Simulate Piezo "knock" Sensor in CircuitLab
A first approximation of a piezo sensor is a voltage source in series with a very high resistance, say \$10 M\Omega\$. They also tend to have significant mechanical resonances that can be modeled electronically, but doing so is not trivial and is specific to the geometry of a particular sensor. Sensitivity, and consequently the amplitude of the voltage, also varies based on the device and application.