COMPACT PULSE OXIMETER USING PIC18F4550

MICROCONTROLLER

Leonardo Zane Vilhegas, Adir José Moreira and Ronaldo Domingues Mansano

Departamento de Engenharia de Sistemas Eletrônicos

Escola Politécnica da Universidade de São Paulo

Av. Prof. Luciano Gualberto, 380 – Bloco A – Sala 46 – CEP: 05508-900, Brazil

Keywords: Oxygen Saturation, microcontroller, PIC18F4550, heart rate, USB.

Abstract: In this paper is propose one compact pulse oximeter system using a PIC18F4550 micrcontroller, which use

of USB (Universal Serial Bus) communication technology. The device has one LCD (Liquid Crystal

Display) 20x4 to continuous check and has the possibility to get one parallel communication with a PC

(Personal Computer) to analysis more detailed. The system is compound for oxygen saturation measures

(SpO

) and heart rate. The equipment is compact and show easy to handle and simple use.

1 INTRODUCTION

In the 80 decade beginning, already had emphasized

the necessity of the improvement and of scientific

development in the biomedic instrumentation area in

Brazil, in order to reach the excellency of the

medical services (Moraes & Vita, 1981).

With this mind and due to great integration

capacity increase of the eletronics devices, as well as

the fast technological advance of the

microcontrollers, a wide development of

applications in the diverse areas of biomedical

engineering was allowed (Moron, 2005).

The oxygen is fundamental and vital for the

correct functioning of each cell of the human body

and its absence, for a drawn out time, it can cause

the deaths of these cells (Webster, 1997). The pulse

oximetry (Moyle, 1994 - Wukitsch, 1987) is a non-

invasive method to measure the arterial oxygen

saturation (SpO

) using two diferents wavelenghts to

determine the relative oxyhemoglobin concentration

(HbO

) and deoxyhemoglobin (Hb) in the blood.

Already, the pulse oximeter is a device that uses an

empirical measurement method and actually is an

important SpO

continuous monitor device where it

offers oxygen saturation results trustworthy similar

the convencional methods.

In this article is propose a development of the

compact pulse oximeter system. The system is

compound for one prototype sensor, one software

interface and one pulse oximeter device with a LCD

20x4, where the device contained one PIC18F4550

microcontroller that produce all control of the

circuit. The PIC contained the USB technology

integrated where the implementation and

compacting was easily.

The primary interface has been the ubiquitous

serial port. Intel developed the USB in the early 90s,

and while many personal computers peripherals now

support this interface. Each USB port can support up

to 127 devices (Lichtel, 2004).

2 MATERIALS AND METHODS

2.1 Overview of the System

The aim of this study is to design and implement one

compact system and with handle facilities and use to

SpO

monitoring and heart rate. The figure 1 shows

the architecture of the considered and developed

system.

The major design requirements of project was

the following: a) It should be portable and easy

mobility; b) it should be easiness to get na supply

source; c) it should have a userfriendly interface; d)

Compacting of the circuit using the PIC18F4550; e)

Possibility to record the data received in a PC; f) it

should collect physiological data of arterial oxygen

saturation and the heart rate (BPM).

194

Zane Vilhegas L., José Moreira A. and Domingues Mansano R. (2008).

COMPACT PULSE OXIMETER USING PIC18F4550 MICROCONTROLLER.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 194-197

DOI: 10.5220/0001053701940197

 SciTePress

Figure 1: Overview of the development system. Device

can be supply by PC or directly by AC 110/220V, using a

converter to 5V.

2.2 Prototype Sensor

In the prototype device of the article hadn`t been use

neither comercial sensor, it was developed one

prototype sensor using comercials LEDs from

CROMATEK and one photodiode TSL251R, from

TAOSINC ,with transimpedance amplifier

integrated. The LEDs has wavelengths of 660nm

(red) and 940nm (infrared). The photodiode choice

was done by yours caracteristics and someone are

about transimpedance amplifier integrated, rise time

about 70us, linear voltage response in respect to

intensity light, spectral response and good angular

displacement, becoming the alignment between

emitter and receptor less critical.

