Pulse generator with variable duty factor

The duty factor of the pulse generator in the diagram is variable in 10% steps from 10% to 90%. With the aid of thumb wheel switch S1, a 4-bit word, S, is added to one input of IC1 and to IC2. Circuit IC1 is a full adder, while IC2 functions as a switch. The binary equivalent of 5 is applied to the other input of IC1. The output of IC1 is linked to the second input of IC2.