Multivibrators produce an output wave shape resembling that of a symmetrical or asymmetrical square wave and as such are the most commonly used of all the square wave generators. Multivibrators belong to a family of oscillators commonly called “Relaxation Oscillators“.
Generally speaking, discrete multivibrators consist of a two transistor cross coupled switching circuit designed so that one or more of its outputs are fed back as an input to the other transistor 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 giving them either a stable or quasi-stable state depending upon the type of multivibrator. One such type of a two state pulse generator configuration are called Monostable Multivibrators.
Monostable Multivibrators have only ONE stable state (hence their name: “Mono”), and produce a single output pulse when it is triggered externally. Monostable Multivibrators only return back to their first original and stable state after a period of time determined by the time constant of the RC coupled circuit.
Consider the MOSFET circuit on the left. The resistor R and capacitor C form an RC timing circuit. The N-channel enhancement mode MOSFET is switched “ON” due to the voltage across the capacitor with the drain connected LED also “ON”.
When the switch is closed the capacitor is short circuited and therefore discharges while at the same time the gate of the MOSFET is shorted to ground. The MOSFET and therefore the LED are both switched “OFF”. While the switch is closed the circuit will always be “OFF” and in its “unstable state”.
When the switch is opened, the fully discharged capacitor starts to charge up through the resistor, R at a rate determined by the RC time constant of the resistor-capacitor network. Once the capacitors charging voltage reaches the lower threshold voltage level of the MOSFETs gate, the MOSFET switches “ON” and illuminates the LED returning the circuit back to its stable state.
Then the application of the switch causes the circuit to enter its unstable state, while the time constant of the RC network returns it back to its stable state after a preset timing period thereby producing a very simple “one-shot” or Monostable Multivibrator MOSFET circuit.
Monostable Multivibrators or “One-Shot Multivibrators” as they are also 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 will remain in this second state.
The timing cycle of the monostable is determined by the time constant of the timing capacitor, CT and the resistor, RT until it resets or returns itself back to its original (stable) state. The monostable multivibrator will then remain in this original stable state indefinitely until another input pulse or trigger 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 transistor 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, RT 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 – 0.7)/RT.
If a negative trigger pulse is now applied at the input, the fast decaying edge of the pulse will pass straight through capacitor, C1 to the base of transistor, TR1 via the blocking diode turning it “ON”. The collector of TR1 which was previously at Vcc drops quickly to below zero volts effectively giving capacitor CT a reverse charge of -0.7v across its plates. This action results in transistor TR2 now having a minus base voltage at point X holding the transistor fully “OFF”. This then represents the circuits second state, the “Unstable State” with an output voltage equal to Vcc.
Timing capacitor, CT begins to discharge this -0.7v through the timing resistor RT, 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 RT CT 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 negative 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, CT the resistor, RT 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 first NOR gate U1 is HIGH at logic level “1”, (NOR gate principals). The resistor, RT is connected to the supply voltage so is also equal to logic level “1”, which means that the capacitor, CT has the same charge on both of its plates. Junction V1 is therefore 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 positive trigger pulse is applied to the input at time t0, the output of the first NOR gate U1 goes LOW taking with it the left hand plate of capacitor CT thereby discharging the capacitor. As both plates of the capacitor are now at logic level “0”, so too 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 timing capacitor now charging up through resistor, RT reaches the minimum input threshold voltage of U2 (approx. 2.0V) causing it to change state as a logic level “1” value has now appeared on its inputs. This causes the output to be reset to logic “0” which in turn is fed back (feedback loop) to one input of U2. This action automatically returns the monostable 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 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 or the 4538B 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 = 0.69RC in seconds.
74LS121 Monostable Generator
This monostable pulse generator IC can be configured to produce an output pulse on either a rising-edge trigger pulse or a falling-edge trigger pulse. The 74LS121 can produce pulse widths from about 10ns to about 10ms width a maximum timing resistor of 40kΩ and a maximum timing capacitor of 1000uF.
Monostable Multivibrators Summary
Then to summarize, the Monostable Multivibrator circuit has only ONE stable state making it a “one-shot” pulse generator. When triggered by a short external trigger pulse either positive or negative.
Once triggered the monostable changes state and remains in this second state for an amount of time determined by the preset time period of the RC feedback timing 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.
In the next tutorial about Multivibrators, we will look at one that has TWO stable states that requires two trigger pulses to switched over from one stable state to the other. This type of multivibrator circuit is called a Bistable Multivibrator also known by their more common name of “Flip-flops”.