2.3 Physiological Acquisition Signals

Device

The compact pulse oximeter device using the

microcontroller PIC18F4550 has the circuit supply

directly by USB port. Using the USB cable, the

device can be conect directly in PC and/or using the

adaptor, where can be connected directly to

electricity network and visualize the data directly on

the LCD.

The USB port has 5V and 500mA, this is enough

to supply the circuit. The circuit is compound for

first condicionning step signal, that proceeding from

the sensor, that have a filter pass-band and a signal

gain. After the signal treatment the PIC is

responsible for diverse other necessary functions of

the pulse oximeter device. The sampling frequency

that the microcontroller works is 1kHz and takes

care of to the necessities of the Nyquist theorem and

prevents the aliasing problem. The prototype also

has a LCD 20x4 to visualize the data received for

the optic sensor. The implementation of the LCD in

the system becomes interesting therefore is not

necessary no complex computational system to get

the value of SpO2 and the cardiac beating. In figure

2 the diagram of blocks of the physiological signals

acquisition system can more be seen detailed.

Figure 2: Diagram of blocks of the compact pulse

oximeter device.

3 RESULTS

All the prototype system had been developed and

implemented. The device (already with protection

circuit), which includes the physiological signals is

compound (120x80x30mm) and light (<300g). In

figure 3 the image prototype device circuit can be

seen. The photo of the pulse oximeter with LCD

connected together with the USB cable and the

eletricity network found in the figure 4. For the

sensor development was done one

photospectroscopy of the LEDs that had been used

in the prototype, to verify if they are in the specific

band wavelenght fo the aplication. In figure 5, the

graphs of the tests realized through with the emitters

can be seen.

Figure 3: Image of the prototype pulse oximeter device

circuit using the PIC18F4550.

The prototype sensor was development with the

concern to keep total isolated of the invironment. In

figure 6 it can be seen the sensor images, the

armored cable and used connector DB9 to realize the

connection with device.

COMPACT PULSE OXIMETER USING PIC18F4550 MICROCONTROLLER

195

Figure 4: Photograph of the device with the connected

LCD, USB cable and adapter for electricity network.

Figure 5: Photospectroscopy: a) LED 660nm (red); b)

LED 940nm (infrared).

The program of use for interface the pulse

oximeter device with the PC was developed through

the utilization of LabVIEW software. The program

has the information about amount of oxygen arterial

blood

(SpO

), of the photopletysmography signal

Figure 6: Pototype sensor: a) Sensor to hardwired to the

indicating finger; b) Sensor with covering against

surrounding light connected with one armored cable and

connector DB9.

and heart rate. The software also diverse other

funtions as for example, to record the data of the

signals received by device, acknowledgment of

emergency, etc. In figure 7 the display of the

software main control developed for interface of the

system can been seen.

Figure 7: Main display of the responsible interface fo the

communication with the device.

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4 CONCLUSIONS

The pulse oximeter device using the microcontroller

PIC18F4550 was developed. In the module are

measured physiological samples of oxygen

saturation in the arterial pulse, of

photopletysmography and heart rate. The software

developed to interface with the computer revealed

very friendly however. With resources limited fo not

being the focus of the article. The data presented in

the LCD are oxygen saturation and cardiac

frequency. The supply device for the USB port using

a PC or the adapter presented great easiness and

fastest in its aplication. The photopletysmography

are not presented in the LCD for the display not to

be graphical.

REFERENCES

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Exercício, São Paulo, Segunda edição, Ed. Manole,

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Lichtel, D.: Implementing a USB Equipment Interface

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Moraes, J.C.T.B.; VITA, G.M.; Desafios de problemas da

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Janeiro, Anais do VII Congresso Brasileiro de

Engenharia Biomédica, p.137-7, 1981.

Moron, M.J.; Casilari, E.; Luque, R.; Gazquez, J.A.; A

wireless monitoring system for pulse-oximetry sensors,

Systems Communications, 79 – 84, 2005.

Webster, J.G.; Design of pulse oximeters. 1.ed. U.K.,

Bristol, 1997.

Moyle J., Pulse Oximitery. 1994: BMJ Publishing Group.

Wukitsch M., Pulse oximitery: Historical review of

Ohmeda functional analysis, Journal of Clinical

Monitoring and Computing, 4, 161–166, 1987.

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