Monostable Multivibrators

Monostable Multivibrators

Multivibrators are the most commonly used of all the square wave generators and which themselves belong to a family of oscillators commonly called "Relaxation Oscillators". Generally speaking, discrete multivibrators consist of a two transistors cross coupled amplifier circuit with a Resistor and Capacitor (RC) network connected across them to produce the feedback tank circuit. Multivibrators have two different electrical states, an output "HIGH" state and an output "LOW" state. One such type of a two state pulse generator configuration are called Monostable Multivibrators.

Monostable Multivibrators have only ONE stable state (hence there name: "Mono"), and they deliver a single output pulse when it is triggered externally only returning back to its first original and stable state after a period of time determined by the time constant of the RC coupled circuit. Monostable Multivibrators or "One-Shot Multivibrators" as they are sometimes called, are used to generate a single output pulse of a specified width, either "High" or "Low" when a suitable external trigger signal or pulse T is applied. This trigger signal initiates a timing cycle which causes the output of the monostable to change its state at the start of the timing cycle and remains in this second state, which is determined by the time constant of the Capacitor, C and the Resistor, R until it resets or returns itself back to its original (stable) state. It will remain in this stable state indefinitely until another input pulse or signal is received. Then, Monostable Multivibrators have only ONE stable state and go through a full cycle in response to a single triggering input pulse.

Monostable Multivibrator Circuit

The basic Collector-coupled Monostable Multivibrator circuit and its associated waveforms are shown above. When power is firstly applied, the base of transistor TR2 is connected to Vcc via the biasing resistor, R2 thereby turning the transistor "fully-ON" and into saturation and at the same time turning TR1 "OFF" in the process. This then represents the circuits "Stable State" with zero output. The current flowing into the saturated Base terminal of TR2 will therefore be equal to Ib = (Vcc - 0.6)/R2.

If a trigger pulse is now applied at the input, the fast rising edge of the pulse will pass straight through capacitor, C1 to the Base of transistor, TR1 turning it "ON". The Collector of TR1 which was previously at Vcc drops quickly to below zero volts effectively giving capacitor C2 a reverse charge of -0.6v across its plates. This results in transistor TR2 now having a Base voltage of -0.6v holding the transistor fully "OFF" at point Vb_TR2. This then represents the circuits second state, the "Unstable State" with an output voltage equal to Vcc.

Capacitor, C2 begins to discharge this -0.6v through resistor R2, attempting to charge up to the supply voltage Vcc. This negative voltage at the Base of transistor TR2 begins to decrease gradually at a rate determined by the time constant of the C2-R2 combination. As the Base voltage of TR2 increases back up to Vcc, the transistor begins to conduct and doing so turns "OFF" again transistor TR1 which results in the monostable multivibrator automatically returning back to its original stable state awaiting a second trigger pulse to restart the process once again.

Monostable Multivibrators can produce a very short pulse or a much longer rectangular shaped waveform whose leading edge rises in time with the externally applied trigger pulse and whose trailing edge is dependent upon the RC time constant of the feedback components used. This RC time constant may be varied with time to produce a series of pulses which have a controlled fixed time delay in relation to the original trigger pulse as shown below.

Monostable Multivibrator Waveforms

The time constant of Monostable Multivibrators can be changed by varying the values of the Capacitor, C2 the Resistor, R2 or both. Monostable Multivibrators are generally used to increase the width of a pulse or to produce a time delay within a circuit as the frequency of the output signal is always the same as that for the trigger pulse input, the only difference is the pulse width.

TTL/CMOS Monostable Multivibrators

As well as producing Monostable Multivibrators from individual discrete components such as transistors, we can also construct monostable circuits using commonly available integrated circuits. The following circuit shows how a basic monostable multivibrator circuit can be constructed using just two 2-input Logic "NOR" Gates.

NOR Gate Monostable

Suppose initially that the trigger input is Low at a logic level "0" so that the output from the NOR gate U1 is High at logic level "1", (NOR Gate principals). The resistor, R is connected to the supply voltage so is equal to logic level "1", which means that the capacitor, C has the same charge on both plates and junction V1 is equal to this voltage, so the output from the second NOR gate U2 will be Low at logic level "0". This then represents the circuits "Stable State" with zero output.

When a trigger pulse is applied to the input at time t_o, the output of U1 goes Low taking with it capacitor C. As both plates of the capacitor are at logic level "0" so to is the input to the second NOR gate, U2 resulting in an output equal to logic level "1". This then represents the circuits second state, the "Unstable State" with an output voltage equal to Vcc.

The second NOR gate, U2 will maintain this second unstable state until the capacitor now charging up through resistor, R reaches the input threshold voltage of U2 causing it to change state, automatically returning itself back to its original stable state and awaiting a second trigger pulse to restart the timing process once again.

NOR Gate Monostable Waveforms

This then gives us an equation for the time period of the circuit as:

Where, R is in Ω's and C in Farads.

We can also make monostable pulse generators using special IC's and there are already integrated circuits dedicated to this such as the 74LS121 standard one shot monostable multivibrator or the 74LS123 re-triggerable monostable multivibrator which can produce output pulse widths from as low as 40 nanoseconds up to 28 seconds by using only two external RC timing components with the pulse width given as: T = 0.7RC in seconds.

Monostable Multivibrators Summary

Then to summarize, the Monostable Multivibrator circuit has only ONE stable state. When triggered by a short external trigger pulse it changes state and remains in this second state for an amount of time determined by the preset time period of the RC feedback components used. One this time period has passed the monostable automatically returns itself back to its original low state awaiting a second trigger pulse. Monostable Multivibrators can therefore be considered as triggered pulse generators and are generally used to produce a time delay within a circuit as the frequency of the output signal is the same as that for the trigger pulse input the only difference being the pulse width. One main disadvantage of Monostable Multivibrators is that the time between the application of the next trigger pulse has to be greater than the preset RC time constant of the circuit to allow the capacitor time to charge and discharge.