Know your Stuff : Elevators

Ever wondered how small things in our lives go unnoticed at times.

In today’s world of skyscrapers, crossing a hundred floors in a matter of few minutes is no big deal, thanks to the – Elevators! But no, we will not be talking about how an elevator works as it is primarily guided by principles that are better understood to the mechanical engineers. The whole set-up is rather complex with many control systems and processors forming the core of the elevator scheme. In this discussion, we will deal with the role of electronics in the modern-day elevator system – the display control.

(Image courtesy : )

For simplicity, we will only be dealing with the fundamental circuitry or concept underlying the displays employed. The actual connections or circuits may-be larger and heavily complicated. To give you an idea, here is a block diagram you can identify with.

Elevator Block Diagram

What exactly are we talking about?

Here we will dig into the details behind the displays that are synchronized with the lift movement, more precisely, how does the display show “1” when we are currently at the first floor and so forth.

(Image courtesy : )

Coming back to the block diagram, let us first define what a keypad matrix is. You can consider it to be the same as the buttons you see in the elevator: 0-1-2-3-4-5-6-7-8-9 and so on. When you press on a particular number, say “1”, a signal is sent to the processing unit that identifies the input as “1” and accordingly generates the corresponding output. So this is how we take the input from the user.

The next part is to process this unit in a form that is better understood by the processor. For this we employ an encoder that converts “1” into “0001” (binary) and sends it to the next block for further processing.

After the processing has been done, it is time to display the data to the user for which a device called the seven segment display or SSD is used. Now, it is the same device that displays time in a digital clock but sadly it cannot be directly connected to the core circuitry. The SSD contains a set of seven LEDs, namely a-b-c-d-e-f-g-h that light up in accordance to the input to show 0-1-2-3-4-5-6-7-8-9. Have a look:

(Image courtesy : )

For this the connections are made via a BCD-to-seven segment decoder that decodes the input into a format that is understood by the SSD. The format of the data output coming from the counter will be in the form of binary coded decimal or BCD. IC 7447 is most commonly used for this decoding purpose. It accepts BCD and subsequently assigns a logic 1 to the LEDs that should be glowing in order to display that particular BCD, for example, if the input BCD is “0110” the decoder output will be a=1,b=0,c=1,d=1,e=1,f=1,g=1. This enables the SSD to deduce that the number 6 is to be displayed. This is how all the digits from 0 to 9 can be displayed using a single SSD.

Coming back to the processing block, it can be explained with the help of a situational example:

Suppose you enter a multi-storey building and wish to go to the 3rd floor. Your first action will be to press the button at the ground floor and wait for the lift to come down if the lift is not at the ground floor initially. Once you enter the lift, you will press the button “3” and read the display that shows: 0-1-2-3, and voila! After the lift has dropped you at the 3rd floor, it will once again go back to the ground floor i.e. now the display will be something like this: 3-2-1-0. So, how does this counting take place? It’s simple, it uses a counter. Are you asking yourself how could it be that easy? Have a look at the circuit we designed ( Click on the circuit to open it in DoCircuits ):

Elevator Control Circuit

Elevator Control Circuit

 The circuit uses three ICs namely: 7485 which is a 4-bit magnitude comparator, 74190 which is a up-down decade counter and 7447 which is a BCD to seven-segment decoder IC. The interconnections made within the circuit would illustrate the working of the display system. First, the input given by the user (after binary encoding) is given as input to the decade counter from the parallel input pins P0,P1,P2,P3. The parallel load pin PL is active low and therefore is given a logic ‘0’. The output from the decade counter is passed to the magnitude comparator which is compared with the input from the P0,P1,P2,P3 pins. The comparator has three outputs viz: greater than, lesser than, equal to. The comparison may be any of these three possibilities and therefore it is essential to decide whether to keep counting up or start counting down based on the comparator output. This decision circuitry is designed as a combinational circuit with the help of logic gates. As can be seen in the circuit, the two outputs from the logic circuit are CE and UD. These are respectively connected to the CE and U/D pin of the counter IC. The CE pin is active low and is called count enable, therefore the counter counts every time this pin receives a logic ‘0’. The U/D pin dictates whether the counter is in up-counter (logic ‘0’ at U/D pin) or down-counter (logic ‘1’ at U/D pin) mode respectively. Finally, the output at each stage from the counter is given to the decoder IC that keeps displaying the corresponding floor number in the seven segment display.

