555 pulse generator - with variable duty cycle

This circuit may look familiar to many readers since it is one of the many variations of circuits on the 555 timer theme. This does not detract from its usefulness however since a versatile pulse generator with a variable duty cycle is an excellent aid for the workshop. Unlike the standard circuit usually adopted (see infocard 19), the resistance between pins 6 and 7 consists of P1, P2, R2, D1 and D2. A closely defined charging time for capacitor C1 is obtained by diodes D1 and D2. This would normally lead to a duty cycle of 50%, if it were not for P2. In this case the duty cycle depends on the relationship between P1 and P2: n = 1 + P2/P1. For example, if P2 = 0 (n = 100%), the frequency will then be: 0.69 f — (2 • P1 + P2 + 4.71(2) • C1 P.C.M. Verhoosel .. with variable duty cycle "see text P1 =frequency P2 = duty cycle elektor july/august 1982 — 7-27 D1,D2=1N4148