A device for detecting the marker dye band which is used to monitor the progression of biological macromolecules in gel electrophoresis. This device mounts external to the gel box, and utilizes a single light detector and a pair of AC activated light sources. The light sources produce reflected or transmitted light signals which, when balanced at the detector, cancel. When marker dye is absent the light signals are balanced, and no signal is detected. When marker dye is present at a specific detection point within the gel, the light reflected (or transmitted) is no longer balanced and a signal is detected.