Our pulse is direct indicator of physiological condition. It may be used to determine workout condition, health status, or even indicate when you are lying. You’ve probably seen that intensive care patients almost always have small thing on their finger which takes reading of of blood oxygenation and so the pulse. ASCAS in his instructable have built simple pulse meter device that connects directly to Arduino.
He had put some effort to make sensor itself. He was looking for suitable clamp that would fit on finger and could carry detector circuit. He found that coat hanger has what it needs. He drilled small hole where he glued a red LED and on opposite side he placed LDR. Then he focused on software. He wanted to see pulse visually, so he wrote a Processing code to capture data from Arduino and display in nice graph.