Home > What is a digital circuit? > Monostable, Astable and Bistable circuits

Monostable, Astable and Bistable circuits

Monostable

The logical state of the output of a basic logic gate remains at logic 0 or logic 1 according to the logical states of their inputs.  As long as this remains constant the output state will also remain constant.  There are however a number of applications in which a momentary pulse (i.e a 0-1-0 or 1-0-1 transition) is required.  A device that fulfills this function is said to have only one stable state and is consequently known as a monostable.  In a monostable circuit the output starts off at logic 0 until a level arrives at its trigger point.  This level can be from 0 to 1 (positive edge trigger) or 1 to 0 (negative edge trigger) depending on the particular monostable device or configuration.  As soon as the trigger is received, the output of the monostable changes state to logic 1.  Then, after a time interval determined by an external timing component such as a 555 timer the output returns to logic 0 and then awaits the arrival of the next trigger. In simple terms monostable means that once the circuit is switched on it will time once and then stop. In order to start it again it must be switched on manually a second time thus having one (mono) stable state.

Astable

An astable circuit is one that is not in either state, it will continuously move from one state to another.  As one side charges up it will reach positive and switch on the other side and go negative itself.  If I set up an astable 555 timer circuit with an LED, the LED will continuously flash or pulse until the power is removed.

Bistable

A bistable circuit has two stable states and can also be used to store state information, this circuit is also known as a flip-flop.  The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs.  Simple bistable devices can be built using nothing more than cross-coupled NAND or NOR gates

coss coupled NAND gate

cross couple NOR gates

These arrangements have two inputs (set and reset) and two complimentary outputs (labelled Q and Q).  A logic 1 applied to the set input will cause the Q output to become or remain at logic 1 while a logic 0 applied to the reset input will cause Q to become or remain at logic 0.  In either case the bistable will remain in its set or reset state until an input is applied in such a sense as to change the state.

Simple NAND and NOR gate bistables suffer from a problem though because it is not possible to predict the output state which results from the simultaneous application of logic 1 to both the set and reset inputs which can cause a ‘dissallowed’ state.

In practice NAND and NOR gate bistables are not usually encountered as there are a variety of integrated circuit bistables available which can be more flexible and predictable in their operation.  The three common bistable types are:

RS (set and reset), D-type (data or delay) and JK, there is also a T ( toggle) type.

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