SPOT
THE MISTAKES!
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One of the most-often criticized man in the world is the person who has changed just about everyone's life.
It is the inventor of the software we all use everyday on our computer.
Maybe things have changed slightly in recent times but the originator who changed our computer from a blue screen with blocky writing, to a full-colour active screen, was BILL GATES with his MICROSOFT packages.
He has been criticized by lawyers, politicians, accountants, professors and engineers, but not one group has banned together and produced a better concept.
Agreed, there were some rival start-ups, but they all wanted $1,000 for a package when Microsoft was only $200.    
Most inventors invent one item. Bill Gates invented one item and then went on to improve and expand it. That takes brilliance.
He could have sat back and counted the millions from the first release, but he went on to produce the next improvement and the next. Each stage was a huge advancement over the previous.
But the one thing no-one realises is this:
His programs required bigger and better and faster computers (operating systems) and he was forcing hardware manufacturers to come up with advancements that were 10 times better and faster and larger in storage capacity, all along the way. 
My first computer had 10 meg hard drive. The next was 80 meg and then it was hundreds of meg. Now we are taking about tera byte.
We went from 5.25 floppy disks to 3.5 and these only stored from 360kB to 1 meg of data. Programs came on 12 discs.
And then the internet was born.
Bill Gates finished up with so much money he now has full time staff dealing with humanitarian activities throughout the world. 
Only an ignorant person would criticize him.
Only a person who has never done any programming.
And especially at a time 30 years ago when there was no hardware to run your programs and no-one to ask, for advice or assistance.
It was like building on sand.

He only had speeds of about 1MHz to 4MHz, a capacity of 64kB and a 10Meg hard drive.
And yet he released programs requiring an upgrade of 1,000% !!
That's forward thinking in a time when IBM said the world would need 8 computers. He said he wanted to put a computer in everyone's home.
That's when they thought he was mad.
And he was wrong . . .  every house has more than 3 computers!!!
The capacity of early computers was so small they could not run any of Bill Gate's programs.
And when you think of it. All his programs were worthless.  You had to buy another computer.
And 30 years ago, the term obsolescence, had not been discovered. Everything lasted until the day it died.
He changed the world.
Not only did he introduce the word "updating," but introduced the term: "buy a computer."
And it was the introduction of "copies" and "clones" and "knockoffs" that brought computers to a cost below $1,000.
Yes, we have an enormous amount to thank him for. Maybe the world would have progressed to the same extent without him, but you can only say this if there were parallel inventors with the same forward thinking. 
Yes. I know he took a lot of ideas from others and he was helped along the way.
I am only using his name as the "Front-Guy" and saying the whole revolution  was brought about by a very small group of thinkers and now this type of thinking has been imparted to millions of others and that's why we have an industry called the IT industry.
 
 
The biggest problem with University Graduates is this:
They "think and expect " their idea, project, circuit will work the first time.
They get lulled into a false concept of the "world of reality" by being involved in markings as high as 99.85%.
And when the circuit doesn't work, they are LOST.
Although my circuits work 100% of the time. They all go through a series of steps and stages of improvement and adjustment until this result is achieved.
Even a microcontroller program will take 100 or more steps to get it finalised and sometimes a PC board will take 3 or 4 changes and improvements to get to the final design.
Most of the time these changes are improvements and even total re-design with fewer, cheaper components.
But, in the end, the result is a robust, reliable project that anyone can build and get it to work as soon as it is turned ON.
As  I have said for the past 50 years, it does not matter if the project does not work the first time. This is when and where you will stat to learn electronics.
It's only after diagnosing, testing and fixing a fault, that you will really learn electronics.
Anyone can build electronics projects and, in fact, out first 6 transistor superhetrodyne radios were build by 14 year-old children on the floor of their apartments in Hong Kong and sold in Australia for $2.50. But if the radio did not work, it would take a capable person to locate the problem.
The same applies today.
Products are made by the millions by either robots and unskilled workers, but the eventual success lies in the technical department to sort out any faults and get it to a stage of being produced with 100% reliability.
So, there is still a need for the "thinker."
And that's what we are providing.
The tools and knowledge to get you into the world of designing and producing products that will assist, help and advance the world, in general.
And the best place to look is the area of aids for the sick and disabled.
The field is enormous and it just takes a little time to associate yourself with those-in- need to see what devices can be invented.
I will leave the rest to you. 
 

