We report the development and test of a novel hydrogen sensor that shows a very peculiar response to hydrogen exposure, due to its micro-structured Palladium surface. The fabrication of the wrinkled Pd surface is obtained using an innovative, fast and cheap technique based on the deposition of a thin Pd film onto thermo-retractable polystyrene sheet that shrinks to 40%2of its original size when heated. The buckling of Pd surface induced by shrinking of substrate produce nano and micro-wrinkles on the entire sensor surface. The obtained microstructured sensor surface is very stable also after repeated hydrogen sorption/desorption cycles. The hydrogen sensing mechanism is based on the transitory absorption of hydrogen atoms into the Pd layer, leading to reversible change of its electrical resistance. Interestingly, depending on hydrogen concentration the proposed sensor shows the concurrent effect of both the usually described behaviors of increase or decrease of resistance, related to different phenomena occurring upon hydrogen exposure and formation of palladium hydride. The study reports and discusses evidences for an activation threshold of hydrogen concentration in air switching the behavior of sensor performances from, e.g., poor negative to large positive sensitivity and from slow to fast detection.