Easy Pulse Sensor (Version 1.1) Overview (Part 1)
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. The original version of Easy Pulse uses the TCRT1000 reflective optical sensor to sense the blood variation in the finger tissue and outputs a digital pulse which is synchronous with the heart beat. Today, we are pleased to announce the release of Easy Pulse Version 1.1, which has some improvements over the original design. The new version provides both analog PPG waveform as well as digital pulse signal as separate outputs. Easy Pulse Version 1.1 board is available for purchase on Tindie. Recently, our Chinese distributor Elecrow has also started selling it for $18.50, and they can ship it world-wide at lower cost.
Quick overview of Easy Pulse
The Easy Pulse sensor is based on the principle of photoplethysmography (PPG) which is a non-invasive method of measuring the variation in blood volume in tissues using a light source and a detector. Since the change in blood volume is synchronous to the heart beat, this technique can be used to calculate the heart rate. Transmittance and reflectance are two basic types of photoplethysmography. For the transmittance PPG, a light source is emitted in to the tissue and a light detector is placed in the opposite side of the tissue to measure the resultant light. Because of the limited penetration depth of the light through organ tissue, the transmittance PPG is applicable to a restricted body part, such as the finger or the ear lobe. However, in the reflectance PPG, the light source and the light detector are both placed on the same side of a body part. The light is emitted into the tissue and the reflected light is measured by the detector. As the light doesn’t have to penetrate the body, the reflectance PPG can be applied to any parts of human body. In either case, the detected light reflected from or transmitted through the body part will fluctuate according to the pulsatile blood flow caused by the beating of the heart.
The original Easy Pulse design was based on the reflectance approach and used TCRT1000 IR device as sensor. It could detect the pulse signal when an user places his/her fingertip on the top of the sensor. While this sensor performed well, it was susceptible to a very small movement of the finger. So, the user should keep the finger very steady to obtain the accurate pulse signal. 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).
The two maxima observed in the PPG are called Sytolic and Diastolic peaks, and they can provide valuable information about the cardiovascular system (this topic is outside the scope of this article). The time duration between two consecutive Systolic peaks gives the instantaneous heart rate.
Here are the features of Easy Pulse V1.1 sensor module.
- Uses HRM-2511E transmission PPG sensor for stable readings
- MCP6004 Opamp with rail-to-rail output capability for maximum signal swing
- Separate analog and digital outputs
- Potentiometer gain control for the analog output
- Pulse width control for the digital output
- Additional test points on board for analyzing signals at different stages of instrumentation
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, a company based in Shenzhen, China, which offers component sourcing, PCB manufacturing and assembly services. It turned out really good. The power supply, Enable, 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 are connected to the raw PPG output signal (VSENSOR) and Stage I output (‘a’), respectively.
The sensor operation and output waveforms of Easy Pulse V1.1 are discussed in the second part of this article. Click here to read Part 2 of this article.
For US customers, Click here to buy Easy Pulse Version 1.1
Easy Pulse version 1.1 boards are now also sold by Elecrow, a China-based company, for $18.50 and ships world-wide. Please visit the following link if you are interested on getting an assembled Easy Pulse V1.1 board at a cheap shipping cost world-wide.