PURPOSE: An encoder device for driving an SRM(Switched Reluctance Motor) is provided to perform an on/off switching operation at a correct position by using a digital logic circuit including a decoder and a counter. CONSTITUTION: An encoder device for driving an SRM includes a disc, a pattern, the first photo coupler, and the second photo coupler. The disc is coupled to a rotary shaft. The pattern has the same period as the width of a position angle corresponding to a phase of an SRM driving inverter. A circular shape of the pattern is formed on the disc. The color sense becomes bright to a center of the pattern within one period. The color sense becomes dark to both sides of the pattern within one period. The first photo coupler is used for outputting the voltage proportional to the light reflected on the pattern. The second photo coupler is located at the predetermined position apart from the first photo coupler in order to output the voltage proportional to the light reflected on the pattern. The first and the second photo couplers are used for outputting two chopping waves having phase differences.