Universities are constantly emailing me in the hope I would recommend one or more of their courses.
https://floridapoly.edu/degree/engineering-masters-degree/
Florida Poly has a HUGE focus on robotics with their Engineering Masters program, and would make a perfect resource to help turn a hobby into a career.

I would not recommend ANY University course.
I have experienced FIRST HAND the course structure, notes and access to practical learning in a number of electronics courses.
Before you even start to think about doing any type of electronics course, spend $1,000 on kits from Talking Electronics and a few robot kits from other suppliers and learn the basics and advance yourself to a stage that you can understand what is going to be taught in the course you are investigating.
These courses cost $30,000 or more and offer little more than glossing over the basics and using some exotic equations to make the course sound impressive.
But, in the end, the graduates have hardly soldered a wire and could not build or diagnose any of the Talking Electronics kits beyond a beginner.
A $30,000 course is $29,000 wasted and you would not even get $1,000 worth of projects to take home.
The last University professor stripped the valuable parts out of the projects and left the rubbish for the students to take home. The PC board he provided was an INSULT and circuit was not designed properly.  No wonder the students did not get a grasp of the subject.
If you want to test the skills of a graduate, take a faulty project to a University and ask the class to repair it for you.
That's when the truth will hit you in the face. No-one will volunteer.


The funniest photo you will get from a University

 
  
Another junk circuit from
Electronics For You March 2018:

None of T.K. Hareendran's circuits work.  He has been dumping his untried, untested, junk circuits into Electronic For You magazine for years and never bothered to reply to my emails.

The 100k resistor will only allow 3.2mA to flow at the top of the wave and this will only occur every half cycle. This reduces the average to less than 1mA !!!
The blue LED drops 2.7v and so the whole circuit only has 2.7v to play with.
The drop across the collector-emitter of the transistor is 0.3v so how do you expect the blue LED in the rainbow LED to illuminate if only 2.4v is available?
But if you look at the whole project, why turn the LED on with a transistor etc when the circuit takes just as much current when not working??
The whole project is a FLOP.
What is the purpose of the 150 ohm resistor?   Who cares about the diameter of the LDR?
Apart from that, the circuit is live and DANGEROUS.
Never make any circuits that connect directly to the 240v.
They are ILLEGAL in Australia and India should wake up to this situation.
He posted a video of the circuit and he thinks: "Just because the circuit works, it must be designed correctly."
The voltage across a LED varies considerably and on some occasions, the circuit will work very faintly.
But I am criticising the lack of understanding and the lack of technical know-how and the lack of "tolerance" in the components.
I remember an Indian flip flop circuit from 40 years ago in an Australian magazine. I built it and it did not work.
I found out later that the variation in gain of the transistors needed the resistors changed to get the circuit to start working. 
And the same with a micro flasher from England. I never got the circuit to work and again it relied on adjusting the components to get the circuit to start oscillating.
It's a fluke if you can get them to work.

He posted a video of his circuit working.
The voltage across a LED varies considerably and just because a particular set of components work in the laboratory, does not mean the circuit is suitable for publishing.
You need to get your values CORRECT and UNIVERSAL  -  so that anyone with any set of components will be able to get the circuit working.
When I design a kit or any project, I build a number of prototypes and prove the fact that it will work perfectly because none of the components are critical.
It just separates the DESIGN ENGINEER from the dodgy ENGINEER.



THIMK !!!
Here is an OR-gate from an electronics forum:

Here is a reply from a technical engineer:

Here is my reply:

The whole point of this website is to teach you how to simplify things and think like an engineer.
Whenever your design a circuit, go through each component and ask: "Is this needed?"
Sometimes, the saving of one or more components can be the difference between profit and loss, when you are selling thousands of items.
But it also shows you are good engineer. That's why you have to thimk !!

