The photo- and acidochromic properties of a new generation norbornadiene derivative have been harnessed for the realization of a three-input keypad lock, where a specific sequence of inputs induces a unique output. Reversible quadricyclane/norbornadiene photoisomerization is reported, and this rare feature allows the first example of a norbornadiene-based molecular logic system. The function of this system is clearly rationalized in terms of the interconversion scheme and the absorption spectra of the involved species.