measure pulse with a sound sensor
You can sense the human pulse cheaply using a sound sensor.
While pulse sensors can now be found even in phones, using a sound sensor works too.
Hold a sound sensor over where your heart is and if the room is fairly quiet you can record enough peaks to calculate your heart rate.
You may want to experiment further as sound sensors vary in shape: for example, hold one near a funnel held to your chest. Or press a stethoscope over the heart area, attach a sound sensor to its earpiece.