SPOT
THE MISTAKES!
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A recent visitor in an electronics forum asked: "Is electronics dying?"
We all know electronics is not dying because everything you hold in your hands is based on electronics.
But the desire to learn electronics - in the hope of making it a career - has changed enormously.
50 years ago you could become a technician - a radio technician, communications engineer, 2-way radio installer, repairer, CB radio stockist, radio station technician, car radio installer or repairman. You could design and manufacture amplifiers for music, guitar, schools, offices. Install 2-way radios for taxis. Service black and white TV's and colour TV's. 
The field and market was enormous.
Servicemen worked from 8 in the morning to 11 at night, 7 days a week. Everyone wanted amplifiers, TV's and sound systems.
Then the Japanese and Chinese came in with products that were cheaper than the cost of a repair.
And servicing went out the window.
Now, nothing gets serviced or repaired.
That's what changed the industry.    
In addition, all manufacturing went overseas.
What are we left with?
Just a few sales.
And this can be done by juniors, just out of school, with 2 weeks training.
You don't need anyone with an electronics background as nothing will be pulled-apart, fixed or serviced.
So, what about design?
This has gone overseas too.
Even though the Chinese had no mechanical or technical ability, some 50 years ago (you just have to look at their farm machinery and cars etc)  they sent their students overseas to acquire the latest technology in EVERY field and are now at the forefront of advanced designs.
Even though some of their designs are clumsy and faulty, they learn from their mistakes and after the third or fourth generation, their product is fully acceptable.
And now it has come to a point where you cannot compete. They have taken over and dominated the market in so many areas that the mechanical and electronics industries are their complete domain.
So, what do you do?
The only markets left are niche markets, that are small-return. Areas such as medical aids and disability aids. There are thousands of people that need devices to assist them in their daily life such as a simple multi-function robot that can be programmed to carry out one or more tasks.
Devices to help people eat, drink, wash, etc and be self-sufficient.
This is one area that has not yet been explored and is an ideal place to start. 

Talking Electronics has produced a number of pieces of test gear to help in designing circuits.
These include a transistor tester, LED tester, Logic Probe, pulser and a continuity tester.  
It would be wonderful to say we have sold lots of these kits, but the truth is: we haven't.
I designed these projects because I needed them and I use them all the time to get a project working.
The LED tester is the simplest and the best.
It tests all type of LEDs because many of the new LEDs are clear and only show the colour when powered.
It is also a continuity tester as the LED on the board illuminates when a low resistance is present.
And the Logic Probe is very useful too. It found a short-circuit in a new PCB that prevented the micro from starting-up. The Logic Probe proved a signal was not present on the oscillator output and the short-circuit was located.
Test gear does not solve all your problems and the most dangerous item to use is a CRO.
Unless you know what to look for, the trace on the screen will contain all sorts of junk and spikes that the circuit may or may not be responding to.
The trace can lead you in the wrong direction and "a little knowledge is a dangerous thing."
However there are many items of test equipment that are essential and that means a multimeter.  This is the first thing you should acquire. Even the smallest and cheapest can be used and you should have both an analogue meter as well as a digital meter because some circuits work better with one and not the other. Simply get both and try them to see what I mean.

Talking Electronics website concentrates on explaining the BASICS of electronics.
xxxxxxxxxxxxxxxxx
 Introduction to Electronic Engineering   By: Valery Vodovozov is an example of trying to explain the basics but using complex equations and failing to provide simple examples.
The book doesn't suit the advanced engineer and doesn't explain to the beginner.
There are lots of text books like this and that's why you should download a free .pdf of the book before thinking about purchasing a hard copy.
We get emails and calls every week from readers who have been with us for over 20 years and some are re-entering the field as a hobbyist in their retirement.
They say we have covered more and explained things better than any of the courses thy have undertaken and one of the most important sections is this section: SPOT THE MISTAKES.
It covers all those things that you cannot explain in a normal article and covers "real life" incidences in fault-finding and circuit development.
That's why we have continued to add to these pages and every time I open up a book on basic electronics, I find more things to add to the list.  

