具有加、减计数以及置数报警功能的计数器，采用三个数码管显示，555产生1秒的脉冲，三块74ls192十进制计数器级联为三位数计数器。-Have add, subtract count, and buy several alarm function of counter, using three digital pipe display, 555 produce 1 second pulse, three pieces of 74 ls192 decimal counter level three