This paper presents a new method to measure a rotor speed using a smartphone camera. An image processing algorithm is proposed to measure the rotation speed, based on the periodic detection of a mark placed on the rotor. Experiments demonstrate our method can estimate rotor speed with a good precision, provided that the camera frame rate is high enough. The method could be very useful for periodic speed measurement in applications where a sensor is prohibited, for example in picohydro-turbine for rural electrification. Since the smartphones are almost everywhere, they contribute to cost and environmental impact reductions in such solutions. The rest of the system is composed of reused components according to the principles of frugal innovation.