Optical Heart Rate Monitor

Pulse Oximetry is a non invasive method of measuring a person's oxygenation
level. It monitors the percentage of haemoglobin (Hb) which is
saturated with oxygen. A probe is attached to the patient's finger or
ear lobe comprising LEDs and photodetector. As the light from the two
LEDs (red - 660 nm and infrared - 950 nm) pass through the body
tissues to a photodetector, it is absorbed by blood and soft tissue.

The light absorption rate at the two wavelengths by the hemoglobin is
different and depends on the degree of oxygenation. The light level
changes as the blood is pumped by the heart. As a consequence, the
oximeter also measures the heart rate in beats per minute (BPM).

In this application note, we create a heart rate monitor by using one IR
LED & phototransistor pair and observing the waveform at the
phototransistor output. This is intended for illustrating a typical
light sensor application and not intended for actual medical use. The
accompanying video also shows the typical low level signals involved
in this type of circuit and how the dynamic range from a 24 bit ADC (EMANT300 USB DAQ)
allows such signals to be observed without further signal
conditioning.

The
TCRT1010 have a compact construction where the emitting-light source
and the detector are arranged in the same direction to sense the
presence of an object by using the reflective IR-beam from the
object. The operating wavelength is 950 nm. The detector is a
phototransistor. Together with the EMANT300 USB DAQ, they form a computerbased optical heart monitor

Fig 1 : Optical heart rate monitor schematic - Connections to the EMANT300 USB DAQ

IR emitter and detector encased in velcro

The
following C# .NET program uses the EmantHRM component. This component
takes the voltage read at the emitter of the phototransistor and
converts it to heart rate in BPM (beats per minute)

private
void button1_Click(object sender, EventArgs e)

{

hrm1.Open();

label2.Text
= hrm1.HwId;

}

private
void button2_Click(object sender, EventArgs e)

{

hrm1.Close();

}

private
void hrm1_onValidHR(object sender, EventArgs e)

{

label1.Text
= hrm1.Value.ToString();

}

private
void button3_Click(object sender, EventArgs e)

{

hrm1.Show();

}

When the console program is first run, the user to place his/her finger on
the sensor.

When a finger is
placed on the phototransistor, the voltage read rises above 0.3V. If
a good contact is made, a sinusoidal type signal is observed. As the
signal rides on a fluctuating DC signal, a simple differential signal
is created. From maths, we know that when a max or min is reached,
its differential value is zero. After this an autocorrelation
function is applied and the peaks extracted to obtain the heart rate.
When the signal is noisy, a wrong BPM will be calculated. Any count
that is obviously wrong is ignored. This is all taken care of by the
HRM class. The HRM class is written in C#. You can improve on it
using the Visual C# 2005 Express.

Clicking on the Show
Waveform button shows the filtered waveform seen at the
phototransistor. You will achieve a good heart rate if your waveform
is as seen below.