An air filter clog detector that provides a simpler and reliable method of determining clogging level of air filters using a grayscale or color light sensor. An example air filter clog detector includes a light source (e.g., light emitting diode (LED)), light detector, and circuitry (e.g., micro-processor controller). The light source is configured to emit light toward an air filter. The light detector is configured to measure a color of light reflected by the air filter. The circuitry is configured to determine whether the air filter is clogged based on the color of light reflected by the air filter. To prevent the air filter clog detector from becoming dirty, the detector can be positioned on the downstream side of the air filter.