Symbol

Just because it came back to mind recently, here's a symbol I propose to represent piezo film as a circuit element. It's based on the existing symbol for a crystal, which is fine because it reflects the capacitive nature of any piezo sensor. But here, we want to show that the sensor is flexible...

Meanwhile, it is worth mentioning that a piezo sensor can be modelled (for example, in a circuit emulation package) as a voltage source that exactly follow the applied mechanical property (stress or strain), connected in series with a capacitor (whose value is the same as the piezo film sensor, which can be measured or calculated from its dimensions). When this arrangement is connected to a fixed resistive load (such as the input of an oscilloscope), a voltage divider is formed. At low frequencies, the impedance of the sensor is much higher than that of the fixed resistor, and so the voltage across the resistor (the input to the scope) is very small. At higher frequencies, the impedance of the capacitor gets much less, and eventually the full "open circuit" voltage developed by the film appears across the resistive input. Unfortunately, it just happens that many real-life applications involve frequencies that are moderate or slow in electrical terms, especially those resulting from human-initiated events. This makes it more challenging to capture the full "potential" electrical signal from the film - we need to use rather high input resistance (often 10 or 100 M ohm, even into the 1-10-100 G ohm ranges for very slow events or small sensors). Charge amplifiers work differently, actually presenting very low resistance to the sensor (so that all the charge flows into the feedback network of the amplifier), but high resistances are still required internally to maintain the desired low frequency response.