The next discussion is not a mistake but an item that needs discussing.

It is a 555 demonstration board.
It is designed to teach beginners how to solder surface-mount components. It should also teach how to lay out a circuit so it is easy to follow.
The circuit is an absolute jumble.
For a start, the power plug is not polarised and if it is inserted around the wrong way, the chip will be damaged.
It should be a 4-pin plug with the “OUT” terminal in the centre, so the GND pin can be at the bottom.
Secondly, the layout should follow the circuit diagram. For a simple project such as this, it would be much easier for the beginner to see each component in the same relative place as on a circuit diagram. This involves turning the chip around so it sits correctly, and placing the parts so you can where they belong, relative to the timing circuit.
These are the skills we are teaching on this website. The whole object of a good design is to make it easy to service the product - not create frustration.

Mat Switch

Here is a circuit from an Indian Magazine: “Electronics For You.”
Take the following circuit for example. It is a MAT SWITCH and is designed to operate a Piezo buzzer for 50 seconds, after someone has put their foot on a mat.
The designer of the circuit has not thought “outside the box.” The circuit uses a 9v battery. When at rest, the circuit draws at least 10mA. How long do you think a 9v battery will last?
What an absurd waste of current.
Now we look into the design faults:
The touch pads consist of conductive foam, such as that obtained from the foam used to pack sensitive IC’s.
The article suggests two small pieces of conducting foam. Where are you going to put small pieces of foam in a mat?
You will need a large piece if you want to guarantee to detect pressure.
The next point is: What is the purpose of the resistor and capacitor on the emitter of the first transistor?
Secondly: What is to prevent the base of the transistor floating and picking up static electricity and false triggering the circuit?
Thirdly: Why is the resistor on the base of the first transistor so low? It should be 10k to 100k.
Overall, the circuit serves no practical purpose. A 9v battery would be flat in a day. And at $2 per battery, how many Indians are going to replace the battery every day!
It makes you laugh!
We have designed a far-superior circuit using a CD 4001 or CD 4011 and it draws one microamp!

ADJUSTABLE VOLTAGE

Here is another “Electronics For You” circuit. It has a very dangerous mistake. When the selector-switch is changed from one setting to another, the output temporarily goes HIGH and this puts a very high voltage on the equipment you are powering.
If you think a 9v AC transformer will provide 12v DC from a 7805, you are kidding yourself. The 7805 needs at least 2-3v for regulation and a 1amp transformer has a regulation problem of at least 1v. The circuit is a real disaster!

VOLTAGE DOUBLER

And yet another “Electronics For You” circuit. In fact every project from their website contains mistakes and designs that are far from the normal. The circuit below is a Voltage Multiplier. But the actual voltage-multiplying section is the part that concerns us.
The output of a 555 is capable of sinking and sourcing about 200mA and it can be connected directly to a “charge pump” as shown in the second diagram. This saves 2 transistors and 3 components. The only problem with using the output of a 555 IC is the voltage swing. The output rises to 2v below rail voltage and only goes to 2v above the 0v rail. In other words, you lose about 4v. This means the rail voltage for the project needs to be higher to get the required output voltage.
The author has used a BC 548 to charge the 100u, and a BD 139 to discharge it. Why use a “power transistor” to discharge the electro? It doesn’t make any sense. Also, why use a 1N 4007 diode for a 12v circuit? The 1N 4007 is a high voltage diode. It looks like the author had a lot of junk parts in his parts-box and threw them into the project. This is the sort of “poor presentation” that I don’t like, since a newcomer will wonder why each of the components has been chosen and will get a false understanding.

STRESS METER

In the next circuit from EFY, we have a Stress Meter. The author claims the transistor is a common-emitter configuration, however it is actually an emitter-follower. But the point of this discussion is the purpose of the 560R resistor and diode.
The author claims they are biasing components, however they don’t serve any purpose at all.
When designing a circuit, you need to go over every component and say “is this necessary?”

