555 variable width monostable circuit diagram

Generally , the output pulse width of a monostable circuit is a fixed value. As shown in the figure, this circuit uses the output pulse, after low-pass filtering and DC amplification, to close-loop control its 555 control end, so that when the trigger frequency increases, its temporary stable width is automatically reduced to reach the output waveform. The empty ratio remains unchanged.

C2 and R2 perform differential triggering on the input signal. When the input frequency is low, after the output is low-pass filtered by R3C3, its DC level is low, and is inverted by IC 2 and then added to pin 5 of 555. Because the Vc control level is high, the output pulse width is large. As the trigger frequency increases, the filtered DC level becomes higher, the Vc terminal decreases, and the output pulse width of the 555 becomes smaller. Such closed-loop control can keep the output duty cycle constant.

The parameters shown in the figure are suitable for the input frequency range of 500 ~ 5000Hz. When designing the circuit, it should be ensured that R1C1<1.1Tmin=1.1/f max, fm ax is the highest frequency of the input trigger signal.

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