555 Timer as Monostable Multivibrator


Posted on Feb 6, 2014

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555 Timer as Monostable Multivibrator
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uits and Projects A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network, connected externally to the 555 timer. In such a vibrator, one state of output is stable while the other is quasi-stable (unstable). For auto-triggering of output from quasi-stable state to stable state energy is stored by an externally connected capaci ­tor C to a reference level. The time taken in storage determines the pulse width. The transition of output from stable state to quasi-stable state is accom ­plished by external triggering. The schematic of a 555 timer in monostable mode of operation is shown in figure. Pin 1 is grounded. Trigger input is applied to pin 2. In quiescent condition of output this input is kept at + VCC. To obtain transition of output from stable state to quasi-stable state, a negative-going pulse of narrow width (a width smaller than expected pulse width of output waveform) and amplitude of greater than + 2/3 VCC is applied to pin 2. Output is taken from pin 3. Pin 4 is usually connected to + VCC to avoid accidental reset. Pin 5 is grounded through a 0. 01 u F capacitor to avoid noise problem. Pin 6 (threshold) is shorted to pin 7. A resistor RA is connected between pins 6 and 8. At pins 7 a discharge capacitor is connected while pin 8 is connected to supply VCC. Initially, when the output at pin 3 is low i....




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