Wearable Thermometer
Sounds an alarm when wearer is exposed to dangerous temperatures.
This project integrates the common electronic part — the thermocouple — into a wearable embroidered patch that can be sewn into any garment. The patch alerts the wearer when they are being exposed to dangerous temperatures by sounding an alarm. The thermocouple is able to respond to either hot or cold temperature fluctuations, and sounds an alarm after 2–3 minutes of exposure.
This piece requires a basic knowledge of embroidery patch-making. I will not explain this technique but I highly recommend this Youtube video, which is how I taught myself the technique.
Materials
- Cotton Embroidery Floss (Various Colors)
- Nichrome Wire 36g
- Nickel Wire 26g
- 2x 2n2222 Transistor or other NPN transistor
- Thick Backing Fabric (I used Denim)
- Conductive Thread
- Resistive Thread
- Arduino Circuit Playground (optional)
Tools
- Needles of Various Sizes
- Scissors (small is good for detail work)
- Embroidery Hoop
- Multimeter
Process
1
Twist 8 to 10cm of Nichrome and Nickel together to create your thermocouple, making sure they are electrically connected along the length of the sensor. Length is an important factor here, as the thermocouple needs to be long enough to have a cold junction and a hot junction. So, you should be able to easily wrap between both sides of your patch.
2
Cut a 0.25cm slit in your prepared hoop with detail scissors. The slit should be wide enough to pass the sensor through, but small enough that it will be easy to secure later. Next, pass your wire through, leaving approximately half of the thermocouple on either side of the fabric.
3
Bend the thermocouple into a U shape (or other shape that will fit your pattern). The key idea here is to have a hot and cold junction on the thermocouple, so it’s important to make sure that the two sides are separated by an inch or so. This makes sure that they are thermally insulated from one another.
4
Couch the thermocouple, paying special attention to secure the slit at this stage with stitches on either side, so there is no risk of it ripping later in the process. Then, add and couch the first transistor, making sure to connect the end of the thermocouple to the base of the transistor as you secure it. Leave the emitter and the collector unsecured, as these will need to be accessible later to complete the circuit.
Note: Before couching the thermocouple, I recommend sketching a plan of the final design. This allows you to make sure all components will be in their place. It is important to make sure the design is large enough to accommodate a board (if you choose to incorporate one) and the two transistors.
6
After couching your sensor and transistor, use satin stitches to fully insulate and cover it the thermocouple. This satin stitch covering will not only disguise the sensor and secure it in the patch, it will also thermally insulate it to make the change in voltage caused by temperature exposure more pronounced.
7
Embroider the aesthetic portions of your patch! I created a dinosaur and used the insulated thermocouple as my outline. I also made sure that my piece would be large enough to accommodate the Circuit Playground, my prototyping board.
8
Complete your circuit by attaching your second transistor according to the circuit diagram below. The two transistors amplify the signal of the thermocouple, making changes large enough for the Arduino to sense. Amplification circuits require a resistor between collector of the first transistor and the base of the second, so I used resistive thread to connect them.
Note: This is a very very basic amplification circuit, simplified for this execution. The simplification is possible because the current from the thermocouple is much too small to overwhelm the transistors.
9
I recommend that you test your system at this point, before connecting a board or other circuitry. Technically, this is a completed soft thermocouple! It creates a signal on the scale of +/- 1V when heated and cooled to an extreme degree. Connect it to a multimeter and subject the sensing node to extreme temperatures to confirm that all circuits are working properly!
Note: This thermocouple is a modified K-type thermocouple (using Nickel instead of Nickel-Alumel alloy because Nickel wire is much easier to purchase). Different metal parings are optimized for different temperature ranges; a list of common commercial thermocouples, their components, and their temperature ranges are shown in the figure below.
10 (Optional)
Attach a board or other logic circuitry. I am using the Adafruit Circuit Playground to test a potential implementation of my patch. There is irony in this, as the Circuit Playground comes with an onboard thermistor. However, this does demonstrate that the amplified thermocouple signal is large enough to be sensed by any Arduino analog port! Here is a link to the code I used for this demo.
11
Finish the patch as desired and incorporate into your sensing application!
Special Thanks To:
Everyone in Soft Object: Fall 2020, Laura Devendorf, Sasha de Koninck, Anette Millington, Liza Tolkin, and Fiona Bell