Wearable Temperature Sensors For Working in Extreme Cold

There are a lot of different kinds of temperature sensors that you could use for a project like this. There are thermocouples, thermistors (temperature dependent resistors), and temperature dependent transistors.

I am using a TMP36 integrated circuit. I chose this sensor because it has a linear output and does not require any calibration. Temperature (in Celsius) = [Vout(in mV) – 500] / 10. It also has a very wide operating range (−40°C to +125°C). It works by using several transistors with different sized emitter layers. These transistors will have different outputs at different temperatures. By comparing them, you can determine the temperature. The output given at the center pin starts at 750 mV (at 25°C) and changes by 10mV per degree Celsius as the temperature changes.

If you would like to see a tutorial on how to use this sensor you can check out this Instructable by Adafruit Industries.

We will be mounting the temperature sensor several feet away from the microcontroller, so we need to connect it to the board with extension wires. I used header pin connector cables from an old computer, which allowed me to easily connect and disconnect the wires so that the gloves will not be permanently attached.

I cut one end of the connector cable and soldered three of the wires to the leads of the temperature sensor. I added heat shrink tubing to help insulate each connection.

To allow the temperature sensor to measure the temperature of your fingers and toes, you need to mount the sensors inside a pair of gloves and shoes.

It is easiest to mount the sensor inside gloves that have multiple layers; I used a pair of gloves that had a leather exterior, cotton insulation, and a fabric lining on the inside.

First I cut a small slot in the fabric lining at the inside cuff. Then I carefully slid the sensor and its connector wire into the glove between the fabric liner and the cotton insulation. Slowly move the sensor down to the end of the middle finger. It may snag a few times in the process, so be patient. When you are done, the connector should stick out a little past the edge of the glove.

To mount a temperature sensor inside a shoe, you can generally just fit it in place before sliding your foot in. You shouldn’t need to modify the shoe at all. If you need some help keeping it in place, you can fit the wires in between the tongue of the shoe and the side.

The TMP36 temperature sensor can be connected directly to pins on the Arduino. The left pin on the sensor is connected to the 5V pin (or the 3.3V pin) on the board. The right pin on the sensor is connected to the GND pin on the board. The center pin is connected to one of the analog input pins on the board.

Use a second set of connector wires to go between the board and the connector wires on the gloves and shoes. The wires can easily be fit between layers of clothing without much discomfort. Connect the cables together in whatever way is most convenient. I was using header pin connectors, so I was able to connect them by just inserting a long pin between the two female connectors.

You need some way for the system to alert you when your fingers and toes are getting too cold. A pair of earbud headphones works well for this because they can easily fit under cold weather head gear. We just need a way to connect it to the Arduino.

The PWM output pins can play a tone on a pair of headphones. You just need to add a 100 kohm resistor to bring the signal down to the appropriate level for the headphones. To connect the headphones to the pins on the board, I added a female headphone connector. Then I soldered on one extension wire and a 100 kohm resistor. To hear the tone in both earbuds, solder the connectors for the end and middle contacts together.

Download a copy of the Arduino Code and upload it to the Arduino: http://cdn.makezine.com/make/Wearable_Temperature_Sensor.ino

Find an insulated project enclosure that is just big enough to fit the Arduino and the 9V battery. Drill holes in one side of the housing that are just big enough for the headphone connector and the wires, then mount everything inside and close up the housing.

Now you can use the system to detect when your fingers or toes are getting too cold (near freezing). You can use up to six sensors at a time. Taking advantage of this, I made two finger sensors, two toe sensors, one sensor for my torso and one sensor that just measured the outside temperature. To do this, the analog input pins are connected to the output lead of the temperature sensors. The 5V pin and the GND pin are connected to all of the sensors in parallel. If connecting the individual wires together is too much trouble, you can make a small circuit board to make all the connections on.

If any of the first five sensors (fingers, toes, and torso) get below the set alarm value in the code, the Arduino will play a tone in the ear piece to alert you.