It’s common knowledge that there is a shortage of clean drinking water in the world, and one of the solutions is SODIS (Solar Water Disinfection), a method that uses sunlight and plastic bottles to disinfect water. Of course, the issue with it is that there isn’t any reliable way to tell when the water has reached a safe level of purity. Now engineering students from the University of Washington have created a simple inexpensive device that does just that. The device uses parts from a keyboard that blinks when exposed to light, attaching it to the water bottle and using it to monitor the amount of light passing through the water. The indicator will blink on and off as long as particulates are still obstructing the light flow and will stop blinking once the water is safe to drink. It’s also smart enough to tell when there is water in the bottle in front of it, so it won’t try to measure data when there’s nothing there. The parts for the device are estimated to cost $3.40, though bulk purchasing will definitely drive the prices down.