I received this PC board today:

It is not mine and was received by mistake from the PC board manufacturer.
But it is a good lesson for everyone.
The board has no markings or component values.
This is a big mistake.
You might think you will remember, but when you have hundreds of different PC boards and lots of different versions and improvements, you cannot possibly remember everything.
All my boards can be assembled without any further information. All the component values are shown on the OVERLAY (called the LEGEND) and the name of the project with TalkingElectronics.com identification.
Some of my customers buy a kit and don't assemble it for 20 years. All these marking are not just for my benefit but for EVERYBODY.
A fully documented board shows professionalism as well as convenience.
We have passed the stage of paying extra for double-sided boards with a legend on top and bottom as well as plate-through holes and tinned lands.  All this beauty is included for FREE.
There is no excuse for not producing a perfect board and something that someone can take way and assemble without frustration.  

One of the websites provided an article on soldering.
It suggested one of the cheap soldering irons that gets "as hot as hell."

Here are my comments:

The soldering iron kit you offer is absolutely WORTHLESS. For the same price you can get a temperature-controlled iron on eBay.
0.2mm solder is a bit fine.
0.5mm solder is the best and here is the way to set the temperature of the iron:
Turn the temperature knob down to very low and turn the iron ON.
Gradually increase the temp and wait for the tip to heat up.
When the solder begins to melt, you have reached the temperature called the MELTING POINT.
Now increase the temperature slightly so you will actually solder at a higher temperature and this will allow the solder to "run" all over the joint and not get cold.
The most important part of the solder is the flux.
It is important not to "burn the flux" but allow it to get hot and it becomes an ETCHANT and "digs away" the oxidised layer on the solder-land - just just soap on our hands.
The solder should be on one side of the wire you are soldering and the iron on the other and you should just touch the solder to achieve THERMAL CONDUCTIVITY to melt the solder and allow it to run all over the wire and the land.
This should always be one in less than ONE SECOND because sometime you are soldering LEDS and they will be damaged if it takes 2 seconds.

The website did not show any valid images on how to solder and no-where did it say to tin only one land for a surface-mount component and then place the component and "stick it in place" before fully soldering the other end and maybe coming back to the first end.

NOTE ON SOLDER:
Some Chinese solder is RUBBISH. It does not melt and the surface is not shiny.
These are the specifications to look for:  63/37    Eutectic  the label with SENIOR SOLDER has been very good.
50gm  or 100gm or 200gm   Must be 0.5mm or 0.6mm    Nothing else.
I have received 3 rolls of rubbish.
To use them I cut off 1 metre of rubbish and 1 metre of good solder and twisted them together with the electric drill and used them to solder large terminal pins. I used up the 3 rolls.
That's why I am so careful when buying solder.  

FM TRANSMITTER
You don't mix the markings on a component with component-values on a circuit.

Sometimes I put additional component-values and component-markings on a PC board to prevent wrong values being fitted. Such as 220 for a 22 ohm surface mount resistor and 4702 for a 47k surface mount resistor. 
The 223 and 102 on the circuit above should be 22n and 1n.
You don't need 470u across the battery because the current will be less than 10mA and 10u will be sufficient.
1uH for the coil is too large.  0.1uH (100nH) is approx correct.   With this simple mistake, the circuit will never work.

A FIRST LAB IN CIRCUITS AND ELECTRONICS
This textbook is filled with mistakes and terrible descriptions as well as teaching quite complex things to beginners, without covering the really-needed basic things.
Here is a positive/negative output supply with the "batteries" identified by a single cell instead of a voltage. The output is marked "+" and "-" but it will be reversed when the pot is at the lowest position.

The text says: "You have made an AC to DC converter."
The input shows DC !!!
If the input is AC, the graph shows the output voltage will follow the input but get clipped at a voltage equal to VDC plus the 0.6v across the diode.

