# playing with stretch resistors

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. This site uses Akismet to reduce spam. Learn how your comment data is processed.