LED DICE

The next circuit uses “gates” (NAND and NOR gates) to drive a set of LEDs to form the spots on a Die or Dice. A CD 4001 or CD 4011 is not capable of delivering enough current to drive two LEDs in parallel. The brightness will be very small. The second circuit uses transistors to drive the LEDs. It only uses two IC’s and has a slow-down feature.

See our LED Dice project for more details on the circuit

Motor Driver

In the next circuit from EFY, we have a Motor Driver circuit:

The motor is driven from a pair of 555 IC’s. Apart from the fact that 555 IC’s can only deliver about 200-300mA, the maximum output voltage is about 2v lower than rail voltage. The “LOW” is about 2v above the 0v rail. This means the 9v motor will get a maximum of 5v from a 9v supply. The motor will deliver almost no torque at all.
I have written to the Indians who have designed these circuits and received no reply.
They have actually put these circuits into magazines with a readership of 500,000. Pity the poor readers!
None of the circuits I have presented, have ever been prototyped or used for a period of time to determine if they contain errors or problems.
This is one of the most important things for a design-engineer to do. He must test everything TO DESTRUCTION!
Actually I am only kidding, but you must test everything for hours and hours and give samples to friends to try.

I could not resist the next EFY disaster:

The circuit has an “H-Bridge” to drive the motor. The top left-hand transistor is an emitter follower. The base must be taken to nearly rail voltage so the emitter can rise and deliver a positive voltage to the motor. But the Q line is limited to a rise of 0.6v since it is connected to the base of the lower right-hand transistor!
This circuit obviously has never been constructed and unless you build a circuit and try it, you will be embarrassed too.

Music Chip

We have covered the next fault on page 4 of this article, but it is worth mentioning again.
It comes from EFY:

When the BC 547 transistor is turned on, the resistance between the collector and emitter is very low. The BC 547 turns on the BC 557 and the voltage drop between the base and emitter of the BC 557 is 0.7v. This means a high current flows across the base-emitter junction of the BC 557 and between the collector and emitter of the BC 547. A current-limit resistor is needed as shown in the diagram below. This will limit the current and yet allow the circuit to operate correctly.

Sometimes it is very difficult to “see outside the box.” That’s why you have to come back to a circuit and look at it over and over again.
I have seen lots of stupid circuits but I have also seen a lot of very clever designs.
To become a good design-engineer, you have to study thousands of circuits and see how and why they work. You also have to see the problems of “others” so you don’t make the same mistake.

12v Adapter

The next discussion is a general mistake. The diagram below has no component values. When you look at the diagram, you have absolutely no idea how the circuit works as the value of each component gives you and idea of the current flowing and how the components are biased. The list of components has been placed on another web page and this makes diagnosis very difficult. A circuit diagram should be as complete as possible. It is difficult to see how you can get 12v out (after regulation) from a 12v supply. The diagram below shows the component values added to the circuit.

PC 12V ADAPTER
Item Quantity Reference Part

1 3 C1,C5,C6 100uF 25V LOW ESR
2 1 C2 100nF
3 1 C3 10uF 16V
4 1 C4 470pF
5 1 D1 LED GREEN 3mm
6 1 D2 1N4148
7 1 D3 LED RED 3mm
8 1 J1 JUMPER 2 PIN
9 1 K1 PLUG-IN TERMINAL BLOCK 2 WAY
10 1 K2 PLUG-IN TERMINAL BLOCK 3 WAY
11 1 L1 150uH TOROID CORE INDUCTOR
12 1 P1 100K MULTITURN
13 1 R1 0R33 3W
14 1 R2 1K5
15 2 R3,R4 10K
16 1 R5 560R
17 2 R6,R7 1K
18 1 R8 180R
19 1 R9 0R33
20 1 R10 82K
21 1 R11 8K2
22 1 R12 5K6
23 1 S1 PUSH SWITCH
24 1 T1 2N3906
25 1 T2 TIP147, BD902
26 1 U1 LM78S40