An inexpensive portable sensor which can detect volatile organic compounds (VOCs) has been developed. The system consists of a commercial blue LED which emits light that is detected by a phototransistor. The phototransistor is coated with a few thin layers of a gas sensing porphyrin. Porphyrins have a highly conjugated molecular structure that displays strong absorption in the visible region of the spectrum. The porphyrin layer undergoes a subtle shift in its absorbance characteristics upon exposure to specific volatile organic compounds. This varies the light intensity received by the phototransistor and results in a change in the voltage across the phototransistor when the analyte is detected. This sensor design can be applied to any available porphyrin–analyte pairing to develop a wide range of VOC detectors. Presented here is an alcohol sensor based upon the interaction of various alcohols with a magnesium porphyrin.