How can a clip on your finger count your heartbeats?

Put the clip on a finger to start.

1. The clip goes on a fingertip.
2. A light shines through the nail, skin and blood.
3. After a few seconds, the screen shows the pulse.
4. A sensor on the other side measures the light. Each beat changes how much gets through.

The finger, the light, the sensor, the screen and the number 76 are added by this page. A real pulse oximeter also shows the oxygen level; this page shows only the pulse.

Turn the model. Find the clip at the end of the cable.