Detecting cost-effectively and accurately the working area for autonomous lawn mowers is key for widespread automation of garden care. Therefore, we propose an active low-cost sensor approach for detecting fluorescence response. The area to be detected is illuminated by an LED and the chlorophyll fluorescence response is observed by a phototransistor. The signal from the phototransistor is further processed by a transimpedance amplifier, an amplifier and a band pass filter and forwarded to a microprocessor. By choosing only low-cost consumer products for construction, high-volume lowest cost sensors can be built. We demonstrate the feasibility of our low-cost approach by evaluating the sensor mounted on an autonomous lawn mower in a garden environment.