One of the brilliant ideas introduced some 30 years ago was the concept of a COMPUTER SWAP-MEET.
This introduced the software developers to the budding computer enthusiasts and started the revolution of sharing software for as little as $5.00 per disk.
It was basically called PUBLIC DOMAIN SOFTWARE and was designed for sharing and improvement by anyone interested in programming.
At the time the internet was in its infancy and speeds as slow as 300 baud was the only way to connect.
There was no Google and you had to type the address accurately to get connected.
What happened next?
Computer clones came on the market and the quality of software exploded.
But what I want to say is this.
At present there is no unifying "hub" or single location where all those interested in electronics can meet and/or receive the latest in technology.
There are many Forums and a number of websites run by manufacturers, magazines or individuals, but nothing for the beginner and experimenter.
"Make" and "Instructables" websites have some features but nothing is indexed or easily navigable. And some of the projects don't work or are poorly designed. 
That's why basic electronics is dying.
Very few magazines have simple projects. In fact I have not seen a simple project in a magazine for the past 12 months. And there are no magazines that back-up any of the projects with a kit of components. 
This makes it difficult, if not impossible, for a beginner to put a project together.
And things are getting worse.
Many magazines are in their last year of publication. Sales figures are falling and just like the death of the daily newspaper, the internet has taken over everything in the publishing world. Even women's magazines, health and all those mass-circulation titles have fallen to miserly levels due to the ease and efficiency of reading everything on the web.
The greatest success of the web is the hyperlink.  Every article and every page has a link to another page and you can wander through a dozen links on similar topics.
And the search-rate is phenomenal. A cat video with 18 million hits in 10 days shows the amount of activity on the web, looking for something to view.
Talking Electronics website has had over 22 million visitors.
Many of these have been invalid landings but the average for interested visitors has been over 7 minutes per page.
The internet reaches every country in the world and readers who could never afford to buy a magazine are now accessing everything for FREE.
Originally websites tried to sign up readers at a few dollars per month, but this was over-ridden by free sites and now everything is FREE.

INDIA
India is trying to rise in the world of technology and the only real thing that is happening in the country is the rise of wealth for many millions of people.
The increase in prosperity has come about from the employment of thousands of people in call centres, where they originally earned $20 per week. When you multiply the effect of thousands earning this money over a period of 20 years and the flow-on effect of money changing hands two times a week, you finish up with tens of thousands of people living a good life-style and feeding their whole family.
This simple beginning has produced and enormous number of middle-class workers and this has been re-enforced by the fact the the money could not be taken over-seas.
But all this has been generated by the simple fact that the Indians could speak English and "pretended" they were "just down the road" from the caller.  
In fact they were working AT NIGHT.
When the truth finally came out, there was an uproar with overseas foreign call centres and staff were gradually re-installed in the originating countries.
India produced a lot of new wealth but technology did not improve.
As far as technology is concerned, they relied on importing and copying from overseas. The main field has been supplying medicines after the patent has run out.
A small amount of electronics manufacture has been tried but failed as well as a number of IC's bearing their own part numbers.
Their electronics magazines try to put up a bold front and instead of saying "made in India" the articles are headed "Make in India."
I have provided articles for one of the Indian electronics magazines for the past 18 months and in this time I have not received one single email from a reader.
Not one person has requested a kit, a printed circuit board or any assistance.
For the past 4 months the magazine has failed to present any more projects and does not even have a single circuit for a hobbyist in the magazine.
Even the largest magazine in India has a number of projects in each issue and a list of comments at the end of the article.  The number of comments is ZERO !!
The magazine had an electronics forum up until 2 years ago and the comments were of such a simple nature that the magazine closed down the forum.
Yes, there has been a rise in the affluence of some of the people over the past 20 years, but you have to understand how this has come about. It has been produced by money flowing into the country for call-centre workers and not being allowed to exit the country.
A single $20 wage has generated 20 x 2 x 50 x 20 = $40,000 of purchasing power over the past 20 years. This is the enormous multiplying effect of money when it is not allowed to be removed from the country.
Now you can see how each worker has created millionaire bosses. And there are thousands of millionaires.
I am not against this or the millionaires, as they cannot eat their money. Their money goes to develop large projects such as housing, factories and employing more workers.
But you need to realise this is all "flimsy" improvement.
It is not technology-based. It is just "being able to speak English" and carry out a simple task.
With the rise in affluence from those who are working and earning dollars, families are buying solar systems for their houses and the country is extending its cell-phone and internet coverage as well as electricity generation. Some of this equipment and infrastructure is being manufactured in India, but most is imported.
For a country so intent on trying to portray "technological improvement" we don't see anything new on the scene as far as transport is concerned.
The train system has not changed or improved since the time when the English supplied locomotives, some 100 years ago and the most common mode of transport - the rickshaw - uses the polluting 2-stroke motor.
India has come a long way with the capital inflow and the aviation scene is one of enormous expansion.
But the electronics scene has not advanced.