Carrying on with our example, the user input here is “3” (for third floor) or “0011” (binary output from the encoder). When this “0011” is fed into this counter IC as shown, the ordinary decade counter will not count up to 9 anymore, instead it will count up to 3. The initial floor is ‘0’ and it moves up to the 3rd floor. Then we have simulated a user input of ‘1’ – 0001 – at the parallel load input. This results in the display now going to ‘1’ akin to the movement of the elevator to a lower floor from the upper floor.

Initially ( Click on circuit to load it )
Initial Condition Elevator Circuit
During Up-count
During Down count

Therefore the lift comes back to its ground state after every cycle unless a trigger is applied to call the elevator to some other floor. Not as easily done as said, but certainly the fundamentals remain the same.

It is an irrefutable fact that some simple electronics runs at the back-end of almost everything that comprises today’s modern world, be it the metro that displays station name and other vital information, your trendy digital watches, the cool display with stock market updates on Wall Street, or of course the elevators!

See for yourself! Try the new additions to the DoCircuits panel—specialized ICs: 74190 up-down decade counter, 7485 4-bit comparator, 7447 decoder and the Seven Segment Display.


DIY – Blinking lights for XMas

Ditch the Walmart lights this time and build your own to make your home the most wonderful during Xmas. Did you think you could have made a better job with the help of electronics. This XMas let us construct our own system of lighting by a very simple yet and effective way. This implies a very simple concept of digital electronics, light control module is made up of a simple “Serial input parallel output Shift register” and a multiplexer for mode selection.

blinking lights DoCircuits

The key components of the system are Multiplexer and a serial input parallel output Shift register as shown in block diagram.

Blicking Light Controller Block Diagram

1. Digital input : For this we use a register which has in it stored a certain sequence depending on the order in which the blinking should occur.

2. Multiplexer for mode selection-A MUX is employed in the circuit for the mode selection process. The input to the MUX is the different digital sequences according to our need. Depending on the input to select line, the corresponding mode gets selected and accordingly light blinks. The input to the select line would be given via a mechanical switch.

3.Serial Input parallel output -shift registers- Four D flip flops are placed end to end so that the output of the first goes as the input to the second and so on and the clock of every D flip-flop is provided with the an input as shown to form a  Serial Input parallel output -shift register as shown in figure ( click on the figure to load the circuit ).

Hence the output is delayed with every flip flop that it encounters. Thus a pattern of blinks is obtained. The output is shown as follows.

Light Output

The output of each stage as shown in output wave form is shifted at every stage as D flip flop provides delay at every stage. These outputs are provided as inputs to different colors of lights and the lights blink according to the digital output at every stage. We can control the duration of blink by adjusting the clock frequency and bit period at input.

For mode selection we will use multiplexer to switch from one digital input to next input. Here we have used a  4*1 as shown in figure ( Click on the figure to load the circuit ).

Xmas Blinking Lights Controller Circuit

Shown below is the output of one of the modes of the light blinking selected by the multiplexer.

Blinking Output

Merry Xmas to you and your family !


Wireless Phone Charger

What is that one thing that has to get in your way to help you charge your phone? The wires. These wires sometimes make it inconvenient to carry your phone while on charging. But now with a small amount of modification a portable wireless charger can be constructed thus making your life hassle-free. Wireless charging is also referred to as inductive charging because it employs the use of magnetic field to transfer energy. This transfer is usually from a charging station, henceforth referred to as a supply, to a rechargeable and moveable device.

wireless charger

The basic phenomenon behind the wireless charging is induction. An inductive charger uses a coil to form an alternating magnetic field in the primary winding which induces a field in the secondary coil (which is inside your cell phone) through the process of mutual induction once these two coils are placed close to each other. Hence, power is transferred from supply to mobile device via magnetic induction. The whole system can be divided into following subsystem we will discuss them all.

