Color Changing Tree

by gracie15

For this project I used thermochromic ink and conductive thread to create a color changing tree whose trunk and branches change color and make it look like spring time is coming!

The thermochromic ink requires at least 100mA of current to heat up to change the color of the ink, so given that I was using a 3.7 volt battery, using Ohm’s law I calculated that the resistance must be at most 37Ω. Therefore I tried to find a way to make the resistance in the conductive thread as low as possible. I discovered by using a multimeter that the thread is less resistant if you double the thread over. So I decided to sew using 4 layers of thread (doubled over twice), just to be safe.  I did a couple of tests with a few pieces of thread using a multimeter just to be sure that it did not have a high resistance.

I began by sewing my pattern for the tree/circuit starting from the trunk and up through the branches. I at first wanted to sew the branches across (breadth first vs. depth first) instead of up and down to give it the effect of growing branches but then I opted not to because I was worried that I would not be able to avoid crossing paths with the thread which would cause the current to take an alternate path.  While sewing I made sure to measure the resistance using the multimeter as I went along to make sure it was not going beyond 37Ω.

After I completed sewing, I measured the trace from the start of the circuit to the start of the switch and got 10.6Ω. I then measured the resistance of the trace from the switch back to the battery and got 1.8 Omega for a grand total of 12.4Ω.  I measured the voltage of my battery and got 3.77V.

Using Ohm’s law, I calculated that the current should be ~304mA but when I attached the battery and measured the current using the multimeter I got 119mA.  I made sure to measure in a place where the multimeter becomes part of the entire circuit.  In any case, this would be enough to change the color of my design.

I then went ahead and painted my tree design using the thermochromic ink, mixing the blue with yellow to form green, and mixing blue with red to make a brownish color.

I measured the current again after the paint dried and it remained ~119mA

Here is my finished design and accompanying video of my tree changing color!

Summary of measurements:

  • Voltage of battery: 3.77V
  • Resistance: 10.6Ω + 1.8Ω = 12.4Ω
  • Calculated current (Using Ohm’s law): 304mA
  • Measured current (using multimeter): 119mA