How to measure heart pulse from body

I want to measure my pulse rate, and turn a light on for a moment for each heartbeat. I’ve seen a few ways to do it. Ideally I’d like it to be wearable and measure the pulse from a sensor attached to or against the chest.

This one:
seems to be the simplest. But also quite expensive.

A much more complex one:

I don’t have much electronics experience, so I’d like something that I could learn from, but not overwhelmingly complex.

There are “not-too-expensive” ECG shields for Arduino, but using a pulse sensor on your finger or ear is easier.
That Pulse Sensor is expensive with 25 dollars for such simple hardware, but it will work.

You could try to make something similar to the Pulse Sensor, but it might not work in the end.