Wireless charger system level

1. Power bank: Power bank is simply a DC source. It can either be your Car’s Battery or any other DC Source. Here we have used 12V DC supply.

2. The colpitts oscillator: The Colpitts oscillator is used here to generate the Sinusoidal Current from applied DC. Its circuit consists of a single stage inverting amplifier and an L-C phase shift network ( Click on the circuit to load the circuit on DoCircuits )

colpitts oscillator

When we apply a 12V collector supply voltage, the capacitors C3 and C4 are charged. These capacitors C3 and C4 discharge through the coil L, setting up os­cillations of frequency given as:

f = 1 / 2∏√[1/LC3 + 1/LC4]

The oscillations developed across capacitor C4 are then applied as feedback to the base-emitter junction and it appears in the amplified form in the collector circuit. This amplified output is then supplied to the tank circuit In order to meet the losses and it will give sustained oscillations. The wave form is as shown below:

oscillator output

2. Amplifier: Here we have used push pull amplifier because of its ability to work in limited power supply. A push pull amplifier has an output stage that can drive a current in either direction through the load. The output stage of a push-pull amplifier comprises of two identical BJTs of which one sources current through the load while the other one sinks the current from the load. The whole arrangement is shown in figure below: ( Click on the image to load the circuit )

Amplifier Charger

The output of oscillator stage is fed to the push pull amplifier and is split into two identical signals with phase difference of 180 degrees using the center-tapped transformer as shown in figure below

Amplifier Output

These signals are then applied to the two identical BJTs whose emitter terminals are connected. Thus amplified signal is then transferred to the receiving side by the use of current carrying wire loop of transformer. The output of this stage is shown in the figure below:

Receiver Stage input

3. Receiver: The receiver consists of a half wave rectifier, RC filter and a constant voltage regulator PVR100AZ_B5V0 IC that will provide a constant Output voltage of 5 volts.

The first stage that is a half wave rectifier rectifies the received wave form which will then pass through the RC network that will act as a filter and will remove the ripples

Receiver Output

Now the output of RC filter is fed to the constant output voltage IC that will provide a constant output voltage of 5 volts and several hundred milli-amperes which will be sufficient to charge your cell phone, music players, tablets etc. The final out waveform is shown below.

DC Output


Introducing in a lead role : Multiplier AD633

Hi folks! This week we have introduced the first in a series of analog ICs – the AD633 multiplier IC. This is a functionally complete, four-quadrant, analog multiplier. Four quadrant means that both operands that are multiplied can take any polarity i.e. +/- and hence multiplication can happen across 4 quadrants. It includes high impedance, differential X and Y inputs, and a high impedance summing input (Z). Thus this multiplier basically does a MAC operation (Multiply and Accumulate). The AD633 is well suited for such applications as modulation and demodulation, automatic gain control, power measurement, voltage-controlled amplifiers, and frequency doublers. The input range of operating voltages for this IC is from +15 V to -15 V. So while designing circuits with AD633 keep in mind that the inputs do not exceed this limit. Typically this IC provides a bandwidth of 1  MHz. [1]

Let’s check out a few applications using AD633 on DoCircuits. First up, given below is an amplitude modulator circuit. ( Click on the circuit to load it on DoCircuits)

amplitude modulation using AD633

Amplitude Modulation using AD633

The carrier and modulation inputs to the AD633 are multiplied to produce a double sideband signal. The carrier signal is fed forward to the Z input of the AD633 where it is summed with the double sideband signal to produce a double sideband with the carrier output. Here is how the AM signal generated looks like:

amplitude modulation using AD633 output

Amplitude Modulation using AD633 output

Another very simple application using the AD633 is the voltage controlled low pass filter. The cutoff frequency is modulated by EC, the control input. ( Click on the circuit to load it on DoCircuits )

Low Pass filter using AD633

Low Pass filter using AD633

Using the above circuit we get the following output:

