MarkT:
100µF is a better choice for decoupling with the original 555 as it has severe shoot-through. A CMOS version of the 555 like the 7555 is much better behaved.
Okay, I've tried with 100µF also, but it did not change the fact, that I killed the poor IRF510 somehow. :S
Anyway, I've done further measurements with a IRF520. I attached the schematic (schematic.png). Based on oscilloscope measurements and simulation comparison, the resistor used for load has around 500nH inductance. I added a flyback diode (soldered directly between FET drain and the power jack socket +), but still, I measure around 30V maximum when the FET switches OFF (555-input_OUT.png). I can't figure out where it comes from, and actually the arduino output pin gets a big kick also. Is it possible that, the the power adapter (the UNO runs from it also) simply needs time to regulate to 12V after ~0.8A ceases?
Thanks in advance.