As soon as you supply sufficient voltage to the gate to turn the MOSFET on, current will start to flow and if the voltage is increased further, the MOSFET will short-circuit the power supply and either the MOSFET will blow up or the power supply will "cook."
There is no current-limiting LOAD resistor.

The 100u is around the wrong way. The Function generator will produce a voltage higher than the base-emitter voltage.

What is a LOGIC LEVEL INDICATOR?
A Logic Level Indicator is generally a LED that shows when an output is HIGH.
The term should be LOGIC PROBE because the output will be oscillating at 100Hz to 10kHz and a Logic Probe will indicate the percentage HIGH and percentage LOW, by the illumination of the LEDs and the pulse LED will show a signal is present. 

He also says:  We see capacitors with 1000 on them to indicate 100 with no zero's to mean 100p.
SHOW ME WHERE ???
In his kit of parts he uses 1/2watt 10% resistors.  Who uses 1/2watt 10% tolerance resistors????

The text book is an absolute DISASTER. 

He has obviously never used a CRO in his life, otherwise he would have included the table:

TIME/DIV Frequency
50mS 20Hz
20mS 50Hz
10mS 100Hz
5mS 200Hz
etc

This is the most important table for a CRO and is placed on the side of the CRO so you can quickly see what frequency is being displayed.

That's why so many visitors to Talking Electronics website said they have learnt more from the website than all the teaching they got at University.

Here's another stupid design:

Wireless Electricity Generator
by D.Mohankumar

The gain of the transistor is only 30 at 100mA and the current capability of the transistor is about 100mA, so it is not suitable for the circuit.  You can't use a "100mA transistor to do a 100mA job."
The power delivered to the LED will be about 300mW at the most. You can't get any more than this from a 9v battery.  Do you think a 9v battery is the "power-plant of the future?" 
That's only the first problem.
You will find the reverse voltage is more than the LED will tolerate.
Here is the specification:
Maximum reverse voltage between ANODE and CATHODE: 5v.
You have a 25 turn to 50 turn transformation ratio so the reverse voltage can be 18v or more.
Reverse spikes can damage a LED much faster than the blinking of any eye.
Just because a circuit works and illuminates a LED does not mean it is a good design.
The whole circuit is designed incorrectly and the turns-ratio should be 25:15 - apart from all the other mistakes.
 

 

It is very complex to design a battery charger and here the problems with the circuit above.
The 3R9 1watt resistor will only allow a maximum of 500mA before the resistor will burn out.
The voltage across the collector-emitter terminals is given as 4v in the data sheet.
If you are detecting 13.8v you are not providing any "head-room" to charge the battery.
The voltage across the 1k5 will be 13.8v + 0.7v + 1.7v = 16.2v   If you have 18v supply, the voltage will be 1.8v across the 1k5.  The current through the 1k5 will be 1.2mA    The maximum gain of the transistor is 100 so 120mA will flow.
If we allow 2v across the transistor the voltage drops will be 13.8v + 0.7v + 1v across the 3R9 plus 2v across the transistor = 17.5v     If the supply is 18v you have no "headroom" so that almost no CURRENT can be delivered at the low supply voltage.

It seems that only 120mA will be delivered and all you need is a resistor permanently connected to the battery and it will be trickle charged all the time.    
This circuit  does almost NOTHING.
It's just a JUNK circuit.

Here's an over-design from DIYODE MAGAZINE:
It is a temperature controller for a USB soldering Iron:


The fact is a USB socket can only deliver 5v at 1 amp. This 5 watts. 5 watts is only good for the smallest joint and will be quite unsuitable for any component that needs to be removed quickly or has large thick leads.
If you have one of these worthless irons and want to reduce the temperature, all you need is 3 x 1 ohm wire wound resistors and an alligator clip. Put them in series with one lead and connect the alligator clip to the other lead and add 1, 2 or 3 ohms to the resistance of the iron to reduce the temperature.  You can also get 3R3 if you want the iron really cold to do plastic work.
For $14.00 you can get a 240v temperature controlled 20 watt iron from ebay.
For $25.00 you can get a temperature controller soldering with a base station that indicates the temperature on 7-segment displays. Everything else is either over-priced or just RUBBISH. 

