The light-absorbing property of hemoglobin is used in the measurement of Heart Beat rate. The light from an infrared light source on the underside of the monitor is shone on blood vessels just under the skin. The light that is not absorbed, but reflected back is captured by a photodetector. The variations in the photodetector signal are related to changes in blood volume inside the tissue. The photodetector produces an electrical signal when the reflected back light strikes it. The signal which we obtained from the sensor is an analog signal and it is converted into a digital signal. Furthermore, the signal can be filtered and then amplified to obtain a perfect PPG (PhotoPlethysmoGraph) waveform, which is synchronous with the heart beat.

Circuit diagram of Heart Beat Sensor

The circuit shows ON/OFF control scheme using a transistor for an infrared light source inside the heart beat sensor module HRM-2511E. The signal from the transistor must be pulled high in order to turn on the IR LED. The photodetector output (Vsensor) contains a weak and noisy PPG signal that goes to a two-stage filter and an amplifier circuit for further processing.

The PPG signal consists of a large amount of DC component, which is attributed to the total blood volume of the examined tissue, and a pulsatile (AC) component (is much smaller in magnitude than the DC component), which is synchronous to the pumping action of the heart. The AC component, which carries essential information, including the heart rate. A typical PPG waveform is shown in the figure below.

In Stage I instrumentation, the signal passed through a passive (RC) high-pass filter (HPF) to block the DC components of the PPG signal. The output from the HPF goes to an Opamp-based active low-pass filter (LPF). In order to achieve a full swing of the PPG signal at the output, the negative input of the Op-amp is tied to a reference voltage (Vref) of 2.0V. The Vref is generated using a Zener diode.

The output from the active LPF now goes to the Stage II instrumentation circuit, which is basically a replica of the Stage I circuit. The amplitude of the signal which is going to the second stage is controlled by a potentiometer (acts as a manual gain control). The Opamp used in this project is MCP6004, which provides rail-to-rail output swing.

The second stage also consists same HPF and LPF circuits. The two-step amplified and filtered signal is now fed to a third Op-amp which provides the required analog PPG signal. The potentiometer P1 is used to control the amplitude of the PPG signal appearing at the output of the third Opamp. The fourth Opamp is used as a voltage comparator. The analog PPG signal is fed to the positive input and the negative input is tied to a reference voltage (VR). The magnitude of VR can be set anywhere between 0 and Vcc through potentiometer P2 (shown above).

 Every time the PPG pulse wave exceeds the threshold voltage VR, the output of the comparator goes high. Thus, this arrangement provides an output digital pulse which is synchronous to the heart beat. The width of the pulse is also determined by the threshold voltage VR. A LED connected to the digital output blinks according to the digital output signal.

Heart Beat Sensor module HRM-2511E consists of the power supply (Vcc), Enable (En), Analog PPG output (AO), and digital pulse output (DO) pins are accessible through the J1 headers. The HRM-2511E sensor connects to the board through a 3.5mm Audio Jack connector (J2). TP1 and TP2 are test pads on the circuit board that is connected to the raw PPG output signal (Vsensor) and Stage I output (‘a’), respectively.

PC-Based Heart Rate monitor using Arduino and Heart Beat (Pulse) Sensor

A PC-based heart rate monitor system can be designed by using an Arduino board and Heart Beat Sensor. The sensor operation was explained in above. The analog output pin (AO) of the pulse sensor goes to analog input channel A0 of Arduino. A mini-USB cable provides the serial communication between Arduino board and the PC. The power supply for Pulse sensor is derived from the Arduino board. The hardware set as shown below.

The analog PPG output from sensor board is fed to an ADC channel of Arduino to convert it into digital counts for further processing, which then transfers the data to the PC through a serial port interface. A PC application is developed using a programming language to display the received PPG signal and instantaneous heart rate.

The programming part of Arduino includes that it takes the ADC samples of the analog PPG signal at 5ms interval and continuously transmits the data to the PC through the USB-UART interface.

PC application

On the PC side, we have to develop an application that reads the incoming ADC samples from the Arduino and process them to extract the PPG signal and heart rate. The below-shown flowchart explains the overall logic of computing the heart rate from the received ADC samples.

The heart beat rate can be computed by knowing the time period of the PPG waveform. Two heart rates are computed from the three consecutive PPG peaks and their average value is displayed as an instantaneous heart rate. The identified peaks are also marked on the display with a cross (X) symbol. The PPG waveform plotted by the PC application on the computer screen shown below.

Thus, this is all about Heart Attack Detection and Medical Attention Using Heart Beat Sensor (pulse sensor) and its working principle. We hope that you have got a better understanding of this concept. Are you keen to build simple electronic projects or any technical assistance, please give your suggestions by commenting in the comment section below. Here is a question for you, what is the Photoplethysmography?