Multivibrators are two-state (high or low) output circuits. These include oscillators, timers and flip-flops. Multivibrators are mostly used in applications which involve timing, pulse generation or pulse triggering of other device and so on. There are three types of multivibrators, these are monostable, bistable and astable.
'mono' means one or single, therefore monostable multivibrator is a single-stable state multivibrator with other state unstable or transient. A triggering signal/pulse is needed for this circuit to change to its unstable state which last for a fixed period preset by timer components of the circuit before returning to its stable state. This circuit finds application in a system where a timing period of fixed duration is needed in response to external event.
Monostable multivibrator circuit
Above is a BJT collector-cross-coupled monostable multivibrator circuit. At the instance of power up, the base of transistor T2 is connected to supplied voltage via the biasing resistor, R2 hence T2 is 'ON' and it conducts. T2 connects the base of T1 to the ground via resistor, R3, turning T1 'OFF'. This is the 'Stable State' with zero output.
If momentary switch, SW is pressed, a short circuit is applied between the collector of T1 and the ground forcing capacitor, C to discharge quickly with base voltage of T2 drop below zero volt, T2 goes 'OFF'. With T2 'OFF', base of T1 is connected to the supplied voltage via R4 and R3, hence T1 is 'ON'. The circuit remains in this state for a period determined by the value of capacitor, C and resistor, R2. This is the 'Unstable State'. As C begins to charge up, base voltage of T2 increases and turns 'ON' immediately it is above 0.7V. The duration of the pulse is given by the formula.
Click here for the monostable calculator
'bi' means two, therefore bistable multivibrator is a two-stable state multivibrator. That is, on receiving of an external signal (triggering signal/pulse), the circuit changes its present stable state to other and remains in this state until next external signal is received. This circuit can be used as an alternate on/off circuit. In fact in my article 'alternate on/off circuit', alternate on/off circuit is called bistable multivibrator and that is it.
Bistable multivibrator circuit
Above is a BJT collector-cross-coupled bistable multivibrator circuit This circuit choose either of the states at the instance of power up. Assuming T1 is on at the instance of power up, its collector is at ground voltage, hence base of T2. As a result of this, T2 is 'OFF' connecting base of T1 to the supplied voltage via R4 and R3. This state is the set state as the output is high. The circuit will remain in this state unless reset switch is pressed.
Pressing the reset switch connects the base of T1 to the ground forcing it to go 'OFF'. At this stage, base of T2 is connected to the supplied voltage via R1 and R2 which turns T2 'ON'. Also the circuit remains in this state until the set switch is pressed.
This multivibrator has no stable state -it continually flips from one state to the other without any external signal being applied. It is an oscillator in the sense that it produces a repetitive output signal. Astable multivibrator is an important circuit in any power inverter, it also finds great use or application in generating buzzing sound of different tones by varying the frequency when connected to a speaker. Another good application of an astable multivibrator is in light flasher.
The most common type of astable multivibrator is the cross-coupled transistor switching circuit. This circuit consists of two witching transistors and cross-coupled feedback network of a pair of timer components which allow oscillation between the two states with no external triggering signal. The timer components are the resistor and the capacitor. The period of a complete cycle is given by the equation below.
Therefore frequency of the oscillation is
Click here for the astable calculator
Astable multivibrator circuit
Though the two sides of the circuit are identical, one of the transistors will conduct before the other due some imbalance in the circuit. A transistor will be assumed to have come up first to describe this circuit.
Assume T2 will conduct before T1. At the instance of power up collector of T1 and C1 (volt on the other plate of C1, i.e base of T2 is 0.6V) rises quickly to towards the supplied voltage (Vcc), while base of T1 (C2) rises from initial zero volt towards 0.6V. Immediately the base volt of T1 rises to 0.6V, T1 starts to conduct forcing the base voltage of T2 (C1) to negative voltage which is (-Vcc + 0.6), hence T2 is 'Off'. Collector voltage of T2 rises quickly to Vcc while base voltage of T2 rises gradually in positive direction exponentially with time constant C1R2 towards Vcc. At the instance the base of T2 rises to 0.6V, it starts to conduct forcing the base voltage of T1 (C2) to negative voltage… and the process goes on and on until the supplied voltage is turned off.
Astable using 555 timer
Monostable using 555 timer
Astable using schmitt trigger
Astable using comparator