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555 Monostable (one shot)

Schematic of 555 monostable or one shot.

    The 555 Timer, designed by Hans Camenzind, is believed to be the most popular IC ever manufactured. It was designed 50 years ago and is still being being manufactured today. Although there are more capable and higher spec parts now available, its ease of use and low cost still make it a good fit for many applications.

    This solver finds the required values for a 555 operating as a monostable multivibrator, or one shot, that produces a pulse in response to trigger input. Entering a value for t (the pulse width) finds R and C. You can optionally enter a value for R or C along with t. Entering a value for both R and C finds t.

    If you don’t like the magnitude of the component values calculated when entering a value for t, your best bet is to pick a capacitor value to use and have the solver calculate the resistor value for you.

    The 555 produces a pulse that varies between roughly 0V and Vcc - 1.7V. The output’s rising edge corresponds to the falling edge of the trigger input as it passes through Vcc. The trigger input must be shorter than t. If not, the output will stay high until the trigger returns to a value greater than Vcc.

    A default value of 5% is assumed for the capacitor’s tolerance. If you wish to use another value, enter it as a percentage (10% = 10).

    For a free running square wave oscillator, see the 555 Astable (free running) solver or the Logic Gate Oscillator solver. For a sine wave oscillator, use the Sine and Cosine solver. The following formula is used in this solver.

copyright © 2021 John Miskimins


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