What I am saying is this:
There is a lot of hype and presentation and flag-waving about the IT presence and performance in India but very little is actually occurring.
It takes a lot of money and know-how to create a product but most of all it takes infrastructure. Infrastructure is the ability to get someone to design something for you, someone to make it and someone to ship it to you  ON TIME.
China, Japan, Taiwan, Hong Kong all have this in place and that's why they can make things.
India does not have this co-ordination in-place and they can only pretend to be performing.
They don't have it in place because they don't need it.
They have never made anything before and it takes years to create this sort of infrastructure.
And now I come to the reality of the situation.
The level of understanding of electronics in India is very low.
You just have to look at the articles and projects submitted to magazines and the comments and feedback from readers.
It is virtually NIL. The magazine does not sell kits, does not provide the printed circuit board and does not have a technical section.
They have refused to accepts any of my projects for the past 3 years and taken no notice of the 35 major faults in their projects. 
There is absolutely no-one in their production-team other than "copy-and-paste" editors that bring the magazine out each month.
And now they have ceased to include even simple projects in the magazine.
Where are they heading?
Nowhere. Because they have not not produced anything in the first place.
The magazine was just "smoke and mirrors."
But let's not be too critical.
India, is after all, a third-world country.
The only thing that shaped India was the English, making Indians speak English as a first language, covering the country with a railway system. buying goods (cotton etc) and educating the population to a point where English-type schools are sort-after by everyone able to afford the fees.    

You have to look at everything realistically. I am not biased in any way. I don't have a "barrow to push." I just see things as they really are.
When you compare India with other 3rd world countries you see they are far above the rest. You have to provide a reason for this. When you see the conflict, corruption and poverty in other countries, you have to look for a root-cause reason.
The one thing that is raising the conditions of hundreds of millions of people is TECHNOLOGY.
Even though we complain about not doing enough, technology is providing food, shelter, electricity, medicine to millions and, while at the same time corruption is drawing-off 90% of the wealth that should be filtering down to the population.

 

 
 

COURSES
You have to be careful when thinking about enrolling in an electronics course and also when buying an electronics text book.
Many of these courses are far too complex and cover things that are not really needed.
Electronics has advanced over the years and building a circuit with gates and has been overtaken by a microcontroller.
Similarly, a lot of problem-solving with resistors in parallel and series can simply be determined by creating the circuit and measuring with a multimeter.
You have to dedicate your time to learning things that you will use.
Many of the chapters in text books are filled with things I have not used in 50 years.
I have not found a text book that isn't filled with 75% of things you will never use and the remainder is very poorly presented.
Very few books have practical examples, using actual circuits from the field.  
Most books are just words, words, words.
That's why I built this website.
It's an example of how a text book should be written.
 
 
THE OUTPUT

Here's a very simple way to work out the voltage you will get across the 32R load (32R speaker).
The first thing you have to understand is this: The energy entering the speaker is not via the transistor but via the 220R load resistor.
The output transistor merely turns ON and "empties" the 1u electrolytic.  If the 220R does not "fill" the electrolytic, the transistor will have nothing to remove. And the only time the electro is "filled" is when the load is not getting its full current from the first half of the cycle. In other words, the energy missed from the first half cycle is delivered during the second half-cycle. It is not as though the second half-cycle is delivering "extra energy," it is just delivering the remains of the first half-cycle.
 How many times has this been explained in a text book???   NEVER !!!!!
So, the LOAD RESISTOR is most important.
It must be a low value to deliver current to the speaker.
When the output transistor turns OFF, the 220R and 32R form a voltage divider and 13% of the 12v supply will be passed to the speaker. The 1u will rapidly charge and this "flow" will reduce during the first half of the cycle. So the maximum is just 13%. If the value is larger than 1u, the electro will fill up slower and more current will be passed to the speaker for a longer period of time.  
When the transistor turns ON, the energy stored in the capacitor will come from the previous half-cycle, (during the time when the energy is being delivered at less than 13%.)
So the overall delivery for the full cycle is less than 13%.
This is how you see a circuit "working" without any mathematics and without using any simulation software.

