Digital <—-> Physical
This device reports a physical volume by reading a digital volume. An air pump and solenoid valve control incremental inflating and deflating of a balloon with air.
In this example, about 100mb of data is moved to the computer's trash, then the trash is emptied. The balloon therefore goes from empty, to slightly inflated, then back to empty. However, as a physical display using a pump and a solenoid valve, it can show and hold any range between empty and full.