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The volume of blood in our fingers changes at different parts of the cardiac cycle (heartbeat). Immediately following ventricular contraction (when the heart pumps blood out of the ventricles and to the rest of the body), the volume of blood in the fingers increases slightly. This change in blood volume can be measured using finger photoplethysmography. In this technique, an LED shines light through a person’s finger and a photodiode measures the amount of light that passes through the finger. The amount of light that hits the photodiode will change due to the change in blood volume in the finger. Interestingly enough, if we plot intensity of light vs time, we get a very characteristic waveform (here and here). The initial sharp rise of our pulse waveform indicates the systolic phase (aotric and ventricular contraction). The smaller secondary peak occurs due to the closure of the aortic valve at the end of the cardiac cycle. This smaller peak is called the dicrotic notch. In this Instructable, I will show you how to recreate the pulse waveform using basic circuit components (resistors, capacitors, transistors, and op amps) without the use of a microcontroller. I didn’t have much inspiration for doing this other than I love finding excuses to play with circuits and I just wanted to see if I could do it. ;) I thought about putting it together a little more to make it some sort of educational tool. I was thinking the circuit could be tweaked in order to show students how the waveform changes when someone has some sort of ailment that messes with their cardiac cycle. I’m not sure if I will still get around to doing that, but I thought I would share my progress thus far. MattDougan and I made this in a bit of a competition to see who could do this better. I won by the way. Lol.”

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