How can I amplify a piezo signal?

Hi, I’m new to the Arduino and recently completed a tutorial to create a piezo heart rate monitor. The piezo sensor needed a 1MOhm resister before it was sensitive enough to pick up a pulse. I wanted to amplify the signal and found a tutorial online. Unfortunately the piezo sensor now doesn’t seem to be sensitive enough to pick up a pulse. This is the tutorial I followed and the schematic – except I am using a knock senor rather than a buzzer.

Can anyone advise me how I could make the piezo more sensitive or a better way of amplifying the signal?