An electronic circuit and method for designing an electronic circuit is provided that includes a first source element, a second source element, a first matching network, and a second matching network. The hollow particles and the solid particles are distributed in the transparent resin layer, while the hollow particles are partially exposed thereon. The controller initiates a misfire detecting operation of detecting an occurrence of the misfire of the engine when a predetermined first condition is satisfied, and initializes the misfire-frequency data stored in the memory when a predetermined second condition is satisfied. Either side of the first optical waveguide is filled with a filling material, but either side of the second optical waveguide is not filled with the filling material. The flat plate structure therefore covers the wear surfaces of the worn skid plate, therein providing new wear surfaces.