SOLDER
I have been using Australian-made solder for the past 60 years and never had a problem.
I though 60/40 solder made anywhere in the world would be absolutely identical to the type I have been using.
But I recently bough Chinese 60/40 resin cored solder and it was absolutely useless. I don't know if the combination of  tin and Lead was exactly correct or if some other "filler" has been included or if the "reject" metals have been added.
The solder did not melt at the normal temperature I have been using for 60 years and the solder did not "run" over and around the joint and the surface of the joint was was shiny.
No-matter what I did, the result was terrible.
I then bought eutectic solder that has the combination in the ratio of 37% to 63%.  This ratio produces a solder with the lowest melting point. This solder worked much better but was still not as good as the Australian-made solder.
You must ONLY BUY eutectic solder and make sure the seller guarantees it to be eutectic as the label on the reel does not indicate this. See above for more thoughts on buying solder.

Here is a TEMPERATURE ALARM from a suppler on eBay.  It is not expensive - but it does not work.
You can imagine the frustration of the hundreds of buyers having the same problem.
The supplier was not interested in the mistakes.

Here is the circuit.

Here are the problems.
The first thing you have to know is the type of thermistor. It has a cold value of 1,000 ohms resistance.
There are two types of thermistors. One is PTC (Positive Temperature Coefficient) and its resistance INCREASES when it get hot. The other type is NTC ( Negative Temperature Coefficient) and its resistance DECREASES when it gets hot.    The type used in the circuit NTC. When its gets hot, the resistance decreases to 600 ohms and less.
Now we can see how the circuit works.
The 10k  10-turn pot is adjusted so that the first transistor is turned ON. The first transistor turns ON the second transistor and the second transistor turns OFF the third transistor. This means the buzzer does not produce a sound.
When the thermistor heats up, its resistance decreases and the voltage between emitter and base of the first transistor decreases and this turns the first transistor OFF.
But the second transistor is still being kept ON via the 39k resistor and thus the buzzer does not produce a sound.
When the 39k is removed, the circuit works perfectly.
But the buzzer only gets a voltage of about 1.8v because this is the maximum voltage that will develop across a red LED.
To make the buzzer louder and prevent the red LED being damaged, a 100R should be included in one of the leads of the LED.
Finally, the circuit did not work for me because the 10-turn pot was "102"  (1k) and the resistance of the thermistor did not go low enough to turn OFF the first transistor.
The CAD package use to produce the printed circuit board created a GROUND PLANE between all the tracks and a beginner could easily leave too much solder on a pad and the solder will touch the ground plane and create a "short."
Lastly, the first and second transistors are connected from the positive and negative rails without a current-limiting resistor. 
When they are both turned ON, a very high current will flow through them. This is especially the case when the supply is increased to 6v. The transistors get very hot.  
Now you can see why you have to be very experienced in designing a circuit. If not, you can introduce 5 faults into a simple circuit such as this, and produce frustration for everyone who buys the kit.

I have told D.Mohankumar to stop putting his JUNK CIRCUITS on the web and messing up young enthusiasts with poor designs and circuits that do not work.
I have covered this circuit before but he continues to put it on the web.
A 1 watt LED takes 300mA at full brightness and this means 300mA will flow through the 10 ohm resistor and drop 3v.
The battery only supplies 4.5v and the drop across the transistor will be about 0.3v. The LED has a characteristic voltage across it of 3.6v when fully illuminated.
Work it out for yourself. The voltage across the resistor will be 4.5 - 3.6 - .3  = 0.7v  When 70mA flows, the voltage across the resistor will be 0.7v.
When 70mA flows through the LED, the voltage across it will be about 3.2v and thus the current will increase to about 100mA. This means the LED will dissipate about 300mW - a lot less than 1watt.
As soon as the voltage of the battery reduces, the brightness will reduce too. 
It's just a poorly designed circuit.
The charging current will be quite small - let's see:  The 4.5v transformer will produce 4.5v x 1.4 = 6.3v.
There is a drop of 2 times 0.7v across two diodes in the rectifier = 4.9v
The voltage of the battery will rise to 5v when it is charging. This means we do not have enough voltage to charge the battery.
The transformer needs to deliver 5v plus 0.7v plus 2 x 0.7v = 7.1v
Another project that has not been tried and tested.
Professor D.Mohankumar is just making a fool of himself, continuing to publish projects that simply will not work.
He may be using a transformer "made in India" and the unloaded voltage may be higher than 4v5. IN this case he will get a tiny trickle of charging current through the 56R.



