An ultra-low power, piezoelectric sensor interface circuit for Weigh-In-Motion applications is presented. It is optimized for energy-autonomous vehicle-weight measuring systems in road monitoring applications and can be combined with any type and size of piezoelectric device. Whenever the output voltage of the used piezoelectric sensor exceeds a predefined threshold level, the circuit is activated, measuring the voltage area integral of the detected pulse. A low-power microcontroller unit can be used for the calculation and use of the obtained weight measurements. The circuit was implemented with a combination of discrete analog circuits for the activation and duration units, as well as an integrated voltage-to-frequency converter for the area-integral calculation.