ENGR337 Lab 2020 Spring
Lab 4 An Oximeter
Name: Donovan Birky
Email: dkbirky@fortlewis.edu

1. Title - An Oximeter

2. Introduction
The goal for this lab was to use passive filters to create high and low pass filters to remove the DC offset and the noise of a signal from an oximeter, to accurately measure one's heartrate.

3. Materials and Methods
The entire goal of the lab is to connect an oximeter to a circuit to be able to read one's heartrate off of the oscilloscope just like an EKG. The oximeter is essentially an infrared blaster and reciever, so that when the reciever recieves less light, more voltage is allowed to flow out of it. This required utilization of two 1NA 128 PA amplifiers. First the signal from the oximeter has to go through a high pass filter, utilizing a 4.7 micro farrad capacitor and a 68k resistor. After the first op-amp, the signal runs through a low pass filter utilizing a 100k resistor and a 458 nF capacitor, then run into the second instrumental amplifier. The output from this amplifier is the signal that will be read from the oscilloscope. A third op-amp, this time a 741 is used to control the voltage provided to the diode, along with a 1k resistor. A reference voltage is created using a zener diode and a 680 ohm resistor connected to a 5V power supply. The output signals can be read off of the oscilloscope, but to get a live EKG, an arduino was used to serial plot the heart rhythym on the computer.

4. Results


Figure 1. Heart rate being measured at around 65 bpm.

                                         
Figure 2. Watch reading showing actual heart rate at time Figure 1 was captured.


                    Figure 3. Heart rate measured showing around 150 bpm.

                          
Figure 4. Watch showing the heart rate at the time Figure 3 was captured.


        Figure 5. The final circuit (including arduino).

Video of the serial plotter of the EKG signal:



5. Discussion
This lab showed how passive filters can be used for a very practical purpose. The keys to success were ensuring that all components were connected correctly, and that multiple op amps were used to prevent any attenuation of the signals. The results showed a clear measurement of the heartrate, that was compared to a smartwatch's readings, and the two matched up quite nicely. Overall, the lab was a success and very fun to do.