An apparatus comprises a transducer having a first output signal and arranged to receive an electrical input. The transducer switches the first output signal between an ON and OFF state. The apparatus comprises a chemical sensing surface coupled to the transducer arranged to receive a chemical input.A signal generator oscillates one or more of said inputs to vary the switching point of the transducer. The oscillating input may be the chemical input and/or the electrical input. The output signal may be a pulse whose period ON or OFF is determined by the oscillating input modulated by the chemical input.