Introduction: Simple Thoracic Impedance


–Safe Circuit

–Find Respiratory Rate


–Keep current across body under 1 microAmp

–Respiratory Rate should be within 15-20 breaths per minute.


•TL072 for constant current source

•1 Mohm resistor for constant current control

•15 kohm resistor in parallel with body


•100 and 220 ohm resistors in series as Gain resistor for INA118

•Wires and electrodes


Function generator

Power Supply

Step 1: Overall Goals

•Circuit is reliable at measuring respiratory rate

•Measured rate falls within standard range of respiratory rates.

Step 2: How Does It Work?

•Breathing changes the impedance of the thoracic cavity.

•If we load the thorax with a constant peak current, then changes in peak voltage correspond to changes in impedance.

•Measuring how these peaks change over time will allow us to determine a respiratory rate.

Step 3: What Does the Circuit Do?

•Current is applied across the body. A 15 kohm resistor is used to draw current and remove DC components.

•Voltage across the body is acquired and inputted into an instrumentation amplifier.

This signal is then imputed into a computer, where we low pass filter the signal and then determine the frequency of the changing peak voltage signal

Step 4: The Blueprint

Our device will be composed of five main components.

1. The Function Generator (sine wave, 1V peak to peak at 20 Khz)

2. The Constant Current Source

3. The Instrumentation Amplifier to differentiate and provide gain

4. The DAQ Assisstant and LabView software.

5. Last but not least.... You!

Step 5: Current Source

You will need nothing more than two resistors and a TL071 or TL072 Op Amp.

Regardless of which op amp you use you will supply +15 V and -15V.

Connect the resistor in parallel with the positive and negative current terminals.

Try to adjust the resistor value to supply between (1~30 microamps).

**Too high of a current can be lethal.

For our design,

Rcurrent= 1 Mohm

Rcurrent is supplied by the function generator.

Rprotect= 15 kohm

Step 6: The INA118

For our circuit, Rgain is set to 330 ohm

Vref is grounded.

The op amp is powered with +15/-15

The output of the op amp is inputted into labview.

Step 7: The VI

The daq assistant is configured at rate of 80 Khz with 8k samples to read

The first low pass has a cutoff frequency of 21 KHz

The filtered signal is then passed through the peak dectector vi.

The amplitudes of the peaks are then compiled into a new signal which is filtered by a low pass with cutoff 0.35 hz to remove ecg signal.

This new signal is then put through another peak detector (using valleys) to determine the frequency.An indexed array is used to determine the distance between subsequent valleys. The locations are subtracted, inverted, then multiplied by the ratio of sampling rate to samples to read and also converted from seconds to minutes

Step 8: Calibration

•Calibrated under normal breathing to a value that falls into standard range.

•0.2Hz~0.33Hz for Respiratory Rate

•~15 Breaths per Minute (.25Hz)

•Work in progress. Currently ECG signals overriding the respiratory signal

•Our VI reads peaks over a large segment in time, so the respiratory rate doesn’t respond to instant changes

Step 9: Future Considerations

•Make mobile using an Arduino and an AD5933

–Have to configure mobile power supply.

–Have to make a PCB for circuit elements