PS: The reason for above mentioned issue is that the Diode tester circuit with its 100V generator are driven directly on the unregulated Vin voltage on PWRIN right after the D1 diode on every Arduino ATmega board. See next circuit => https://www.arduino.cc/en/uploads/Main/arduino-mega2560-schematic.pdf
Only if the autor had used a stabilized voltage to feed the entire tester AND had used the right calibrated voltage at which only correct curves would be produced, it would have been a very good project. Now it is sadly not and the article(s) are wrong saying 5V and 12V work. Why the author, who wrote a very interesting Arduino program for a very interesting tester didn't stabilize his entire Diode tester circuit beats me. Maybe he just couldn't find the right voltage at where every diode upto 100V worked? And calibration now is a real problem! But at the moment there is no equally interesting project that can do what this tester can do for the affordable price it costs. And after all the software/firmware is completely free!

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Important Note: I just noticed a bad thing about the mentioned problem of The Diode Curve Plotter not showing any information in the second square bottom box on the right on the 2.8 inch touchscreen. Only when you use an external power supply inserted on your Arduino Mega board instead of using the 5V USB port to power your Diode Tester will give correct resulting curves and both working information showing boxes!
Because powering the Diode Tester over 5V no matter how high the given current will NOT work and only shows wrong results (completely wrong curves) and again an empty second square box on the bottom screen right! So keep this in mind! Which makes this devices sadly not useable by powering up using an USB powerbank! Which makes it less portable by giving completely wrong results and still the missing square box information!

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In manual settings I therefore chose the lowest power value possible which was about 50mW and it showed that position on the curve for my green Led under I/V test.

So that was for the first time the second box showed any information. And I now know that I have at least 5 other good Arduino Mega boards that also work without anything being damaged previously. (the jumper setting after programming was done to prevent blowing up the 2560 ATmega chip warning!)

Why the explanation in the magazine talked confusingly about using the reverse option for the second box to appear I do not understand because the reverse option never does really show anything selectable in the negative x- and y- axes part of the curve by pointing on the curve in that (left bottom) quarter of the plotted graph. That quarter of the screen never is selectable!
Not in the normal I/V plotted curve screen and neither in the reversed plotted curve screen!
So I still do not see the value of reversing the graph?
And the article in that respect was very unclear in the way those square information boxes will appear when running the Diode tester program.

And the article should have explained that only the positive going plotted curve (the top right quarter of the touch screen) can be selected by using the cursors. But it never did.

So the PE/Silicon Chip "Diode Curve Plotter" finally really works and building one is not the problem. Only not knowing that the negative part of the left bottom quarter of the plotted curve, where x and y are negative, never can be selected with the cursurs was the problem. But this Arduino tester now does work and is added to my other useful Testers.

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ALSO READ THIS! About the still not correctly working Diode Curve Plotter aka Zener diode tester the problem is even worse! According to an email I got indirectly from the author you also can easily destroy your ATmega 2560 board! It wasn't mentioned anywhere in the PE and neither Silicon Chip article, but he said that if you place the jumper on the Diode tester board apparently before programming the Arduino Board, it could kill your Arduino board because of the higher external voltage. But the strange thing here is that nowhere in the article clearly was mentioned either when you should or shouldn't place your Diode Tester Board with or without jumper on the Arduino Mega 2560 Board while programming or pre-programming safely. And they even admitted that the article should have mentioned this more clearly. Although I explained the persistent issue with the always there and always empty second square box on the bottom screen right and the not working cursors in the left bottom curve of the negative x- and y-axe going graph, still no plausible solution to solve the issue was given. So better avoid this project and do not build this lame Diode Curve Plotter! You will be very disappointed. The free Practical Electronics/Silicon Chip Arduino software download comes with a price because it just doesn't work as was presented!

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Thanks Andrea! I'm almost ready with finishing the in the article mentioned Arduino Diode Curve Plotter. A few ordered parts are still in transit but the 2.8 inch LCD touch screen and the software already work splendidly! I only needed to run the Touch screen calibration.ino file to change the wrong configurated Touch screen when the original x and y values in the program were used. And after finding and changing the by the program give right x and y values into the original Zener Diode Tester.ino program it is working correctly! Everything about doing that can also be found free on the PE online webpage in the software download for this tester.

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Congratulations Albert, always better and more practical ...

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Gracias Jorge. Buena suerte construyendo herramientas Arduino.
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Thanks Jorge. Good luck building Arduino tools.

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