Revenue bonds, authorized by section 18-502, may be issued by ordinance duly passed by the mayor and city council of any city or the board of trustees of any village without any other authority. General obligation bonds, authorized by section 18-506, may be issued only after the question of their issuance shall have been submitted to the electors of such city or village at a general or special election, of which three weeks’ notice thereof has been published in a legal newspaper published in or of general circulation in such city or village, and more than a majority of the electors voting at the election have voted in favor of the issuance of such bonds.