Downey Mayor James Santangelo was charged with a misdemeanor conflict-of-interest violation for a July, 1984, vote in favor of a redevelopment project that would have benefitted an area in which he owned several parcels of land, including a real estate office. If convicted, Santangelo, who will be arraigned Aug. 20 in Los Angeles Municipal Court, faces a maximum of six months in jail and a $10,000 fine, and could be barred from holding public office for four years, Deputy Dist. Atty. Herbert Lapin said. The 380-acre Firestone Boulevard redevelopment plan, which had been approved by the Downey City Council on a 3-2 vote, was invalidated last September after Los Angeles Superior Court Judge Norman Dowds said Santangelo should have abstained from voting since he had "a material and financial interest in the decision. . . ."