 

I am highlighting these faults and showing how to work out the current that will flow in the charging process and the illumination of the LED.
These values are much less than expected by Professor D.Mohankumar and that's why it is important to do a little bit of calculating before designing a circuit, so you know if it will be operational. 
In this case the circuit is a DUD.

Not only do we get a lot of bad engineering from Indian websites, but many of the world-wide electronics magazines also have terrible faults.
Here is a typical example:

A 3-amp diode will drop about 0.7 to 0.9 to 1.1v across it when full current is flowing.
For the diodes in this bridge, it amounts to about 1.5 watts per diode  because only two diodes are conducting at any one time.
However the total for the bridge can amount to 6 watts and the printed circuit board simply will not dissipate this amount of heat when the diodes are closely spaced.
I have had many TV's where 1 amp diodes on a PC board have eventually blackened the board and melted the solder and created faulty connections.
If you want long-term reliability, the board must have sufficient trackwork to keep the diodes to a point where you can hold your finger on the leads for at least 10 seconds. This involves spacing the diodes further apart - much further apart.

I am absolutely horrified that a magazine would allow a contributor to send in a construction project, showing the final design with no overlay.   I would probably go further to say the board had been created with a "dalo" pen and etched with Ferric Chloride.

This means the reader has no opportunity to get the board made and really the whole project is a worthless endeavour.
There are just so many things wrong with presenting this sort of rubbish, that it makes me wonder why the publishers produce an electronics magazine in the first place.
When they do have a printed circuit board available, the cost is so high that you could but the whole project read-made for less cost.
The subscriptions to these magazines is so small, they do not publish the figures !! Going by the number of copies in the newsagents, the sales can be counted on a few fingers. They are only crippling themselves by not catering for the beginner/experimenter and providing Printed Circuit boards at a reasonable cost and a kit of parts. 

Here's a circuit from Electronics Monthly February 2019 - an Indian magazine with no technical staff and no understanding of circuit design:

The battery shows a single cell whereas it has to be a voltage greater than 7v for the 7805 to work.
Why include a voltage regulator.  The circuit will work perfectly from 6v to 12v.
The 10u and 1u do not show the polarity of the electrolytic.
The "globe" above the 1u has no connections and no-one really knows what it is suppose to be.
R1 is 100k. (according to the parts list)
This value is FAR TOO HIGH.
To turn off the first transistor, the base voltage must be less than 0.5v 
In the dark, the resistance of the LDR is very high. Normally it is about 300k.
So, let's work it out.
Suppose the LDR goes to 100k. The voltage at the mid-point of the LDR and 100k resistor will be 2.5v.  Too high.
Suppose the LDR goes to 400k, the voltage at the mid-point will be 1v. Too high. It has to go to 800k !!!
Suppose R1 is 100 ohm. The LDR will have to go to about 1k to turn ON the transistor. This is possible with a bright - focused - light.
When the first transistor is turned ON, the wasted current through it will be 50mA and the transistor will get warm. This is a bad design.
The speaker symbol is not really a speaker but an active buzzer and the diode is not needed.  
When the first transistor is OFF, the base current through the second transistor is 15mA.  It only needs 3mA to 5mA to operate the buzzer.
Just another badly designed circuit that should not be presented in an electronics magazine.

Here's another DIYODE MAGAZINE disaster.

This is a PROBE intended for both TTL and CMOS circuits.

