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Topic: Mistake in Arduino Projects Book: 220 Ohm leads to 12.7 mA at LED (Read 3014 times) previous topic - next topic

grafzahl

Hi Forum!

I recently bought the Arduino Starter Pack and, next to the wrong data sheet for the red LEDs, I think I found a rather disturbing mistake in the manual.

On page 30 it says that 5V/22Ohm=0.0227...A, leads to the current (23mA) used by the LED.
But the LED leads to a voltage drop of ca. 2.2V, so the current should be around (5V-2.2V)/220Ohm=0.0127...A. I verified this, of course.

I am really a beginner with electronics and this was the first time I used a resistor!
Am I missing something important, or did the author of that book know even less about electronics than I do?

- Michael

grafzahl

I just found out, while looking for a pdf version of the manual, that it is common knowledge that the manual is full of errors.

Just one other question to those who are experienced with Arduino.
Is the whole Arduino stuff of this quality?

Flomach

Your are right. There is a voltage drop when a LED is lighten up. It is more around 1.6V for a red LED.

Your are also right, you have to count this drop doing the Ohm's law calculation.

But anyway, this book and these projects aims the complete beginner, who doesn't even know why using a resistor is needed. The first project is really simple but it is also a lot of information for the complete newcomer.

Seriously, Arduino has developed so many stuff, do you really think they forgot to mention that drop ? Don't you think they decided not to mention it to make things easier to understand ?

Concerning your second post, you can change what you dislike, everything is licence under "Creative Commons", why not write and sell your own book, with your own card and you own projects ? Second option, why don't you make a complete list of errors you notice and send it to Arduino so they can correct the ones who need to be ?

Easy bashing like yours is so casual nowadays.
Criticism is very easy but it never comes with proposals so what's the point ?

AndersJJ

Your are also right, you have to count this drop doing the Ohm's law calculation.

But anyway, this book and these projects aims the complete beginner, who doesn't even know why using a resistor is needed. The first project is really simple but it is also a lot of information for the complete newcomer.

Seriously, Arduino has developed so many stuff, do you really think they forgot to mention that drop ? Don't you think they decided not to mention it to make things easier to understand ?
I think that this is a serious error, as the calculations are easy verifiable with a multimeter. There is no reason to omit this even for noobs, as it gives the wrong value by a large margin.

Coupled with the other errors in the book, I am not sure that this is an intentional simplification. Rather I think it is an honest mistake.

What is regrettable is that there seems to be no interest in fixing the text, which has lead to that I can't recommend the Arduino starter kit when I present Arduino and teach courses on it.

Raybrite

 You must be careful when measuring voltages with a multimeter in a digital circuit. There is a parallel resistance formed by the multimeter which will change the voltage reading you will get and sometimes even stop the circuit from working when it is connected.
 If you want to be more accurate use a TVM (Transistorized Volt Meter) or as we used to have a VTVM (Vacuum Tube Voltmeter) to measure these circuits. The input impedance on these is more like 10 meg ohms. That will not load the circuit.
 Hope this clears up a lot.
Ken

ShapeShifter

If you want to be more accurate use a TVM (Transistorized Volt Meter) or as we used to have a VTVM (Vacuum Tube Voltmeter) to measure these circuits. The input impedance on these is more like 10 meg ohms. That will not load the circuit.
These days, a typical digital multimeter (even a very cheap one) has a 10MOhm input impedance. Your concern is valid for an analog meter that does not have an amplified front end, but those are rare these days. (While state of the art once, I haven't owned one for over 30 years - I'll bet a lot of the kids starting out now have never seen one.)

Raybrite

I checked a digital multimeter that I an going to try to get on payday. Not expensive but has 5 Gigaohms input resistance.
 You are correct that most kids today do not know about circuit loading and input impedance.
 I was a Precision Measurement Equipment Tech in the Air Force and later went on to teach it a few times.
 Generally as long as they stick to the digital meters they should be okay. Just don't get a one with a meter on it or you are looking for problems.

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