Rongge

Here is a simple oscillator circuit that varies the duty cycle over a wide range without affecting the frequency. It is a variation of the simple 555 astable oscillator. Initially, I told a reader that there was no standard 555 circuit that could do this, but then the grey matter started working. The use of an air-variable capacitor for frequency control is a mind-blower—nothing short of a time warp! 555 Duty Cycle Control Schematic Overview When potentiometer R1 is centered, operation is obvious and the duty cycle is 50%. However, as R1 is rotated in either direction the charge time and discharge times vary accordingly. The two sides of R1 have independent steering diodes (D1 & D2). C1 & C2 make up the timing capacitor. Pins 2 & 6 of the 555 are the upper and lower thresholds of the input comparators. The charge /discharge voltage is taken from pin 3 because it has rail-to-rail voltage swing, and the open collector output (pin 7) cannot do this. The rectangular waveform o...