Look at the resistor connected between the probe and 0v rail.
Suppose you are testing the input pin of a microcontroller that has internal 47k pull-up resistor.
The line will detect a high when 2.2v or higher is presented to the pin.
When the probe is applied, the 47k pull-up will be acting against the 10k (forming a voltage divider) and the voltage will be right down at the 1v to 2v level. You have to take into account the 18k pull-up resistor and this will raise the voltage above 1v.
The PROBE is very badly designed as the input impedance of a probe should be 1M. This effectively puts NO LOAD on a circuit AT ALL - especially when the lines are running at high frequency. .
But if you are dealing with a fairly low frequency, the biasing resistors can be as low as 100k. But when they are 10k to 18k, the probe is quite worthless and may produce a false reading.
You will be totally unaware that the probe is at fault and be looking all over the circuit for the problem. A VERY DANGEROUS PROBE.   

Here's another DIYODE MAGAZINE disaster.

The resistor symbols are too big and it makes the circuit look childish. The breadboard layout had no parts identified and these have been added by me to make the layout useful. The block diagram of the 555 might be ok to show the placement of the wires, compared to the wires on the breadboard but the whole purpose of a CIRCUIT DIAGRAM is to show the output lines of a chip in standard locations on the circuit so you can instantly see what the circuit is doing.
This is the whole purpose of an EDUCATIONAL MAGAZINE and this is where DIYODE MAGAZINE falls down completely. 

Here are a couple of circuits from an Indian website. The first circuit is a traffic light. The red light should have the same ON time as the green LED. The green LED has no current-limiting resistor.

A resistor is needed to limit the current to the electret mic. The chip numbers are needed and pin 2 should not be left floating. The corrections are in RED. The circuit has so many mistakes that it may or may not work correctly. Build it and find out the mistakes yourself.

LIGHT SWITCH

Here is another junk circuit from ELECTRONICS FOR YOU.

Why is it a junk circuit?
Basically because a resistor is turning ON the relay and not a transistor.
In a correctly designed circuit, a transistor in one of the stages passes a current to the output stage to turn ON the output transistor.
A poorly designed circuit allows the resistor in the output stage to turn ON the transistor.
Let's see why this circuit is badly designed.
If we remove the first two transistors, the output transistor will be fully turned ON.
In other words, the first two transistors simply TURN THE OUTPUT TRANSISTOR OFF !!!
Now, here's the complex part that no-one understands.
Suppose the relay requires 100mA.  Suppose the output transistor has a gain of 100.
The base current will be 1mA. The voltage across the 1k5 will be 1.5v  
In other words, the output transistor will begin by not turning ON and the current through the 1k5 will be (6v/about 2k) = 3mA. This current will turn the transistor on fairly strongly and the emitter-collector voltage will be less than 1v. This will allow less than 1mA to flow through the 1k5 and the transistor will turn OFF slightly until it settles at a voltage across the emitter-collector leads that allows 1mA to flow through the 1k5. 
The base-emitter voltage will be 0.6v when this occurs.
This means the circuit will lose  1.5v + 0.6v = 2.1v and the relay will get 4v from a 6v supply.
If the supply is about 10v, this loss will not matter. But it is just the wrong way to design a circuit.
Secondly, when the relay is not activated, about 10v is across the 1k5 resistor and if the circuit is battery operated, this is 7mA wasted current and a correctly designed circuit will consume less than 1mA when not activated.   
Now, for the other mistakes:
What is the purpose of R4 (4.7 ohm) ???   It serves no purpose.
What is the purpose of the 470u??   It will have 100 times more effect if placed in the second stage of the circuit.
The output transistor is an EMITTER FOLLOWER and it has a high voltage across its terminals when turned ON, compared to a COMMON EMITTER stage. 
You only need 2 transistors to get the required amplification.
And finally, a qualified engineer will design a circuit according to the commonly-accepted way, so anyone looking at or fixing the circuit, will see how the circuit works.
Here's a correctly-designed circuit:


The correct way to design a circuit

This circuit is simpler, takes less current and delivers more current and a higher voltage to the relay.
You can see exactly how it works and is much easier to fault-find and fix.
There is a standard way to design a circuit and produce a diagram that all other electronics engineers will understand. If you want to use PNP transistors, simply invert the 2-transistor circuit and have the emitters connected to the positive rail. Use PNP circuit-symbols and the job's done.

