€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.

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2 Responses to €5 PPG – photoplethysmogram amplifier / Arduino circuit

  1. chiath1 says:

    Hi Ted,
    I am trying this set up for a project in school. However when I replicate this I was unable to get the same graph of the pulse as you. For my set up it only showed a one-off reading when i placed my hand on the sensor. It peaked as the IR was reflected then it went to 0 again.
    Do you have any tips/advice as to how I can improve it?
    Thanks!

    • batchloaf says:

      Hi chiath1,

      Sorry for the delay responding.

      This circuit is designed to amplify very small changes in the amount of infrared light reflected by the finger. Any sudden large changes will “saturate” the amplifier, meaning that its output will go all the way to the maximum (or minimum) level. This should happen when you first place your finger on the sensor because the change in reflected light is very large (between no finger on the sensor and a finger directly on the sensor). It takes the amplifier a while (about 20-30 seconds maybe) to settle back down when it saturates and return to amplifying the very small changes that occur when the heart beats.

      What I suggest you do is place your finger on the sensor (pressing lightly on it) and hold it very still for about a minute (or until you see the signal on the screen come out of saturation). What it looks like when it’s saturated is the signal is just dead flat, either at the top or the bottom of the graph. When it comes out of saturation, the signal will more like wander around a bit. Hopefully, it will wander to a resting level somewhere in the middle of the graph and you’ll then see the little pulses that occur each time the heart beats.

      If that doesn’t work, maybe I could get you to send me a clear photo of your circuit and I’ll check that everything’s where it should be?

      Ted

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