Upon a four-fifths vote of all the members of the board, it may issue negotiable promissory notes bearing interest at a rate of not exceeding 7 percent per annum; provided, however, that said notes shall be payable from revenues and taxes levied for purposes of the district other than the payment of principal and interest on any bonded debt of the district; and provided further, that the maturity date shall not be later than 20 years from the date thereof and that the total aggregate amount of such notes outstanding at any one time shall not exceed 1 percent of the assessed valuation of the taxable property in the district, or if such assessed valuation is not obtainable, 1 percent of the county auditor’s estimate of the assessed valuation of the taxable property in the district evidenced by his certificate.