Easy Pulse Sensor (Version 1.1) Overview (Part 2)
In Part 1 of this article, we briefly discussed about the principle of Photoplethysmography (PPG) and its applications in retrieving vital information about the cardiovascular system. The Easy Pulse sensor allows you to measure the pulse rate from fingertip using the transmission mode PPG. The Easy Pulse Version 1.1 uses the HRM-2511-E sensor that fits comfortably onto fingertip. Inside the sensor there is an IR LED that illuminates the finger from one side. A photodetector placed on the opposite side and facing towards the IR LED detects the transmitted light through the finger. The little variations in the transmitted light intensity are synchronous with blood volume changes and hence with the pumping action of the heart. The on-board electronics filters out the noise from the PPG signal and amplifies the signal so that it is readable by a microcontroller. In this part, we continue our discussion of Easy Pulse Version 1.1 and analyze the output signals at various stages of instrumentation.
Although the HRM-2511E sensor fits on almost any of the five finger tips, we have found that the sensor performance is better if used on the middle or index finger. The flexible elastic Silicone rubber case helps to attach the sensor to the finger. 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.
Testing the Easy Pulse (Version 1.1) sensor board
The HRM-2511E sensor is plugged into the Easy Pulse board socket J2, and a jumper is placed between second (VCC) and third (Enable) pins of J1 to turn on the IR LED. A +5V power supply is applied between the VCC and Gnd pins of the Easy Pulse board. 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 in the circuit diagram described in Part 1, whereas TP2 connects to the output from the Stage I amplifier (see Part 1). 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. The following pictures show measured PPG waveforms at these test points using Digilent’s Analog Discovery tool.
Easy Pulse sensor is designed for hobby and educational purposes to illustrate the principle of photoplethysmography. The new version of Easy Pulse (Version 1.1) sensor uses a transmission type sensor (HRM-2511E) that fits on finger tip and provides more stable PPG readings. The Easy Pulse V1.1 sensor provides both analog PPG and digital pulse outputs. The pulse rate information can be derived from any of the two outputs by measuring the time period of the signal. We will discuss about this more in upcoming tutorials.