Low Pass Filter using AD633 Output

Low Pass Filter using AD633 Output

Now to show how you can control this low-pass filter we can sweep the control voltage (Vdc) from 0.01 V to 0.02 V (this is done by selecting Frequency Domain Analysis and enabling the sweep settings. Then vary the Vdc values as given) and plot the frequency response as shown:

low pass filter using AD633 output sweep

Low Pass Filter Using AD633 output sweep

So where can we find such a low-pass filter? In some popular electronic music styles, “filter sweeps” have become a common effect. These sweeps are created by varying the cutoff frequency of the VCF (sometimes very slowly) [2]



Rock and Roll- and mix it…. Part I

Glimmering lights, banging heads, screaming crowd, jumping fans, enthusiastic star performers, and above all the blasting sounds; you might want to think that the vocalist is setting the stage on fire, but no, actually it is the inconspicuous someone who sits silently with a large board containing hundreds of taps and switches, mixing sounds coming from several inputs from the stage, so that only the best of it reaches the audience.

Now, what happens in a sound system? One may find this block diagram quite familiar:

The sound input produced by either humans or instruments in this case is first taken through a microphone that is a transducer, i.e. it converts sound energy into electric energy for the internal processing, after which the electric energy is again converted back into sound energy through the speakers (also transducer).

Here we will broadly discuss about two important aspects in this processing system viz. the audio mixing and the audio amplification. Apart from these, the system may also contain other processes like the music recording or playback but for our discussion we will try to understand one by one, the electronics behind the working of an audio mixer and an audio amplifier which are indispensable for live performances and concerts. However, many more intricacies are involved when we talk of any real life application of any circuit. Therefore the following explanation is based on certain assumptions and requires other software/hardware aids for complete functionality.

Audio Mixer

As clear from the name itself, the function of an audio mixer is to mix or combine multiple sounds which are taken as inputs from its multiple channels and send them to single (or multiple) channels. This is done to adjust or add the volume levels, tuning, frequency matches, placement, reverb, equalization and other dynamics of the incoming sound signal.

In professional terms, this device is also known as the sound board or mixing console that guarantees that only the best sounds reach to the audience. Beyond doubt, the operator (better referred as the sound engineer) of such a device has a real tough job and it requires a great deal of patience to adjust all the inputs, and observe every detail so as to get the maximum out of mixing.

Now, one might be surprised to see that how only transistors and potentiometers can be used for the purpose of audio mixing. Here is how we do it at DoCircuits ( click here to load the circuit or click on the circuit below ):

audio mixer circuit

For simplicity, there are two input channels to represent sound signals that may be coming from a drum, guitar, keyboard, vocals, or may-be a pre-recorded sound track. The potentiometers are used to adjust the volume levels for each of the input. The next stage is an amplification stage using a transistor. The output is taken from the oscillator for this simulation as:

Just to make the mixing process clearer, see the simulation result for two substantially different frequencies:

For inputs: 500 Hz, 10 KHz


For inputs: 500 Hz, 20 KHz

It can be easily understood that more matched the input frequencies are, we get a uniform sound pattern. One may try to change the inputs and simulate the given circuit to observe the changing outputs of the audio mixer. Surprising isn’t it, how the basic electronics involves and evolves itself around us, as simple and elementary blocks. Some audio mixers contain inbuilt preamplifiers that are used to amplify the incoming sound. Here comes the requirement of amplification as the microphone processed signal is very small to be passed directly to the loudspeakers. In our next article, we discuss the fundamentals behind the next stage i.e. the audio amplifiers.

Till then, keep reading and keep yourself open to electronic fascinations around you.

System Modeling using DoCircuits

The other day my friend and I were supposed to design an AM modulator – an ideal one – that can work out for practically any carrier and message (modulating signal) signal. One way to do it would be to design an AM circuit the traditional way using BJT et al. Well what’s wrong with it? One is the accuracy. We won’t get an accurate representation of the AM wave. It will depend on so many other things like the BJT biasing, its properties etc. And also we would have had to take the trouble to design the circuit! (Ouch!) So we decided to model the circuit using ideal components like the multiplier (after all the AM operation itself is a function in multiplication).

