The use of photonic crystals as sensors is based on the change of their spectral characteristics on exposure to various analytes. A new approach based on a recently discovered asymmetry in deformation of the photonic crystals lattice allowed to create cheap, high-sensitive and fast chemosensors which do not require spectral instruments. This effect shows itself most strongly in gaseous ammonia. The sensitivity of such a sensor is several parts per million (ppm), while the response to an increase in concentration is less than 200 ms.