To reduce size, minimise the parts count, and simplify assembly, we do not use discrete focussing optics to guide the exciting or emitted light, although the chosen LED has a focussing lens integrated within its housing. If required, additional lenses could be straightforwardly incorporated into the housing for better light management and enhanced sensitivity. By replacing the short-and long-pass filters with band-pass filters matched to the emission peak of the LED, the system may be straightforwardly adapted to absorption-based measurements. (The ability to switch between fluorescence-and absorption-based measurements is the reason why we have chosen to arrange the light-source and detector in a face-toface configuration rather than at 90 to one another, although it would be simple to adapt the system to orthogonal fluorescence detection if required). For the purposes of cholesterol detection, the LED is chosen to have an emission peak at 525 nm, which coincides with the absorption band of the resorufin dye used in the enzymatic assay. The Full Width Half Maximum (FWHM) of the LED's emission spectrum is approximately 25 nm, but a long-wavelength tail extends beyond 570 nm (Fig. ). The emission tail is attenuated by the short-pass filter, which has a cutoff at 550 nm (Fig. ). The emission band of resorufin lies predominantly in the range 550 -650 nm, with a weak tail that extends to longer wavelengths (Fig. ). The long-pass filter has a cut-on at 570 nm (Fig. ) and therefore blocks most of the short-pass-filtered excitation light, while efficiently passing the emitted light. The LED is modulated at ~410 Hz, using a 30-mA constant-current driver circuit (see Fig. ) that is toggled on and off with a 50 % duty cycle using a digital output pin of the microcontroller. For the 30-mA square-wave modulation used here, the time-averaged output power of the LED is just over 1 mW. The amplified silicon photodiode (OPT101) has a broadband optical response from 400 to 1000 nm, which fully covers the emission band of resorufin (Fig. ). The set-up may be readily adapted to assays based on other dye molecules by swapping the LED and optical filters as needed.