The Easy Pulse Sensor Module

The Easy Pulse sensor is designed for hobby and educational applications to illustrate the principle of photoplethysmography (PPG) as a non-invasive optical technique for detecting cardio-vascular pulse wave from a fingertip. It uses an infrared light source to illuminate the finger on one side, and a photodetector placed on the other side measures the small variations in the transmitted light intensity. The variations in the photodetector signal are related to changes in blood volume inside the tissue. The signal is filtered and amplified to obtain a nice and clean PPG waveform, which is synchronous with the heart beat [1].
Easy Pulse Version 1.1 uses a more robust sensor (HRM-2155E) that operates in transmission mode and fits tight around the fingertip, thereby it is less prone to motion.



The HRM-2511E sensor is manufactured by Kyoto Electronic Co., China, and operates in transmission mode. The sensor body is built with flexible Silicone rubber material that helps to keep the sensor tightly hold to the finger. Inside the sensor case, an IR LED and a photodetector are placed on two opposite sides and are facing each other. When a fingertip is plugged into the sensor, it is illuminated by the IR light coming from the LED. The photodetector diode receives the transmitted light through the tissue on other side. More or less light is transmitted depending on the tissue blood volume. Consequently, the transmitted light intensity varies with the pulsing of the blood with heart beat. A plot for this variation against time is referred to be a photoplethysmographic or PPG signal. The following picture shows a basic transmittance PPG probe setup to extract the pulse signal from the fingertip.



The PPG signal consists of a large DC component, which is attributed to the total blood volume of the examined tissue, and a pulsatile (AC) component, which is synchronous to the pumping action of the heart. The AC component, which carries vital information including the heart rate, is much smaller in magnitude than the DC component. A typical PPG waveform is shown in the figure below (not to scale).



Here are the features of Easy Pulse V1.1 sensor module.
Circuit diagrams

The following circuit shows the ON/OFF control scheme for the infra-red light source inside HRM-2511E. Note that the Enable signal must be pulled high in order to turn on the IR LED. The photodetector output (VSENSOR) contains the PPG signal that goes to a two-stage filter and amplifier circuit for further processing.


The PPG signal coming from the photodetector is weak and noisy. So we need an amplifier and filter circuits to boost and clean the signal. In Stage I instrumentation, the signal is first passed through a passive (RC) high-pass filter (HPF) to block the DC component of the PPG signal. The cut-off frequency of the HPF is 0.5Hz, and is set by the values of R (=68K) and C (=4.7uF). The output from the HPF goes to an Opamp-based active low-pass filter (LPF). The Opamp operates in non-inverting mode and has gain and cut-off frequency set to 48 and 3.4Hz, respectively. In order to achieve a full swing of the PPG signal at the output, the negative input of the Opamp is tied to a reference voltage (Vref) of 2.0V. The Vref is generated using a zener diode. At the output is a potentiometer (P1) that acts as a manual gain control. The output from the active LPF now goes to Stage II instrumentation circuit, which is basically a replica of the Stage I circuit. Note that the amplitude of the signal going to the second stage is controlled by P1. The Opamp used in this project is MCP6004 from Microchip, which is a Quad-Opamp device and provides rail-to-rail output swing.



The second stage also consists similar HPF and LPF circuits. The two-step amplified and filtered signal is now fed to a third Opamp, which is configured as a non-inverting buffer with unity gain. The output of the buffer provides the required analog PPG signal. The potentiometer P1 can be used to control the amplitude of the PPG signal appearing at the output of the buffer stage.



The fourth Opamp inside the MCP6004 device 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 below). Every time the PPG pulse wave exceeds the threshold VR, the output of the comparator goes high. Thus, this arrangement provides an output digital pulse synchronous to heart beat. Note that the width of the pulse is also determined by VR. An LED connected to the digital output blinks accordingly.

The following picture shows the Easy Pulse Version 1.1 board. The boards were manufactured by Elecrow



The following picture shows a correct way of placing the HRM-2511E sensor on the index finger. The IR LED illuminates the finger from the top.



Initially, the potentiometers P1 and P2 are set to the midpoint. The sensor is plugged into the index finger. Although the J1 header pins provides final PPG output signal, it is possible to analyze the signal at various intermediate stages through test pads TP1 and TP2. TP1 connects to the VSENSOR signal pin, TP2 connects to the output from the Stage I amplifier. Connect an oscilloscope channels to TP1, TP2, AO (4th pin of J1), and DO (5th pin of J1) to observe the PPG waveforms at various stages.

Signals from several nodes of the circuit.
VSENSOR:



Stage I's output:



Final adjusted analog output AO:



Final DO:



To make it compatible with our XADC on the Basys3 board, the Op Amp's power supply needs to be 1.8V (minimum for MCP6004), Vref needs to be 0.5V. A 0.5V reference can be provided by a dedicated reference IC or use a voltage divider coupled with a voltage follower. Here, we will directly supply it from a bench-top power supply.




Tasks:

Task 1: Assemble the Easy Pulse Sensor module. Use a bench-top power supply to provide a 0.5V reference voltage to make sure that the AO output has a bias at 0.5V and the largest voltage is less than 1V. VCC = 1.8V. Show oscilloscope snapshots. (20 points)
Task 2: Design a simple voltage divider to reduce the DO to 0-1V. Show oscilloscope snapshots. (10 points)
Task 3: Use the oscilloscope to record your resting heart rate. Then, briefly run or jump to raise your heart rate and take another measurement to confirm that the device accurately detects both values. Show oscilloscope snapshots. (10 points)



Reference
[1] Embedded-Lab Easy Pulse Sensor Lab