Solar Powered Pulse Oximeter and Heart Rate Meter Using Atmega644

Introduction
Pulse Oximeter is a non-invasive medical diagnostic device used to detect the oxygen saturation of the blood. Heart rate meter detects the number of beats per minute of the patient, normally referred to as bpm. The pulse oximeter is designed using an infrared and a red LED, projected alternatively on the finger and detection of the transmitted light by a photodiode/phototransistor. The output of the photodiode is given to a transimpedance amplifier, and further amplified and filtered before giving to the Atmega644 microcontroller.

The ADC in the microcontroller will convert it into digital form, and later display the value of SpO2 and heart rate on a 16*2 LCD screen. The goal of the project is to develop a low cost pulse oximeter and heart rate meter powered by solar energy. The idea was conceptualized due to the fact that the oxygen saturation of blood is one of the most important parameter to be monitored, and the pulse oximeter being low cost and solar powered, allows it to be accessed by the developing and under developed nations.
High Level Design/Background
Pulse oximetry has become a standard procedure for the measurement of blood-oxygen saturation in hospitals, clinics, etc. Pulse oximeter can directly detect hypoxemia, deficiency of oxygen saturation in the arterial blood. Early detection of hypoxemia can reduce the gas poisoning by CO2 or CO, tissue damage, etc. Thus, the oxygen saturation of the blood can quickly and accurately be monitored non-invasively using pulse oximeter. Pulse oximeter works on the principal of absorption and reflectance/transmittance of light by multiple components like skin, muscle and blood vessel. Absorption due to tissue, skin or muscle remains fairly constant, whereas absorption due to arterial blood varies. Arteries expand due to the pumping of the heart, expanding the arteries and inturn increasing the tissue between the LEDs and the photodiode, thus increasing the light absorption. Using this principle, heart rate can be detected. Absorption of oxyhemoglobin and the deoxygenated hemoglobin form differs significantly with wavelengths (i.e.) oxygen is transported in the blood by hemoglobin, and, depending on the binding of oxygen to the hemoglobin, absorption of light takes place at two wavelengths as shown below.
Light from two LEDs with different wavelengths i.e. 660 (RED) and 940 nm (IR) are made to fall on the finger. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated hemoglobin absorbs more red light and allows more infrared light to pass through. The ratio of absorption at the two wavelengths is used to determine the fraction of saturated hemoglobin. Pulse Oximetry can be done using two methods, reflectance oximetry and transmittance oximetry. In case of reflectance oximetry, the two LEDs and the photodiode are on the same side. Here, the light moves through the skin, muscle and blood vessel, and is reflected back from the bone. Reflectance oximetry has low signal to noise ratio and difficult to set up. In case of transmittance oximetry, the two LEDs and the photodiode are on the opposite side of the finger. Here, the transmitted light is detected by the photo diode, and is found to have higher signal to noise ratio.
Both the approaches were tried in this project and it was found that the noise in case of reflectance method is higher as compared to the transmittance, also, the amplitude of the waveform is more in case of transmittance method.
The output of the photodiode is very less in amplitude, and also very noisy. Before giving to the microcontroller, high amplification and filtering is required to get the desired signal. Two band pass filters are used for the signal processing. The microcontroller is required to perform the analog to digital conversion of the signal, and calculate the peak amplitudes of the signal to generate the heart rate and SpO2. The values are displayed on a 16*2 alphanumeric LCD. Background Math: The ratio of the absorbance due to red led to that of infrared led can be formulated as:
R = ((Vmax(Red)-Vmin(Red))/Vmin(Red))/(Vmax(Infrared)-Vmin(Infrared))/Vmin(Infrared)
and oxygen saturation of blood can be formulated as:
SpO2=HbO2/(HbO2+Hb)
and SpO2 = (10.0002*R^3 )-(52.887*R^2 )+(26.871*R)+98.283
Logical Structure:
Following is the logical structure/flow process of the project:
Hardware/Software tradeoff:
1. The sensor probe does not have a flexible diameter, thus people with thicker finger might not be able to use this probe.
