermochromic pigment that is on top of it. We may even be able to provide it with 4.5 volts instead to get it even hotter. How much energy would that take?

Heating element made from conductive thread

hitehrmo

 

 

Using Conductive Thread as a Heating Element

Thread some conductive thread and knot the end.

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Stitch a design.

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Grab some fabric paint medium and thermochromic pigments.
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Mix it up in a cup.

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Paint over your design. The medium will determine how opaque, thick, etc. the end result will be.
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Let it dry, use a hairdryer or carefully use a heat gun to quickly dry it.
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Stitch a second design, you can layer multiple designs to reveal different colors beneath the top layers.
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Heating Element Circuit
1 x MOSFET
1 x heating element with thermochromic print design
1 x breadboard or perf board
1 x switch
1 x microcontroller
1 x battery source (3.7 LiPo or 2 AAs)
alligator leads

Hook up the circuit as pictured.
Print

Use a breadboard for the transistor if you need.
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Use alligator leads to connect to your heating element.
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Code for Heating Element Circuit

Copy and paste below code, change pin number if necessary. Alter this final code to meet the requirements for the Dynamic Textile assignment.

Conductive Fabric Heating Element

Use a Silhouette Cameo CNC cutter to cut a design from iron-on conductive fabric. 

Don’t forget to do some tests to see if the resistance of the element you are designing will work with the power source and produce enough power to produce heat!