€5 PPG – photoplethysmogram amplifier / Arduino circuit

The photoplethysmogram (PPG) is a signal that measures changes in blood volume in some part of the body (e.g. the fingertip) by shining light into the skin and detecting small changes in the level of light absorption that occur due to the blood vessels enlarging and contracting. One common application is heart rate measurement. When the heart beats, blood vessels around the body swell slightly due to the increased blood pressure. This results is variable light absorption over the course of each cardiac cycle.

This circuit is a €5 PPG system that uses a TCRT5000 reflective infrared sensor, an LM358 opamp and an Arduino Nano.

The following Arduino code samples the analog voltage on pin A7 (the sample rate is approximately 100 Hz) and prints the values via the serial connection. The signal can therefore be plotted using the Arduino development environment’s Serial Plotter tool (located under the Tools menu). Normally, the Serial Plotter dynamically scales the vertical axis to fit the displayed signal. To prevent this (and maintain constant scaling), this program actually outputs two additional dummy signals – one which is always 0 and another which is always 1023. These hold the vertical axis limits at constant values.

//
// Photoplethysmogram (PPG) example
// Written by Ted Burke, 3-4-2019
//

void setup()
{
  pinMode(2, OUTPUT);
  Serial.begin(9600);
}

void loop()
{
  int du;

  du = analogRead(7);

  Serial.print("0 1023 ");
  Serial.println(du);

  delay(10);
}

This is an example PPG signal recorded using the above circuit (I rested my fingertip directly on top of the TCRT5000) and displayed in the Serial Plotter:

Since the PPG is a very low frequency signal, you may wish to reduce the gain of the amplifier at higher frequencies, which will tend to reduce the visible interference and smooth out the signal. This can be achieved by placing a 100nF capacitor in parallel with the 100kΩ resistor. The following signal was recorded with that capacitor in the circuit.