So anyone can recall the equation for an AM operation? We did our research and ended up with this:

y(t) = [1 + m(t)].c(t)

= A[1 + Mcos(2πfm t]sin(2πfc t)

M= modulation index

fm= modulation frequency

fc= carrier frequency

A= amplitude of carrier

This equation is used for a sine carrier and sine modulating wave. Let’s see how by modelling this equation and making a few changes we can model an AM waveform for a sine wave modulating signal and square carrier signal. If I were to directly implement the above equation using ideal components it will look like this: ( Circuit Here )

AM Waveform

The carrier is supposed to be a square wave of amplitude 6 V and frequency 10 kHz. From this frequency the square wave TL and TH are set as 0.05 ms. Since for the square wave source U value is a peak-to-peak amplitude, we add an offset of -6 V to get the amplitude from +6 to -6. From the equation it is known that an offset of A should be added to the modulating wave. “A” here is the amplitude of the carrier wave. Thus the sine wave modulating wave is given an offset of 6 V. The amplitude of the sine wave is determined by the modulation index. Say I want a modulation index of 50%.  Thus the amplitude of the sine modulating wave is set as 3 V with a frequency of 1 kHz. These two signals are multiplied with the help of a multiplier.

There you go, we have modelled an AM system from the equation for AM. Whether we are successful is yet to be seen. Simulating the above circuit gives the following output:

AM Waveform Output


We seem to have succeeded but let’s verify if the modulation index matches if calculated from the plot.

M= (54-18)/(54+18)  – modulation index is given by (Vmax-Vmin)/(Vmax+Vmin)

After calculation the modulation index is found to be 36/72 = 0.5 or 50%. Thus our method is pretty accurate.

This example is just one of many types of systems that can be modelled in DoCircuits. Of course you can extend the above method to all the different types of signals like sine, triangular, square etc. both as carrier and modulating wave. But what is the idea behind it all? It all started by getting the equation for a system and trying to implement it by modelling it in DoCircuits. Basic operations like addition, subtraction, division and multiplication are available in the ideal components panel and can be modelled very easy at present and further operations will be added in the future.


Red, Yellow, Green….Go !

Hi folks! We’re back this week for another interesting circuit application. This week let’s try our hand at a simple digital application which has one of the most widespread uses all over the world. Yes, I’m referring to the ubiquitous traffic light!

traffic light controller

It’s a silent part of everyone’s lives doing its part in maintaining order on the road throughout the world. The traffic light system is one of the systems which over the years have improved ever so slightly. It still has – and will – its three lights, red, yellow and green. Each light is lit for a certain period of time and in a certain sequence. The yellow light is lit in between the red and green. When the red is lit in one lane the green is lit in the other lane stopping traffic in one lane while allowing traffic in the other. What we are talking about is a ‘traffic light controller’. So let’s design a circuit keeping this simple logic in mind.

The image below shows the digital equivalent for the traffic light controller.

traffic light controller circuit

Click on the image to load the circuit or click here.

The above circuit can be used to signal the two sets of traffic lights – named R1,Y1,G1 and R2,Y2,G2 where the R, G and Y stand for red, green and yellow respectively. We have used logic probes in place of the lights. The D flip-flops are connected in series as a 10 bit ring counter. A clock signal is applied to each flip-flop which transfers a ‘1’ bit across each flip-flop. This results in initially G1 on and R2 on. As the bit shifts through the counter, this in turn shifts traffic lights G1 to R1 and R2 to G2. And the process continues. Thus when traffic in one lane is stopped by the lights the traffic in another lane is allowed to pass. Analyze the circuit according to the logic. The output plot is as shown below:

traffic light controller waveforms

But it would be nice to see the lights of your circuit in action right? Just after clicking on “Run” to simulate the circuit, come back to “Build” and on the circuit you can see the lights shifting in sequence.

But how will this traffic light controller be in real life? Although the lamps will be more powerful, the logic to run the system will be pretty much the same. A 555 timer will be used to generate a clock signal of the required time delay. Then a decade counter IC 4017 (this has 10 bits and each bit goes high with a clock signal in a round-robin fashion) performs the function of the ring counter in this circuit. And a NOR gate IC would be used for the NOR gates.

Hope you had fun designing and learning about your own traffic light controller!

