SPOT
THE MISTAKES!
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Electronics is such a big topic. 
I have produced lots of articles to try and cover the subject and maybe covered  1%.
Text books are filled with useless circuits and mathematical questions that I have never come across in 50 years of designing modules.
All the things you want to know are missing from text books. That's because the writer has never designed a circuit in his life or fixed any faults or problems.
The same with University Professors. Students come out of a $70,000   5-year course and never soldered a resistor!
One University student applied for a job at my business and said "Oh, I don't do any soldering."
Remember this: When you come out of your 5-year course, the interviewer will ask you for your experience . . . you have  none.
That's why you have to start NOW and have electronics as your hobby and take projects to the interview to distract the interviewer from asking awkward questions.   
NIGHT LIGHT
Here is a circuit from the web, from an "engineer" who doesn't understand circuit design. He claims the "circuit works" and that makes the circuit a good design.
But if you look at it carefully, the circuit takes the same current when operating as when it is resting. This is a waste of battery-current. And the circuit takes DOUBLE the necessary current.

The circuit above takes 9mA all the time via the middle 1k resistor and the LED takes 8mA when it is illuminated.

The improved circuit takes 2 microamp when light falls on the Light Dependent Resistor and the first transistor is turned OFF. This makes it "disappear" from the circuit and all we have is two 10k resistors connected between the base and emitter of the PNP transistor and these turn the transistor OFF. So nothing else in the circuit is taking any current. A huge saving on current.


COMMON EMITTER AMPLIFIER
YouTube is filled with instructional videos explaining the Common-Emitter amplifier and similar related circuits.
Most of these demonstrations are quite impractical, as they use component-values that I have never seen in reality and none that I would recommend.
But more important, you have to "look outside the box" and understand that you cannot design a circuit without knowing the characteristics of the input signal and the impedance of the stage that is connected to the output.
You cannot specify a signal that comes from a sinewave generator as it is completely different to that found in a normal circuit.
In fact there is nothing you can specify or predict, as the results will be completely different to reality.
It is absolutely pointless going into the fine detail of including base-emitter voltage drop and using a value of 100 for the gain of a transistor as you are finishing up with a "dream" value for the stage.
Even learning the fundamentals of knowing how to find the gain of a stage is an unnecessary exercise as it will bear no resemblance to the actual value. 
I have never used a mathematical calculation in my life as I know it produces NOTHING.
University students are being led "up the garden path" with questions and computations that are just a waste of time.
But a University cannot charge $70,000 and tell the student to build the circuit and measure the results. So they have to lull the student into  false sense of achievement by complicating the issue with mathematics. It's a SCAM. And after the COVID lockdown, a lot of the University myths have been uncovered and debunked. Even Elon Musk has mentioned the ineffectiveness of University teachings. In some areas they are effective as no student has the technical equipment to carry out medical operations, but in many another areas he has said the only way to learn is on-the-job.
And the fastest way to learn is called REVERSE LEARNING. A medical student learns the fastest when he attend a dying heart patient and sees how the operation is carried out. An electronics person learns more by diagnosing a faulty device and seeing how to locate the fault.
 

To make a long story short . . .  In 50 years I have never used any of the internet mathematics to design a circuit. It is not relevant and does not apply. It is remote from what is needed and does not take into account external influences. It is a complete waste of time putting pen-to-paper. But telling the "Professors" is a waste of time.  Nothing has changed. I sent my concepts to all schools and colleges in Australia, over 40 years ago and got no reaction. I explained that the only way to learn electronics was through building projects.  Not one school or University bought a single kit in 40 years. The reason  . . . .  when the project did not work . . . because of student incompetence . . . . the instructor did not have the skills to fix it in front of the student.  It's as simple as that.  The blind leading the blind.   Talk is CHEAP.  Blabbering in front of  blackboard is easy.  Proving your blabberings is difficult.

CHINESE SOLDER
Don't buy Chinese Solder.  I have bought 10 different types from China and none of them worked. For some, I had to increase the temperature and the solder still did not stick. For others, the solder poisoned the tip of the iron and I had to remove all the solder and apply Australian solder.
I don't know what is wrong is wrong with the Chinese solder as some was 63/37 and is supposed to be low melting solder. None of the solder gave a nice shiny result and I have now stopped my experimentation. It was  just a waste of time and money.
The only way I could use-up the solder was to twist the rubbish with my good solder and melt the two at the same time and my good solder would make a result that I could use.