2. The charge capacity of the battery is not monitored currently.
3. Larger solar panel has been used currently, eventually a smaller panel will be incorporated.
4. Brightness of the LEDs is currently constant, complex algorithm can be incorporated in which the brightness of the LEDs will be determined by the thickness of the skin.
5. Too many wiring currently, shift to a printed circuit board will make the design more compact, with lesser wires.
Standards:
There are no standards that we need to worry about in this project.
Intellectual property:
There are no patents or copyrights associated with this project.
Hardware Design
The hardware design for this project can be divided into four major parts; sensor, amplifier-filter, microcontroller and power supply.
. Sensor: As discussed earlier, the sensor probe consist of a red and an infrared LED focused on the finger. The transmitted light through the finger falls on the photodiode/phototransistor. The variations in the blood volume causes variations in the light absorption and hence the transmission. The light source and the photo detector are placed on the opposite side of the finger. The ratio of the two absorptions will give us SpO2 value. The Infrared LED used was a LTE-4208 160-1029-ND Emitter IR 5 MM 940 NM Clear. OPT101 has been used as an integrated photodiode and transimpedance amplifier. It is a monolithic photodiode with on-chip transimpedance amplifier. Output voltage increases linearly with light intensity. The integrated combination of photodiode and transimpedance amplifier on a single chip eliminates the problems commonly encountered in discrete designs such as leakage current errors, noise pick-up, and gain peaking due to stray capacitance. Transimpedance amplifier is used to convert current into voltage, as the output of photodiode is current.
2. Amplifier and Filter: We know that any bio signal has very less amplitude, and thus very likely to be super imposed by noise and interference hum. As the analog to digital convertor has high sampling rate, and can sample milli volts of signal, any kind of super imposed noise will disrupt all the readings completely, as even the noise will get sampled and will be digitized. Thus it is very critical and crucial for filtering the signal and get a pure noise free one. Also, for the efficient sampling and digitization, the analog signal must be amplified. Desired signals fall in the range of 0.1-3Hz, thus two band pass filters are used to eliminate all the signals except the band frequency of 0.7-3Hz. The gain for the first amplifier (A1) is set as about 150 and that for the second amplifier (A2) is 10. Thus total amplification A = A1*A2 = 1500. Dual packaged Operational amplifier LM358 chip has been used for the amplification and filtering as they are efficient, cost effective and were available in the lab.
3. Microcontroller: Atmega644 is an 8 bit microcontroller and has been used in the project. Atmega644 has 10 bit ADC which is used for sampling and digitizing the input analog signal. Hardware also includes the interfacing of the Atmega644 to the LCD display.

4. Power Supply: The goal of this project, as mentioned before, is to power this device using solar cells. 12V/1.5W solar panel was given by Prof. Bruce Land. The output current of the solar panel in bright sunlight is about 120mA. The solar panel is used to charge 4 NimH rechargeable batteries having charge capacity of 2500mA. Batteries are used to power the device during low light or no light conditions, i.e. when the solar panel is not efficient enough to drive the electronic circuit. The output of the solar panel is given to a decoupling capacitor of 0.2uF to filter out the high frequency noise. It is then given to a linear voltage regulator LM340LAZ-5.0 which gives out a constant DC voltage of 5 V. The output is further given to a decoupling capacitor of 0.01uF and later given to a schottky diode IN5817. A diode is very crucial element in the design, as it acts as reverse charge protection, i.e. it will prevent the battery to discharge backwards into the regulator and hence into the solar panel, thus avoiding damage to those. Schottky diode is used due to very low forward voltage drop, i.e. about 0.2-0.3V. The output of diode is about 4.8V and is appropriate in charging the rechargeable batteries. Two switches have been used, one in between the solar panel output and the voltage regulator and second between the output of the batteries and the Vcc terminal of main electronic circuit.

Parts List:

 
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