Cook up Your Own Function Generator

Hi folks! We’re back to discuss another circuit using DoCircuits. This week we will see an interesting circuit which most design engineers, test engineers, hobbyists, lab technicians and students can relate to. Yes, let’s build a simple function generator. It won’t be a very high-end design but easy enough to be constructed by you in the lab – real or virtual!

Function Generator Block Diagram

Function Generator Block Diagram

So what is the principle behind this function generator ? It is made up of various parts which are all op-amp circuits. The first part of the circuit is an astable multivibrator. This will generate a square wave which will oscillate between positive and negative saturation. This square wave is passed on to an integrator. The integral of a constant say ‘c’ will be c*t where ‘t’ is time across which the integration is taken place. This means that a positive constant will give a positive ramp and a negative constant will integrate to a negative ramp. Adding them together we get a triangular wave. We got our square wave and triangular wave – if only there were a way to obtain a sine wave too from this setup. Well there is. What will happen if you integrate the above triangular wave? A triangle wave consists of positive and negative going ramps. A ramp is a function that increases linearly with time. If you integrate a ramp, you get a function that increases as the square of time which has the shape of a parabola. So the integral of a triangle wave is a series of positive and negative going parabolic shapes. In other words, yes you guessed it right you will get a pretty accurate sine wave. Alternatively I can approach this mathematically. We have to integrate the ramp c*t – which would result in c*t2/2. As you can see integration reduces the amplitude of the result. This can be adjusted by inserting an amplifier at the end.

Let’s take a look at the circuit:  ( Click on the image to load up the circuit on DoCircuits )

Function Generator Circuit

Function Generator Circuit

Starting from the left hand side, the first portion is an astable multivibrator, the output of which is a square wave. R0 is the feedback resistor and C0 is the timing capacitor. The frequency of this square wave can be varied by varying the RC values namely the R0 and C0 values. Note the initial value of capacitor C0. It is set to 1 V. In real life the oscillations will be started by the offset voltage inherent to op-amps which would charge the capacitor and in turn push the output to positive and negative saturation. But since we are using an ideal opamp this ‘irregularity’ is introduced by giving an initial voltage to the capacitor.

This square wave is applied to an integrator as shown which in turn converts the square wave to a triangle wave.

Further the triangular wave is integrated again through another integrator resulting in a sine wave. The output of this integrator is connected to an inverting amplifier with gain given by -R10/R9. Varying this gain you can control the amplitude of the sine wave.

Let’s see the output when you simulate the given circuit: ( Click here to load and run the circuit )

Function Generator Output

Function Generator Output

So without the help of any external input source using only the op-amp and the supply provided to it we have generated three standard waveforms. This is the principle of working of the basic function generator you find in your lab.


How stuff works : Your ever helpful cell phone charger

 In today’s mobile age, our cell phones keep us connected to everyone 24×7. Without mobiles, we’ll all be back to the Stone Age. But irrespective of their class, they all run on a battery which goes down at the end of the day. The smarter your phone, the sooner it runs out of charge.

Have you ever wondered about the one thing that keeps your phones going? Yes, we are talking about your cell phone chargers here. Cellphone chargers are nothing by simple AC to DC converters, i.e. they take the regular AC supply of 220 volts coming to our homes, and give a constant DC output voltage of around 5 V (approx). In this article, we are going to talk about the internals of a cellphone charger and even create a working circuit.

The cellphone charger extracts the power from the home supply (AC 220V) and converts it to a DC level of required voltage. The voltage output is fairly constant which means it is regulated. The output voltage remains constant whether the load current changes or there are fluctuations in the input AC voltage. This is achieved in a series of steps:

Step 1: Step down the high input of 220V to a working output voltage. This is achieved with the help of a transformer

Step 2: Convert AC signal into a DC signal using rectification

Step 3: Smoothen the output of the rectifier by filtering the ripples from DC rectification

Step 4: Generate a steady output signal with the help of a regulator

The circuit below gives a high level view of the working of a “regulated power supply”.

The components used are very common and simple. Most of you know what goes on inside of them.


