Just so you have some extra information (I actually build a heart rate monitor and pulse oximiter) skin color and transparency makes a big difference, same with the color of the LEDs. I'm pretty pale and one of my friends is pretty tan, my signal was much stronger than his. The IR (infrared) ones work better as they are able to penetrate the skin better and there are clearer absorption lines from oxidized blood. I also tested different locations to place the device, I did notice most are fingertip or wrist (fitbits and smart watches), but found I got substantially stronger signals by placing the device near the upper forearm (god did this help in debugging). This was true for everyone I tested (unfortunately only white people) and helped smooth out the data a little, but it was still noticeable.
On how these devices work, it is typically two LEDs the produce light that bounces through the skin, some light is absorbed by blood (different absorption by different oxidation and density) then the reflected light is captured by a photodiode. All of this is on one surface, they are very basic. The density changes rapidly and that's how you capture the pulse (easy to get btw) and the oxidation is based on absolute levels (THIS IS THE PROBLEM!!!!). After building one I learned to not take these things too seriously, especially fingertip and wrist mounted ones.
I encourage others to build them. You can do most of it in a weekend if you have some soldering experience (best if you can do surface mount soldering), some programming, and are comfortable doing data analysis. And no, I do not expect my device to be on par with the ones in hospitals, but I was quickly able to match readings from a finger based device that I bought from the local drug store.