One "engineer" on the Forum replied:
As I understand the task, the amplifier should work in the quasi-linear range with a gain of 100.
Hence, the transistor is neither turned (switched) ON nor OFF.

This is not so.
Here is the answer:
The output transistor will be fully turned ON. The base bias resistors are trying to put 2v on the base and this is more than enough to turn the transistor ON. The base will rise no more than about 0.7v due to the output transistor being in a common-emitter configuration.
The 10k will deliver about 1mA to the base and if the gain of the transistor is 100, this will produce 100mA through the 220R load resistor. The voltage across the resistor will be 220 x 0.1 = 22v which is clearly more than the supply voltage, so the output transistor will be FULLY TURNED ON.

As far as the output stage turning OFF, you will have to do some complex calculations to determine if the 1u will turn the stage OFF and this is beyond our simple explanations.
   


WHAT'S WRONG WITH THESE:

CLAP SWITCH

Here is a kit from India for a CLAP SWITCH. 
The circuit is a common design and it works very well. But the components are placed on the board UP-SIDE-DOWN !!!!!
The positive rail is at the bottom of the board and no resistor values are identified.
I have learnt NOTHING from this kit.  How do you expect a beginner to learn anything ???
The kits costs $8.00 in India.

The kit has been stolen from a Chinese manufacturer:

The kit costs $2.00 from Hong Kong !!!!!!  

4 CHANNEL TO 2 OUTPUTS
A reader of a forum requested a circuit to take 4 outputs of a receiver and connect them to 2 inputs of his microcontroller.
Here's what an electronics engineer came up with:

It does not let you know when no input is present and is very complex in design.

Here is my circuit from a project: 4 Channel Remote Control. It is much simpler than the circuit above and can be expanded to 6 inputs.

Picbuster replied: 
Place a resistor over the 100nF. Mandatory avoid DC restoration over 100nf (avoiding an 'open/floating' GP0 input.)
To start with, the 100n across the input does not create a "floating input."
Picbuster  replied with more incorrect statements:
"The input is floating with only a cap connected to it."
"How to discharge cap after loading it via the diode? ( it remains high until discharged by mpu's input impedance."
An input is considered to be floating when it can pick up stray voltages and rise and fall very quickly and the micro detects values that it cannot deal with. When a 100n is connected to the input, it charges and discharges very slowly and does not charge via stray voltages or interference from electromagnetic waves.
The 100n capacitor is discharged by making the pin an OUTPUT and taking it LOW to quickly discharge the 100n.



TIMER
 

The original circuit is so messy that I could not work out how it worked. It has been re-drawn and now you can see the mistakes.  The 100k pot can be turned to zero ohms and when the switch is pressed, the transistor will be damaged. The SCR is IRF530n and its gate voltage is about 2v to 4v.   It may work at 3v.
The IRF530n is actually a POWER MOSFET and needs 2v to 4v to turn on so it is not really suitable for this circuit. The main point at the moment is to draw a circuit so you can how it works.
Once you see how it works, you can see the gate voltage of the SCR (or MOSFET) must be less than 2.5v for the device you are selecting as this is the maximum voltage available.

CLAP SWITCH - another Clap Switch !!

This is a badly designed circuit because the mic does not have a load resistor and it is effectively across the 6v to 9v supply.
Pin 2 is floating and this pin has a high impedance. The voltage on the pin is unknown.   

Wasim Khan from http://www.electricaltechnology.org emailed me to say he didn't have a clue what I was talking about, so here is the corrected circuit:

The electret mic needs an operating current of less than 1mA and the original circuit did not have a load resistor to limit the current.
Pin 2 in the original circuit is floating and although the 555 may work, pin 2 must be held above 33% of rail voltage so it does not activate the chip until required.

TRAFFIC LIGHTS

Another badly designed circuit from the same website.
The 2u2 has no polarity. The timing capacitor has no value. The 100k pot does not have the wiper connected. The green LED does not have a dropper resistor.  The sequence for the 4017  is 3  2  4  7 10  1  5  6  9  11.    Pin 12 is HIGH during the time when 3  2  4  7 are HIGH. The LED sequence is correct.

