This paper describes a new mechanical implementation of a folded pendulum based inertial sensor, configurable as seismometer and as accelerometer.1 The sensor is compact, light, scalable, tunable (< 100 mHz), with large band (10-6 Hz÷10 Hz), high quality factor (Q > 1500 in air) instrument and good immunity to environmental noises, guaranteed by an integrated laser optical readout. The measured sensitivity curve is in very good agreement with the theoretical one (10-12 m/√Hz in the band (0.1 ÷ 10 Hz). Typical applications are in the field of earthquake engineering, geophysics, and in all applications requiring large band-low frequency performances coupled with high sensitivities.