Water and wastewater analyses for environmental control require considerable expenses, especially with respect to instrumentation. The importance of purchase of low-cost laboratory equipment has been recognised by environmental initiatives which are active in poor regions. Photometers are versatile instruments in water and wastewater analysis. Unfortunately, they are expensive. This site provides a manual for the construction of a low-cost photometer operated with a tricolor LED. The following are examples of nutrient analyses which can be performed with the low-cost LED photometer with the wavelengths 430, 565, and 627 nm, resp.: ammonia according to the German standard method (following a similar principle as the phenate method given in the American Standard Method 4500-NH₃ F with photometric determination at 640 nm), ortho-phosphate using the stannous chloride method which can be found in chapter 4500-P D in the "Standard Methods", and nitrite by colorimetric method (Standard Method 4500-NO₂^- B). Also the cadmium reduction method for nitrate (Standard Method 4500-NO₃^- E) would be feasible with the low-cost photometer, as this method utilises a wavelength of 543 nm which is close to one of the low-cost photometer LED‘s emission maxima (565 nm). However, any photometric methodology applying absorption maxima close to one of the three mentioned wavelengths can be realized with this low-cost photometer.