Here is a comment from a reader:

Hi Colin
I think it is fantastic that badly designed circuits made by others and put on the internet, are shown for their errors.
It is a great teaching source for those who want to learn about the in’s & outs of electronics, in such a practical & clear way.
Yes, it can also prevent one from building a circuit, only to find out that it does not work.
Keep up the great work and website. 

Regards
Craig Adkins
PC Hardware & Software Support

TOGGLE

Here's a SET / RESET circuit made with relays.
The LED is turned off by SHORTING ACROSS THE POWER SUPPLY !!!!

TSOP 1738
Here's another faulty design from Electronics For You May 2016. They never test the circuits. That's why most of them don't work.

There are hundreds of new and exciting chips and products being developed and reviewed on the web, but you have to be very careful about including anything in your designs.
Many of these new devices are not stocked by any supplier and sometimes you have to buy a whole roll (3,000 pieces) to get your order fulfilled. Take the PR4401 inverter chip to drive a white LED from 1.5v. 
It is worth 5 cents but costs 70 cents and the supplier shows a stock of 994 pieces aster 3 years !!!   He has sold 6 pieces !!
Most of these recently developed chips are too expensive and and will be deleted from inventory after a few years.
When designing a product, you have to think of a life-span of 20 years. Many of the items I produce are 25 years old and they can still be produced because I have used readily-available components.
All the kits using special parts have been deleted as the chips are no longer produced.
If you are not careful, this can cripple your business.
A new product or chip may look tempting but others will see the item too and if sales are less than expected, you will see the product on eBay for $1.00.  Just look at the Arduino modules. A $10.00 product is now $2.00!!  
Lots of these new chips will disappear  . . . . POOF!!!   
    

Here's another poorly-designed circuit from "Professor Mohan Kumar:"

The second transistor is not needed. If the first transistor is capable of delivering 30mA for the buzzer, it will also be able to illuminate the LED:

All these points, discussions, criticisms are referred to as "second-order understanding."
Second-order understanding is when you take a circuit and see if it can be simplified or improved so someone with greater understanding cannot criticise it.
You will never find this concept in any text book.
It's wonderful to churn out mathematical formulae but an equation will not design a circuit for you.
It's like a person buying a CRO to design a circuit by seeing the waveforms.
You have to know which components to change to get greater amplitude or higher frequency or shorter mark-space ratio.  You have to know what the waveform will look like before viewing it, otherwise you will be "tricked" by what you see.
The same with mathematical results.
You must write down what the result should be and see if the equation conforms your understanding.
The same applies to the circuit above.
If the first transistor can deliver 30mA to the buzzer, why not include the LED ???
It's simple "electronic understanding."

There is a constant stream of new produces and chips on the web but you have to be careful.
Most of these are very exotic and perform wonderful tasks, but they are very expensive and very few suppliers stock them. On top of this, there is no guarantee that the item will still be available in 3 years time.
The other big problem is copyright. if you design something that takes off and has a long=term future, you will find others will copy the idea and use cheaper components.
No only will they under-cut you but your copyright and/or patent will be worthless.
Chasing up an infringement will cost $50,000 and the chance of finding the manufacturer will be zero. Look at all the clones that come onto the market after a few weeks.
People work 25 hours a day to copy things and they generally have a much bigger distribution market. They will out-strip you 10-fold
Here are t he two points to note:
Don't waste your effort, money and stock in exotic components.
Don't waste time in paying for a patent or registered design. You are only alerting the copiers to your idea and giving them 4 weeks "head-start."  
 
 
SOLAR LIGHT
Here's more rubbish from "professor" Mohan Kumar:
 

The circuit will NOT work.  A 3v solar cell through a diode will no charge a 3v battery. The "floating voltage" produced by 3v battery when it is being charged is nearly 4v and you need a voltage higher than this to charge it.
Secondly, a white LED has a characteristic voltage across it of between 3.2v and 3.6v.
The 3v battery will be less than 3v via the transistor and the LED will NEVER illuminate. Just another untried, untested, junk circuit from an Indian Professor.
What is the purpose of the 100u electrolytic???   It does nothing.
He has been informed of his mistakes for over 18 months and he still keeps adding more junk to the web each week.  When will he learn??
NEVER !!!!!

