Archive for the ‘What is a digital circuit?’ Category

Monostable, Astable and Bistable circuits

March 5, 2011 Leave a comment


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.


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.


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.

What is the difference between an analog circuit and a digital one?

February 24, 2011 Leave a comment

Basically, Analog signals operate on a  continuous range of  voltage levels, where as a digital circuit can be looked at as a string of data 0 and 1 (binary numbers) or off and on.  The key difference is that analog technologies register and use waveforms as they are, whereas digital technologies will convert the analog waveforms into a binary set of numbers.

Digital has only two states, on or off or 1 and 0 whereas analog is not going on for on or off it is in order of the actual level.

For instance an audio signal may range  anywhere from 0V to 1000 mV and an infinite number of places in between.  A digital circuit will operate on only one of it’s two states which are typically represented as 0 and 1 and these on/ off states can be represented by different voltages in different systems.  But any given digital signal will only have two distinct values.  A typical digital signal will dictate that anything below 1.0V its considered off and anything above 4.5 V is on .

Since most physical quantities such as velocity, temperature and audio will vary constantly, an analog circuit provides the best means of representing them.   However  digital circuits can often be preferred because of the ease in which their outputs can be manipulated by computers and they are considered more robust and less subject to transmission errors.   For example an audio signal that is represented digitally can be transmitted as a sequence of 1s and 0’s and can be reconstructed without error providing the noise picked up is not too great to prevent the identification of the 1’s and 0’s.  A more precise signal can be obtained by using these binary numbers to represent it. 

This information storage can be easier in digital circuits compared to analog.  The noise immunity of a digital system will allow the data to be stored and retrieved without degradation, whereas an analog circuit can be affected by things such as noise, age and wearing that can alternate the information it is trying to store.  As long as the noise is below a certain level, a digital circuit will be able to recover the information without error.

One disadvantage of a digital circuit is that it can use more energy than an analog circuit to complete the same task.  This in turn would produce more heat and so in the case of a portable or battery-powered unit such as a mobile phone the use of a digital system may be limited and they would instead use a low power analog circuit to amplify and tune the radio signals it receives.  Digital circuits can often be more expensive particularly in lower quantities.

In this graphic it appears to give the impression that an analog signal is better than the digital one due to its smooth continuous flow, however this is not necessarily the case.  By increasing the number and decreasing the size of the time divisions in the digital circuit it would be possible to make it nearly as smooth as the analog. 

What is a digital circuit?

February 22, 2011 Leave a comment

Before I learn how to design a digital circuit I need to get a greater understanding of what a digital circuit actually is:

A digital circuit is a circuit that functions on a number of different logic gates. The logic gates differentiate power signals. The power signals are then transferred to different parts of the digital circuit through other gates to create an output signal directly pertinent to the energy level at the moment of signal input.

Most digital circuits are comprised mainly of smaller analog components that, because of the logic gate occurrence, only operate within a certain frame of voltages. Usually these operate at extremely low voltage signals. Digital circuits also require that the analog components located throughout the circuit not be placed in a manner that will allow them to perform analog functions. This usually means there is a logic gate both before and after the analog component.

Well this seems like a straight forward answer to the question of what is a digital circuit,  however it poses numerous questions in it’s answer.  The first being what is a logic gate? And what are analog components?

The basis for this section is for me to become competent in basic digital circuit design and in this instance I need to strip a digital circuit back to basics to fully understand the components and processes involved before I begin to attempt to design them.  Please see the sections ‘logic gates’ and ‘components’.