In the circuit, rCl is a two-input four-NAND gate CD40 1 1, in which NAND gates L and 2 form a multivibrator, and NAND gates 3 and 4 form an RS trigger circuit to shape the pulse generated by the multivibrator, and then output a clock pulse from the output end of NAND gate 4 to the Cp end of IC2.