CHARGE CONTROLLER
Here's more rubbish from "professor" Mohan Kumar:

Mohan Kumar says:
The charging current will be:
12V / 127 = 0.094 Amps or 94 mA.

This is NOT TRUE.
The charging current will be: THE VOLTAGE ACROSS THE TWO RESISTORS DIVIDED BY 127 OHMS.

The voltage across the resistors will be 12v minus the voltage produced by the 9v battery when it is being charged. This will develop a "Charging Voltage" of at least 10 volts.
This means the voltage across the 127 ohms will be 2v AND NOT 12v.
The maximum charging current will be: 2 / 127 = 15mA.
The maximum voltage across the 27 ohm resistor will be 0.4v and the transistor will never turn on.
Another untried, untested circuit for this Indian Professor."
 

NIGHT LIGHT
Here's another disaster from "professor" Mohan Kumar:

This time we are talking about a 240v LIVE circuit. The 225 will pass up to 150mA and this will cause over 325v to be developed across the 47k resistor. The 47u 25v electro will BLOW UP!!!
The current required by the circuit is only about 10mA to illuminate the LED so the 225 capacitor should be 220n (224).
The 47k R3 should be 100R.
To turn OFF the circuit, the resistance of the LDR must be reduced to 1,000 ohms because the base voltage of T1 must be 0.5v to turn the transistor OFF.  This is created by the voltage divider of the LDR and 22k pot.
The 1k8 resistor will simply slow down the change from OFF to ON and ON to OFF. It's not a good place to put a resistor. 

BOAT LIGHT

When ever you have a potentiometer in a circuit, make sure it does not damage any of the components if it is turned fully clockwise.
In the circuit above, the LDR will see full rail voltage when the pot is turned and if the LDR is in bright sunlight, its resistance will be very low.  It may get damaged.

 
EMERGENCY LIGHT
Here's another disaster from "professor" Mohan Kumar:

He doesn't test anything. He does not understand electronics AT ALL and yet he still keeps adding his faulty circuits to the web.
This circuit does not work.
The gain for the BD140 is up to 250, but this is in a test circuit and pulsed at a duty cycle of 2%. This is totally unrealistic and when the transistor is placed in a real circuit, the gain is less than 100.
When the transistor is turned on via the 2k7, the base current will be 3.4/2700 = 1.2mA.  This means the maximum collector current will be 120mA.
The characteristic voltage drop across a white LED is a minimum of 3.2v and this allows 4v - 0.2v across the emitter-collector junction - 3.2v across the LED = 0.6v across the 47R resistor.
The current through the 47R will be 12mA.
Why use 500milliwatt LEDs ???
Most white LEDs have a characteristic drop of 3.4v to 3.6v and this circuit will not work AT ALL.
 
PHONE CHARGER
Here's another disaster from "professor" Mohan Kumar:

There is no current-limiting resistor between the 12v supply and 9v rechargeable battery. The battery will BLOW UP !!!
There is no switch between the 9v battery and regulator. The regulator takes 10mA all the time and will flatten the battery.
The output of the regulator is 5v minus 0.7v = 4.3v.  It is NOT 5v.
The mobile battery will see 4.3v minus the voltage across the red LED (1.7v) = 2.6v.   The mobile battery will NEVER get charged.
I don't know how Mohan Kumar became a "Professor."    It just gives teachers a bad name. None of his circuits work and none have been tested.  He says I don't know what I am talking about.   Let the 22 million readers of Talking Electronics website be the judge.

Here's a 555 circuit from an "electronics engineer" in a forum:

The output is required to deliver 1 amp.
Two faults with the circuit.
The output of the 555 does not rise to 11.4v to turn OFF the PNP transistor. It rises to about 10.3v.
When the output is HIGH the voltage across the 1k base resistor for the NPN transistor will be almost zero, even though the emitter is minus 2v compared with the 0v of the 555. The NPN transistor will never turn ON !!!
That's why you have to build a circuit and not rely on a simulation package.  



