Today we’re building a simple voltage divider where the output is read using an analog input on an Arduino Nano.  I’m following a similar project that uses a thermistor instead of conductive cord.



We use the equation Vo = R / (R + 10K) * Vcc to measure the expected voltage output at various resistances.  The conductive, resistive rubber cord has a resistance of 350-400 Ohms per inch (not very exact but fine for our prototyping needs).  My fixed Resistor is a 10k (already in the equation above) and Vcc is 5v from my laptop to the Arduino.

Here is the simplified math:

ADC value = Vi * 1023 / Vcc

ADC value = R / (R + 10K) * Vcc * 1023 / Vcc

ADC value = R / (R + 10K) * 1023

R = 10K / (1023/ADC – 1)

We will test using the following code from the tutorial linked above.

We now want to measure two things:

breath volume

We can measure this by measuring the amount of time it takes to reach a peak (maximum value) from the baseline resistance.

breath rate

We can measure this by measuring the peak-to-peak time.

So far I’ve slightly modified the code so that it will tell me when the cord is measuring breath in and breath out or holding breath.