Optical sensor for the detection of arrhythmias

This is the sixth version of the optical sensor “Theremino Pulsometer”
and it collects all the improvements experienced in recent months.


Then sensore OCG (Note 1), to be applied on a finger can provide more information than what one normally thinks. In addition to heart rate can be observed arrhythmias and classify the main types. It may also be observed in the low-pressure effects on the amplitude of the signal and even periodic changes in heart rate caused by breathing.
Pulsometer V6 connections

(Note 1) The name exact for the detection Optical would "photoplethysmography" but it's hard to remember and too poorly composed. Given that optical variations are detected instead of electrical, the equivalent of "Electro" is not "PhotoPlethysmo" but "Optical" and then from now on we will call them ECG and OCG.



Arrhythmias often cause no symptoms. But you may feel a strange sensation in the chest, For example:

  • Palpitations
  • Weight chest
  • Dive to the heart (often described by patients as well)
  • Increased awareness of the heartbeat
  • Feeling missing heartbeat

Many of us have had these feelings and almost no one went to do an ECG to check. As an ECG is necessary to book, it takes hours and it is almost certain that in the short examination time does not observe significant events. He says Murphy's Law but it is also due to the fact that an ECG is done in times, environments and different states of mind from those that could cause arrhythmias.


Principle of operation

An infrared LED illuminates your finger from the tip. The infrared light passes through the skin and is more or less attenuated depending on the blood pressure. A photo-transistor (which is seen in transparency) collects light through a hole.

The gray block is made of spongy material is a soft base for the finger and at the same time isolates the sensor from the external light. The light can only pass through the finger and then from the hole.

With this arrangement you get a better signal with respect to the sensors that work with LED and photo-transistor from opposite sides of the finger and also with those who have both of them at the bottom.

This sensor (when used with the amplifier we design) It generates a fairly constant amplitude signal even with people who have dark skin or lacking in transparency, or that painting their nails and even with people who have low blood pressure.


Sensor Construction

The elastic part of the sensor is constructible with a piece of pipe insulation for heating and a screw 3mm.

Further details on the mechanical construction in the PDF file Download from here.

electronics Construction

Both the sensor PCB that what amplifier are very simple and can be constructed “do-it-yourself” on a breadboard.

The numbers are used to count the holes and facilitate the positioning of the components on a Stripboard. Then with the same wires that are cut by the resistors will make the few necessary connections.


Wiring diagram

The resistor R1 sends a current of about 16 mA to the sensor LED.

The resistor R2, of higher value compared to similar schemes, It provides a very low current to the photo-transistor (less than 5 uA). With this arrangement, when you remove your finger and the LED light is no longer filtered, the voltage on the transistor drops from the normal two volts (circa) at a very low value and the transistor goes into saturation. In this way they eliminate the noise due to external light which might generate false signals.

The capacitors C1 and C3 from 1uF determine the low cut-off frequency. It is recommended to use the value of 1 uF or lower them to 470nF, as explained in the following pages.

The sections IC1A, IC1B, IC1C IC1F and amplify the signal of more than a thousand times (66 dB with a maximum Qtr1) and at the same time filtering out high frequencies effectively so as to completely eliminate the mains frequency interference.

The resistor R9 unbalances the last section so that the output at rest is below half of the voltage and the LED1 remains off. The IC1E sections and IC1D square off the signal and make the LED blink with each beat.

The regulator U1 stabilizes the 5 volts coming from the USB in a 3.3 Volt stable, low-noise.

Disposition of the components

This can be useful when placing and welding components.

The Qtr1 trimmer must be small ones (7×7 or at most 8×8 mm). There are three extra holes that allow you to use different trimmer models, or rotate it 180 degrees, to do so by turning the signal to increase time.

All resistors can be a quarter of a watt, but if you can find them by an eighth of a watt it is better. It will be easier to position them and are even more beautiful to see.

The resistors R3, R5 and R7 are positioned standing, by bending one of the two legs along the body. To avoid that the long legs can touch each other is good to arrange all three with the body on the same side. In the image it is seen that the bead at the top has the black edge and that side is the body of the resistance.

Note that the C5 and C6 capacitors are polarized and you have to rotate with the "+" as shown in this.

For this circuit it is not necessary to use precision components. No need for capacitors mylar, you can use small ceramic and economic.

The C1 and C3 are to be polarized, and preferably from 470 nF the 1 uF.


Software for reading the sensor

To view the ECG, but mainly for research of arrhythmias, using the application Theremino ECG Download from here.



For those who wish to learn the details of the operation and the details of the mechanical construction, we have prepared a PDF file Download from here. In the PDF file you will find links to download the plans of printed circuit boards.

In This Page Theremino the site you can find more information and also the English version of the documentation file.


3 replies
  1. Just4Fun
    Just4Fun says:

    Interesting study the sensor and the wiring diagram.
    It was a lot that I could not use the CMOS inverter series 4000 (strictly type “unbuffered”) mode “analog” (The first time I read the related Application Notes on the datasheet of the National Blue went to high school… 🙂 )

    • theremino
      theremino says:

      The advantage with respect to compactness is operational.

      With the operational components should be added to bias the inputs instead here the inputs are polarized alone. In addition with the opamps it is more difficult to pass the slopes of a single-sided PCB, given that there would be two inputs instead of one. And finally there are operational in a sestupli houses 14 pin.

      Of course, the characteristics of the operational could be much higher as both amplification as noise. But for many such projects are so comfortable that the 4069UB.


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