2 LAMPS

Here is a question from the web.  No-one got the answer correct and no-one understood how to approach the problem.
For a start, you have to know the characteristics of a filament globe. The resistance of a globe is about one-sixth when it is cold and it increases six time when it is fully illuminated.
This means it takes 6 times more current to get it to start to glow. If you cannot supply this current, it will never start to glow.
We are not going into any mathematics because we don't have to.
The 25 watt lamp will only allow a small current to flow in the circuit and this will be less than 25% of the current needed to get the 100 watt to start to glow. Thus it will never start to glow and its resistance will always remain low.
Now we look at the 25 watt globe. The supply is 200v and it is a 230v globe. So the supply is on the low side.
Now we look at the effect of the 100watt globe.
It has a resistance of about 25% of the 25watt lamp so the initial voltages across the lamps will be divided into 5 parts with 4 parts across the 25watt lamp and one part across the 100watt lamp. This puts 160v across the 25watt lamp.
We now have to find out if the lamp will start to illuminate. Only a demonstration will show this.
After it starts, the resistance of the 25watt lamp will increase, whereas the resistance of the 100watt lamp will remain the same and thus the 25watt lamp will see a higher voltage and maybe it will glow brighter.

ELECTROLYTICS
Here is a very simple circuit called a Capacitor Discharge Unit.
When the switch is in the down position, the 2,200u electrolytic is charged and the solenoid pulls the core into the solenoid. This is because the core is a magnet. When the switch is in the up position, the core is pushed out of the solenoid.  This is because the current flows in the opposite direction.

You would think the strength of the activation is the same in both directions, but when the switch moves into the up direction, the 2,200 is directly across the solenoid. And the full voltage of the electrolytic is passed to the solenoid.
However when the switch is down, the 2,200 is charged via the power rail and since the initial current is very high, the power rail will not be able to deliver this high current and the voltage will dip.
To reduce this dip, two 1,000u electros have been added, but we come to a technical situation where a 2,200u electrolytic is taking energy from 2,000u storage and if the 2,200u electro has zero voltage across it and the other has say 24v, the instantaneous initial voltage will be just 12v across each.   That's why the impact in one direction is not as strong as the other.

PCB

What a terrible Printed Circuit Board artwork to put in a magazine.  Many of the lands are touching nearby tracks and you could not make the board without creating lots of mistakes.
The secret is to firstly keep all components away from other items and when you are going to place a track through the pins of an IC socket, the width of the track is reduced.
In addition, the value of the components must be included on the overlay as you might be making the project 2 years after the board is manufactured and you will never remember the values.
All my 1,000 PC boards are fully labelled so they can be assembled years later without having to reference to any other information.
This is just a sign of professionalism. Something I have not seen in any magazine, website project or any product on the web. Even the indicator LEDs needs to be identified so you know how to read the output of the project.
I was the first person in the world to put good quality Printed Circuit Boards on the cover of my magazine when the other magazines had no overlay, no tinned lands and no solder mask. Just a paper-bakelite piece of rubbish.
These are things you need to know to turn you into a quality engineer and real expert.
The board above is not labelled, I don't know what it does. 
I went to an electronics shop 50 years ago and they had hundreds of unlabelled boards.  It is no wonder none of them were sold.  They were all just junk.

TRANSISTOR INPUT IMPEDANCE
The circuit shows a BC547 transistor with base and collector resistors.  
A BC547 transistor identifies as a common NPN transistor with no special features and almost any NPN transistor can be used.
Both resistors have very high values to show the effect of this demonstration.
We are commonly told the input impedance of  a common-emitter stage is 2k, 5k, 10k or some other low value.
But the input impedance varies enormously according to the collector current.
When the collector current is very low, the base will respond to an extremely small current, in the order of a microamp or less. In the circuit shown, the transistor turns ON fully with a 10 meg base resistor. This is one microamp base current.
This means you have no idea of the sensitivity of a stage and when you are detecting very small currents. The input (wires) will have a high impedance and this will make them susceptible to interference such as hum and spikes. You will need suppression-capacitors to reduce this noise. 
In summary, we are saying a transistor has a very high sensitivity (very high input impedance) when detecting microscopic input currents and this impedance will fall dramatically to 2k or less when the transistor is operating at maximum collector current.
In other words, a microscopic base current will not drive a LED or motor or globe or any other high current component in the collector circuit. The collector load can only take about 200 times more current than you are delivering to the base.

