monostable multivibrator 555 timer

Posted on Feb 5, 2014

As its name indicates it has one stable state and it switches to unstable state for a predetermined time period T when it istriggered. The time period T is determined by the RC time constant in the circuit. Monostable mode of 555 Timer is commonly used for generating Pulse Width Modulated ( PWM ) waves. This is the circuit diagram of 555 Timer wired in Monostable mode.

monostable multivibrator 555 timer
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8th pin and 1st pin of the 555 timer are used to givenpowerVcc and Ground respectively. 4th pin is the Reset pin of 555 Timer, which is active low so it is connected to Vcc to avoid accidental resets. 5th pin is the Control Voltage pin used to provide external reference voltage to internal comparators. Since it is not used here, it is grounded via a capacitor C` (0. 01 µF) to avoid high frequency noises. When a negative trigger is applied on the Trigger input of 555, output goes high and capacitor starts charging through resistor R. When the capacitor voltage becomes greater than 2/3 Vcc, ouput goes low and capacitor starts discharging through the Discharge pin of 555 Timer. Time period of the unstable state is given the tye expression, T = 1. 1RC. When a negative trigger is applied to the Trigger pin of 555 Timer, output of lower comparator will become HIGH and output of upper comparator will be LOW, since the capacitor voltage is zero. This makes the output HIGH. After the negative trigger, output of lower comparator becomes LOW and that of upper comparator remains LOW. Since both inputs of the SR Flip Flop are LOW, output will not change, so the output is HIGH. When the capacitor voltage will become greater than 2/3 Vcc, output of upper comparator becomes HIGH and that of lower comparator remains LOW, so the output becomes LOW.

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