Introduction to Multivibrator

A multivibrator is an electronic circuit that is employed to implement a range of simple two-state systems like oscillators, timers and flip-flops. It is characterized through two amplifying devices (electron tubes, transistors, or other devices) cross-coupled through resistors and capacitors.

There are three categorize of multivibrator circuit:

  • Astable, in this category the circuit is not stable in either state it continuously oscillates from one state to the other. Because of this, it does not need a input (Clock pulse or other).
  • Monostable, in this category the one of the states is stable, but the other one is not the circuit will flip into the unstable state for a specified period, but will finally return to the stable state. This circuit is also termed as a one shot.
  • Biteable, in this category the circuit will stay in either state indefinitely. The circuit can be flipped from one state to another. This circuit is also termed as a latch or a flip-flop.


The associations of the trigger signal, the voltage on C and the pulse width within monostable mode.

In the monostable mode, the 555 timer works like a "one-shot" pulse generator.

The pulse starts while the 555 timer receives a signal at the trigger input which falls below a third of the voltage supply. The width of the pulse is ascertained through the time constant of an RC network that contains a capacitor (C) and a resistor (R). The pulse ends while the charge at the C equals 2/3 of the supply voltage.

The pulse width may be lengthened or shortened to the requirement of the particular application through adjusting the values of R and C. The pulse width of time t that is the time, it takes to charge C to 2/3 of the supply voltage.

The pulse width of time t, that is the time it takes to charge C to 2/3 of the supply voltage, is provided through

t = RC ln (3) ≈ 1.1RC

In which R is in ohms, t is in seconds, and C is in farads.


The 555 timer works like a basic flip-flop in bistable mode. The reset and trigger inputs (pins 2 and 4 correspondingly on a 555) are assumed high via pull-up resistors when the threshold input (pin 6) is just grounded. So configured, pulling the trigger momentarily to ground works like a 'set' and transitions the output pin (pin 3) to Vcc (high state). Pulling the reset input to ground works like a 'reset' and transitions the output pin to ground (low state). No capacitors are needed in a bistable configuration. Pins 5 and 7 (control and discharge) are left floating.


The '555 timer 'puts out a continuous stream of rectangular pulses containing a particular frequency, in astable mode. Resistor R1 is connected among VCC and the discharge pin (pin 7) and other resistor (R2) is connected among the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins which share a common node. Therefore the capacitor is charged by R1 and R2, and discharged only by R2, because pin 7 has low impedance to ground throughout output low intervals of the cycle, so discharging the capacitor. In the astable mode, the frequency of the pulse stream relies on the values of R1, R2 and C:

1951_Astable Mode Homework Help.jpg

The high time from each pulse is expressed through

high = In (2) . (R1 + 2R2) .C

And the low time from each pulse is expressed through

low = In(2) .2R2.C

In which R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.

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