In this assignment you will build a plethysmograph, use the DAQ toolbox to interface to the computer, then write software to extract your heart rate from the fluctuating voltage.
The circuit below uses a infrared emitter to shine light into your finger tip. The amount of light scattered out is related to the amount of blood flow and hence phase of the heart beat. The light scattered is measured by a phototransistor, amplified and filtered by the two opamps. You can try reflection or transillumination. I found transillumination of my thumb to give the best signal. It is important that your finger does not move relative to the phototransistor or emitter.
You can see a slight glow in the IR emitter (on the left) because the camera is slightly sensitive to infrared light. The Phototransistor is the black device just to the right of the thumb in the last image. The black color is caused by a filter in the plastic body of the phototransistor which blocks visible light.
You will need to make a movie of your scrolling data. The following code (taken from the Matlab command
help addframe) shows how to make an AVI file which can be played using Windows Media Player. An example AVI shows a startup transient as I put my thumb into the sensors, then several seconds of heart beat, then a transient as I removed my finger. At about 25-28 seconds into the recording, I did a gentle Valsalva maneuver which reduces heart stroke volume. You can see the reduced signal during these few seconds.
fig=figure; set(fig,'DoubleBuffer','on'); set(gca,'xlim',[-80 80],'ylim',[-80 80],... 'nextplot','replace','Visible','off') aviobj = avifile('example.avi') aviobj.compression='cinepak'; aviobj.quality=100; x = -pi:.1:pi; radius = 0:length(x); for i=1:length(x) h = patch(sin(x)*radius(i),cos(x)*radius(i),... [abs(cos(x(i))) 0 0]); set(h,'EraseMode','xor'); frame = getframe(gca); aviobj = addframe(aviobj,frame); end aviobj = close(aviobj);
Quitbutton should end the data acquisition and close the figure.