Any bonds hereafter issued under the provisions of sections 10-501 to 10-509 shall be for the payment by the county issuing the same of the sum specified therein, made payable at the office of the county treasurer, and to run not more than twenty years nor less than five years, with interest payable semiannually; the bonds and coupons shall be signed by the chairman of the board, and countersigned by the county clerk of the county; Provided, such bonds may be made redeemable at any time after five years, at the option of the county board.