LOW VOLTAGE CUT-OFF

Here is another poorly designed circuit from Mohankumar

An AC transformer does not have "+"  And it is not necessary to indicate the start of any of the windings.
1N4007 diodes are 1,000v diodes.   You can specify 100v or 200v or 400v but 1,000v diodes are used when the voltage is very high.
The diode across the relay is not needed because the transistor will turn ON and OFF very slowly due to the 1,000u charging and discharging slowly and the relay will not produce any back emf.
Value of pre-set pot is not given.
3v zener diode is not needed.  It makes no improvement and has no purpose.
The 5k6 resistor is in the wrong position as it needs to be above the pot to prevent the high voltage from the transformer damaging transistor.  
The circuit is a COMPLETE DISASTER.

FLICKERING LED
Here is another terrible design from DIYODE magazine:




Here is a simplified version of the circuit. You do not need the two transistors as the output of the 555 will illuminates the LEDs.

I do not like the layout above as I have to try and work out the what the circuit is doing.
In the following circuit I have started to draw the two 555's and the timing components.
I stopped when I realised the circuit is so badly designed that it is not worth finishing.
The first 555 simply turns the LED on brighter and the second 555 simply flashes it at a regular rate. This is nothing like a flickering candle.


The unfinished FLICKERING LED circuit

Here is a much better design and it is much simpler. It only needs two flickering/flashing LEDs in series with the high bright LED and the result is the combination of the effects of the two flashing LED. You do not need the 220R resistor as the flashing LEDs have inbuilt resistors. This gives a really random flickering effect.
It is pointless using 15 components to perform an effect that only needs a few components.


You can buy a flickering candle LED from
Talking Electronics.com for 10 cents.
It has an inbuilt resistor and works from 4.5v to 6v.

If you are going to do something, do it PROPERLY.

POLICE SIREN
Here is another design from DIYODE magazine, that needs improvement:

The 555 IC has the capability of driving a speaker via a 10u electrolytic and the transistor is not needed.
Why use a 470u electrolytic when a 100u can be used, providing the resistor is increased to 10k.
That is the purpose of the 10k resistor, connected to the 470u electrolytic?
 When the circuit is drawn with the pins in the correct positions, you can see exactly what the circuit is doing.

SOLAR STREET LIGHT

Why is this design so inefficient?

The specifications are very deceptive.
Their advertisement is vague, dishonest and deceptive.
The facts and figures are these:
There are 6 panels around the post and the total output of all the panels is 100 watts.
This is 16 watts per panel.
Only one will get direct sunlight and 2 will be at 30 degrees to the direct rays of the sun. The two side panels will produce about 15% less, according to this accurate website:
https://www.civicsolar.com/support/installer/articles/solar-array-tilt-angle-and-energy-output
This gives 16 + 14 + 14 = 44watts.
But the angle of incidence must also be taken into account because the panels are vertical and sun is high in the sky. The rating must now be de-rated by another 15% to 20% making the maximum output about 35 watts.
However we do not know if the 100watt rating has been determined in the laboratory with direct sunlight and the final result is a mystery.
The only thing we know is 100 watt (direct) becomes 35 watts in reality.    
Allowing 5 hours per day, this is sufficient for a 50 watt LED light for a few hours each night. 
A 100watt incandescent globe produces 1800 lumens.  (A 50 watt LED light produces 5,000 lumens, so it is equal to 250watt globe).
A 12v 20 A-hr battery will store the energy. ($40.00)  or (2 x 10A-hr batteries).
The lights are far too high. By reducing the height by half, the brightness will increase 4-fold. Some parks have low wattage lights at 3 metres and they are much-more effective.
This design is really inefficient and I am surprised no-one has picked up the ineffectiveness.

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