The detection and quantitation of weak, slowly varying fluorescence signals from trace concentrations of chromophores is an essential task in many fields of science. Important examples include: biochemistry, where low-concentration biomolecules are quantified using fluorogenic assays; synthetic chemistry, where reaction kinetics are often determined by monitoring changes in fluorescence intensity due to consumed or generated fluorophores; and environmental science, where fluorescence is used to detect and quantify pollutants, either directly or indirectly via their interaction with probe dyes. In common with other optical signals, fluorescence is commonly monitored using a photodiode coupled to a sensitive current meter, with the current generated by the photodiode typically being proportional to the intensity of the incident light. At low fluorophore concentrations, the weak fluorescence signal is frequently masked by noise and/or strong background signals, preventing reliable detection and measurement. In such circumstances, the fluorescence signal may often be extracted by modulating the light-source (and therefore the fluorescence signal) at a fixed frequency 𝑓 mod , and using a signal processing technique known as "lock-in" detection to eliminate unwanted interferences at other frequencies. The lock-in technique is simple to implement and, as shown here, can give good performance even with inexpensive hardware, making it an attractive choice for low-cost, high sensitivity fluorescence detection.