Anex (Exercise 1: heart rate monitor)

Why a heart rate monitor? (pulse oximetry)

It’s a pulse oximetry, which is based on the reflection of the red light into the blood.
This exercise is a part of a further project that I’ve, but now will be very helpful to understand how the light analogue sensor works.

At the beginning I’ve tried to copy this model or others from internet  (
But the idea is to improve my knowledge so to copy is not an option, but the idea is quite good, I used the Newton method, (think and  try if error re-think , and repeat till it works :))
Is quite important to read the datasheet of each component.


.- Led Red high bright
.- Photo transistor (mine is BPW40 really cheap 0.46 €)
.- Operational amplifier LM358.
.- Some Resistors.


An image is better than 1000 words, so a video is much better 🙂

How does it works ?

pulse oximetry

pulse oximetry

The red light is reflected by the blood in the finger and with the sensor I can see how much light is reflected. (it’s not exactly like that , but to avoid other issues which are out of our studies)
With the operational amplifier I adapt the signal from the light sensor (BPW40) to values acceptable for A/D converter

The circuit

This circuit is made by the Newton method, think, probe and error, till works

pulse oximetry circuit

pulse oximetry circuit

I’m so glad with the result but for sure it could be improved, so any comments, doubts, don’t hesitate and let me know.

Gutting the circuit

The fototransistor + 100 ohm resistance give us a very low signal no more than 20 mv ( I choose the circuit from the datasheet)
with the first OA I amplify the signal to be near to 2 Volts

After I have an divisor adjustable to give a value little low than the 2 volts (need to be adjusted)
With the second OA, I amplify the difference which is bigger with each beat, and the result can be read by an analog-digital converter, in my case with an arduino.


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