By using a pulse-width-modulation (PWM) input signal generated by a microcontroller to drive the solenoid, a field-effect transistor (FET) switch connected in series with the solenoid and a voltage source (car battery) can be quickly turned on or off. The average voltage applied to the solenoid depends on the ratio of the 'on' time of the PWM waveform to the pulse period. Changes in pulse width and variations in the solenoid's mechanical load can cause changes in the average current flowing through the solenoid. This average current indicates the total motion of the solenoid, and thus also represents the liquid pressure and flow rate.