A spin coater is widely used for thin film coating in nano/microtechnology. In this paper, a spin coater with inexpensive mechanical and electronic components was fabricated based on an open-source Arduino microcontroller. To measure and control the spin speed of the spin coater, two sensor types including two infrared (IR) light-emitting diodes (LEDs) or a Hall Effect (HE) integrated circuit (IC) were used. The spin coater was tested for coating polydimethylsiloxane (PDMS) polymer in the spin speed range of 1000-9000 rpm (revolutions per minute). An optical interferometric method was used to determine the thickness and optical transmittance of the spin-coated PDMS films. The results show that the performance of this spin coater is similar to that of a commercial model and therefore it can be used in the laboratory and for student education. The total cost and power consumption of the spin coater are less than $30 and 5 W, respectively.