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Noob question isolating homemade programmer
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Fixed_Until_Broken:
LOL you are correct in that assumption Ian.m My first thought about this a couple days ago was to throw some little dip switches on but then thought no that is lame especially when the arduino has plenty of pins to drive something. Plus now I get learn something new! 
Fixed_Until_Broken:
I think I got the resistors right. I put a 10 ohm at collector so that way it shouldn't be able pull enough current to damage the transistor. Is the attached picture how you would do it? I don't have the parts yet to test it on the bread board.
David Hess:

--- Quote from: Ian.M on March 07, 2020, 05:13:09 pm ---You probably don't need to switch the ground.  You can switch target power (Vcc) by using a low Vgs threshold* P-MOSFET driven by an Arduino pin, but protecting it is harder.  Use a beefy MOSFET and a polyfuse of no more than 5% of the MOSFET's current rating and it will probably survive being shorted.
--- End quote ---

That will not prevent a short if the IC clip is rotated 180 degrees allowing the body diode to conduct; isolation on power and signal pins must work with both positive and negative voltages.
Ian.M:
Yes.  That's why I was advocating 50mA polyfuses in all signal lines, with Schottky clamps on the programmer side and another appropriately rated polyfuse in the Vcc line.

You CAN'T prevent idiocy, and when you attempt to prevent the resulting damage, the world invariably comes up with a better idiot, that does something so stupid no-one anticipated it.  The best one can hope for is to prevent damage as often as reasonably practical, and when you can't, limit the cost of the damage.
Fixed_Until_Broken:
Gotta love amazon free next day I got the transistors in I am going to play with them and do some testing. I still haven't ordered the diodes yet because not really sure what a good value to start at would be.
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