The transformer contains two huge copper coils, one between the two terminals of the input power supply and other between the two terminals of the output. Here we use a step-down transformer which means it will convert high voltage to low voltage. The number of turns of the coil inside will determine the voltage supported at input and output both.

i.e. Vin/Vout=Nin/Nout

Vin = Input AC voltage

Vout = Output AC voltage

Nin = Number of turns at the input terminal of transformer

Nout = Number of turns at the output terminal of transformer


Now comes the rectifier part. This converts the AC voltage output of the transformer to a DC voltage. It just reverses the polarity of one half of the period of the AC signal. This will make both parts have the same polarity. Here we use a full wave bridge rectifier to convert the AC signal to DC.


The output from the rectification stage is DC, but hardly constant. So, we use capacitive filtering to smoothen the output. In this example, using a simple low pass filter at the output of the rectifier, however in real life, higher order filters may be used, which would give a much more smoother output.


The filtering significantly smoothens the output, but even after that small ripples remain. If we use this directly to charge our phones, the constant fluctuation in the voltage may damage the device. It is very important to have a steady output voltage with minimal fluctuations. This is where the regulator stage kicks in.

Here we have used a simple zener diode based regulator. The tendency of a zener diode is to have a fixed voltage between its two terminals when reversed biased. So when input voltage changes, the current through the zener diode also changes inversely so that the output is constant. This regulator is quite simple to create, but its is that it wastes a lot of power. So, the cell phone chargers typically use IC voltage regulators, such as IC 7805, IC 7806, IC 7812 etc.

Combining all the steps explained below, here’s a working circuit for “regulated power supply”. You can even go ahead and run it with DoCircuits and see it working for yourself !!

Try it on DoCircuits !

All the symbols have their usual meanings. Function generator is used as a power input source to the system. OSC1 and OSC2 are the CRO’s placed at the input and at the output terminals and can be used to study the changes between them.

So, hope you will now appreciate the small charging device at your home a little better, and if it breaks down, don’t hesitate to open it up and pry upon the internals. Even if you want some unique voltage supply, you can custom build it so easily now. But like any other technology, people are revolutionizing this as well. We’ll leave you with a glimpse of the same:

Please do share your comments and feedback

Memories : What’s in your computer hard disk ?

Some time back we saw the working of a register for saving data. This week let’s extend the concept a little further and talk about a 4-bit memory array. This memory array is made up of basic memory cells. Each memory cell can be used to store one bit of data, either a 1 or 0. This memory array circuitry is designed for the reading data from and writing data to the memory cells. The addressable unit of such an array is referred to as a word which is a collection of bits, each bit being stored in a cell. Thus a W x b memory has W number of words each word having b number of bits. The array is called as a random access memory in the sense that each memory location, (that is a word) has a unique wired-in addressing mechanism. As a result, corresponding to a given address of a word the bits of that word can be accessed randomly and the time to access any location is equal to the memory cycle time.

Consider the circuit as shown below.

The decoder is used for row select. For example to select the first row – or word – A0 and A1 are made 00, for the next row 01 and so on. Similarly A2 and A3 are used for selecting the columns while reading the data. As you can see these two lines are connected to select lines to the MUX0 which has inputs from all the columns. The data line as shown here is given to a demultiplexer which has the same select lines as the multiplexer described above. Again the select lines of this multiplexer choose the required column to which the data is to be written. So using this column select the data is written to or read from a certain column, but the particular cell is selected from the row select – the decoder.

Let’s go through a write cycle. The data line contains the particular bit to be stored – say ‘1’. The RW line should be 0 so that write is selected. Thus the data is given to the demux. The output lines of the demux are connected to the data lines of the flip-flops. But note that the data gets latched on when the clock goes high. This clock is provided by the row select lines. Once the particular row gets selected the data gets latched.

Now let’s see a read cycle. As shown the multiplexer is connected to the outputs of the flip-flops. When the RW line is made high – to ensure that the read function is enabled via the AND gate. Thus the Read signal reads the particular bit which is pointed out by the column select and row select bits.

Now since you know the working of this memory array why don’t you simulate the circuit to see how data is written and read from this memory? Click here to load this design.