Pressure Sensor to Measure Respiration

Multiple sleep tracking mats are available on the market. These "nearable" devices quantify biometrics like respiration rate and heart rate by sensing pressure changes underneath an individual's chest while sleeping. Using low-cost materials, including a pressure-sensitive conductive sheet called Velostat, we will create a simple pressure sensor to measure respiration.

• Pressure-Sensitive Conductive Sheet (Velostat)
• Arduino Uno
• USB 2.0 Cable Type A/B
• Breadboard
• Jumper Wires
• 100 olm Resistor
• Alligator Clips x 2
• LED
• Cardstock
• Copper Tape (2 inch thickness)
• Scotch Tape

1. Cut two 4 inch x 4 inch pieces of cardstock.
2. Cut one 4 inch x 4 inch piece of Velostat.
3. Cut two 4.5 inch pieces of Copper tape.
4. Apply the two pieces of Copper tape to the two pieces of cardstock, leaving approximately half an inch of the tape overhanging to fold over itself and create a tab. Be careful, the Copper tape can crinkle easily. I recommend removing the backing on the tape progressively, as you apply the tape to the cardstock.
5. Rotate one of the cardstock squares so that the Copper tape on that square is perpendicular to the Copper tape on the other square.
6. Using the Scotch tape, secure the Velostat between the two pieces of cardstock so that the Velostat is contacting both pieces of the Copper tape. Make sure the two pieces of Copper tape are not contacting each other. The final sensor should have the two tabs on adjacent sides of the square sensor.

Your pressure sensor is now complete.

Velostat is a piezoresistive material that decreases in resistance as pressure is applied. We will create a voltage divider with the Arduino to measure pressure applied to the Velostat. R1 is our 100 olm resistor and R2 is our Velostat pressure sensor. Assemble the voltage divider using your Arduino, breadboard, jumper wires, and alligator clips. Wire the LED to the digital pin. (Click on the Arduino schematic above to see what pins to connect.) In the next step, we will run MATLAB code to record and plot Vout and to turn on the light if a certain amount of pressure is detected.

The MATLAB code uses the MATLAB Support Package for Arduino Hardware and is available on GitHub at gwusk/wearable-bioinstrumentation. Once you have installed the support package and downloaded the pressureSensor function, connect your Arduino to your computer and call the function.

Congratulations! You can now record and plot voltage data from your pressure sensor. You can also adjust the threshold to control the LED. Try lying on the sensor to measure respiration. Adjust the threshold so that the LED turns on when you are situated on the sensor. I would recommend placing the sensor on a bed and covering it with a blanket for comfort. Remain still and breathe. Can you see your inhales and exhales in your data, like you can in the 5-minute and 30-second samples provided?