WHY DIODES AND BRIDGES FAIL
Diodes and bridges fail when the current increases.
The main problem is this:
A diode rated at 1 amp is really a 700mA diode. The voltage-drop across a 1 amp diode at 700mA is 750mV. This gives a wattage dissipation of about 0.5 watts.
When the current increases to 1 amp, the voltage across the junction rises to 1.1v to 1.2v. This fact is never mentioned anywhere and that's why you have to test a circuit and see what is really happening.
At 1 amp, the diode is dissipating more than 1 watt and if the printed circuit board does not have large solder-lands, the diode will eventually fail.
The same reasoning applies to bridges.
When a bridge is connected to the output of a transformer, the waveform is sinusoidal (AC) and as it rises to a peak, that's when a very high current flows, because as the voltage falls, very little current flows. To get an average of say 1 amp, the peaks must be a lot more than 1 amp.
During the peak, the current might be 3 amp and the voltage across the diode 1.2v. This is 3.6 watts for a short period of time.   Now you can see why a diode can fail.

EARTH
Here's another disaster from "professor" Mohan Kumar:

Mohan Kumar says:
When the wiring is proper, a potential difference develops between the Neutral and Earth lines and T1 turns on to light the LED.

This is INCORRECT.
Plugging this circuit into the wall socket will NOT illuminate the LED because the Earth and Neutral will be at the same potential.
At NO TIME will the earth be at a higher potential then the Neutral.
I am amazed that Mohan Kumar is still alive, with his mis-understanding of the mains.
How can he teach this RUBBISH to his students ???

Here's fraud from

Ultrasonic Pest Repeller Electronic Pest Control Rodent Mouse Anti Mosquito Insect EU US Plug ABS 100-240V

 Here's what's inside:

The item has NO ultrasonic components.   The LEDs simply illuminate !!!!
I am having enormous trouble getting a refund.  That's because Alibaba does not have paypal.  They debit your credit card. Another FRAUD from Alibaba.  

R2 ????
Here's another circuit from "professor" Mohan Kumar:

What is the purpose of R2 ????      It does nothing.    That's why you need electronics experience before putting this type of rubbish on the web.  

Note that switching threshold is determined by a 470k potentiometer (VR1) that causes the output to toggle with the preset threshold values. The light source (LED2) automatically switches on when it gets dark and switches off when there is sufficient ambient light.
The 100µF capacitor (C1) provides a bit of hysteresis to prevent the circuit from jittering near the threshold level.

The 100u has no effect on the hysteresis or the threshold values. These are determined by the 555 and the 12v supply.
The 100u simply prevents the voltage on the top of the LDR rising or falling rapidly. 
The 470k pot forms a voltage divider with the LDR and it changes the level of light needed to make the chip change states.

POWER SUPPLY
Here's another disaster from "professor" Mohan Kumar. And it will BLOW UP  !!!!!!

Mohan Kumar does not know how to calculate the voltage across the various components.
He does not know the basics of the power supply.
The whole design revolves around the current delivered by the two 1u capacitors. This is 150mA.
The circuit is a CONSTANT CURRENT design and the 150mA will pass through the 470R resistor, the zener and the 10R resistor.
The voltage across the 470R resistor will be .15 x 470 = 70 volts.  The 25v electrolytic will BOW UP. You can see it has already blown up in Mohan Kumar's photo.
The wattage dissipated by the 470R will be V x I  = 70 x 0.15  = 10.5 watts.  You can see the resistor has already got very hot in the photo.
The dissipation of the zener diode is  V x I  = 12 x .15 = 1.8 watts.   The zener in the photo is only 400mW.
Mohan Kumar has done no computations and just because he has tried it for a few minutes, does not make the circuit acceptable.   It is an absolute disaster. 
This power supply WILL KILL YOU. If it doesn't electrocute you, it will burn the house down.

SENSOR KIT
This is one of the reasons why kits have such a bad name:

The kit is built on MATRIX BOARD  !!!!!    The author hasn't even bothered to make a PCB for the kit.
And the kit sells for $45.00  !!!
And the instructions say to use the circuit diagram as the diagram above is not correct  !!!
RELAY, SPDT, 12VDC, 360Ohm, 15A,120VAC
Relay can turn on/off AC appliances (300W max)
Why specify 15 amp relay for 2 amp load ?

The complexity of this project necessitates a PCB, especially as the mains is connected to the board.
Look at the poor layout and the power lead ALL OVER THE BOARD !!

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