How to add a capacitor and switch correctly to piezoelectric circuit?
Without reading through the Instructable I think what you want is this:







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Multi-piezo rectification.

The piezo output will exhibit a very high internal resistance. Once they are loaded the output voltage will collapse. In practice the capacitor loads the piezo so that each piezo pulse will add a little charge to the capacitor. The max voltage the capacitor sees will depend on the rate of charge versus the rate of discharge.







simulate this circuit

Figure 2. Six-piezo rectification. All rectified outputs joined in parallel.