You can increase the input impedance by connecting one transistor on top of another. This is called cascading.
This will increase the input impedance by a factor of at least 200. And when you add another transistor the impedance is increased by another 200 times.
This produces an enormously sensitive circuit and the diagram on the left will detect static electricity.


 

555 IC MISTAKE

The whole idea of drawing a circuit is to present the components in the simplest way possible.
The original circuit has been improved but the placement of the 555 pins is not ideal. However the major mistake is connecting a 470u directly to pin7. This pin has a low-current transistor inside the IC that "shorts-to-ground" and the energy in a 470u can blow-up the transistor.

ELECTRONICS MAKER MAGAZINE 

India's second largest electronics magazine has ceased providing construction projects in the magazine.
Why do they keep the title ELECTRONICS MAKER if there is no "maker" in the magazine.
A few years ago I provided them with articles for 18 months and did not get one single reply from any of the 40,000 subscribers. That's not-one-in-720,000 views.
India is just fooling itself with slogans like: "Make in India."
I don't know where the fault lies, but the University lecturers on the web have very little understanding of basic electronics and what is needed to be taught.  They just fill the whiteboard with mathematics and equations and come with up with answers that are far more accurate than the data !!!
I have never seen them explain how a circuit works, and certainly no mention of how to test it. 
And now the magazine does not have any articles for the beginner or the constructor.
They are just allowing China to take over the field.  And they are. China is designing many electronics items and they show enormous skill in programming and buying microcontrollers that are only available to the Chinese market, for a price that is one-tenth of our purchase price.
The Western World needs electronics engineers to design products because the Chinese do not know what we want or how to design things aesthetically. 
Unfortunately everything is going "down-hill" and China is rising.
However the recent downturn in China-economics has seen millions laid-off and more than 10 million apartments either abandoned or unfinished with millions of purchasers unable to reside in their unfinished dwellings.
And this is only the beginning of the collapse.

UNDERSTANDING THE 555 IC
The circuit shows an IR transistor connected to pin 2 of a 555 IC. Technically the 555 will trigger when the voltage on pin2 is lower than 33% of the supply voltage.
But there is no resistor supplying a voltage to pin2. The 555 will react when the voltage on pin2  is lower than 33% of rail voltage as there are some resistors inside the chip that cause the chip to start-up in a "set-mode" but you cannot use them in your designs.
When the Infrared light is turned off, the chip requires a HIGH on pin6 to change state but the voltage on pin2 must be above 66% of rail voltage to this to occur, as pin2 controls the action over the voltage on pin6.

The solution is to place a high value resistor between pin2 and the supply rail.

The IR transistor creates a voltage divider with this resistor and we call this a HIGH IMPEDANCE or LOW IMPEDANCE SET-UP, depending on the value of the resistor.
If the resistor is 10k, we can say it is low-impedance, because this is a low-value for this type of set-up.
It will require a high level of IR light to change the state of the 555.
On the other hand, a 100k will produce a high-impedance input and the 555 will respond to a very low level of illumination. This is called sensitivity. The following circuit shows a sensitivity pot with a 10k "stop resistor" to prevent damaging the transistor. 
Pin6 needs to be connected to a resistor or switch to complete the circuit.

COMMON EMITTER STAGE

Too much time and effort has been put into describing and analysing the Common Emitter Amplifier.
The facts are these: The gain of a transistor can be anywhere from 150 to 450 and it is a waste of time producing a mathematical result for the gain of the stage.
The gain will depend on the impedance of the stage connected to the output and if this is unknown, nothing can be calculated.
However the mistake in the circuit above is the biasing.
You cannot connect the base to the supply rail. If you replace Rb with an adjustable resistor, you will find the collector voltage is equal to the supply voltage and as you decrease the resistance, it will immediately fall to nearly zero.
You have no opportunity to set the collector voltage to mid-rail and thus the output voltage will be "clipped" or "bottomed."
In other words it will be distorted and this type of circuit is useless for a demonstration. In fact it is useless for anything.


 
TIMER
We have covered lots of faulty circuits from Mr Mohan Kumar as he does not test any of his designs and most of them do not work.
The main problem with this circuit is the 10k resistor on the base of the T2.  This will allow very little current to flow and depending on the resistance of the relay it may not "pull-in." The resistor should be 1k to 2k2.



 


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