PICKit 4 - An easy fix for the bad button design - Page 1
Forum section: electronics / general

author: scopeman
author_rank: Frequent Contributor
timestamp: September 16, 2021, 12:26:47 am
content: Well while working on the PTG issue (Programmer-To-Go) I came across a problem with the novel push button design on the PICKit 4.The problem is you could push as hard as you wanted and you would hear a click but the switch contacts never closed. Once in a while if you applied pressure in an exact spot the button would work but only sometimes. As you can imagine this was the first time I got the PTG working and it was flaky I thought that I had some other problem with the target I was trying to program, as the switch would click every time.Taking the unit apart and testing the push button with the board out of the housing the button worked and the contacts closing perfectly evey time. So I put the acrylic light pipe in place and when I pressed down on it the switch would click but no electrical contact was made.The problem with the arrangement is the button for the switch needs to be pushed slightly below or very near the height of metal frame of the switch. The bottom surface of the light pipe is flat so even pushing it down on the switch won't activate the switch. A mechanical engineering design flaw for sure!It would have been a FPF (Face Palm Fail) if Dave had reviewed it for sure.I gave serious consideration to just replacing the switch with one with a long pushbutton and drilling a hole through the enclosure cover but thatwould have been ugly and after all I just bought the thing new. Besides I did not have any suitable switches in stock.So I made a modification to the light pipe by putting a small protrusion on the light pipe so the switch would activate.Here is the fix:I used piece of round ABS plastic rod as the actuator and modified the light pipe.I put the light pipe in place on the PCB aligning the plastic guide pins with the holes in the PCB and then using a Sharpie marked the dead center of the switch on the light pipe.I then drilled a #60 pilot hole dead center where I marked it through the light pipe. I did this using a pin vise to hold the drill and drilled the plastic by hand as I had better control that way and drilling the plastic was easy.Once I had that hole I drilled a larger hole, also using the pin vise, that would allow the ABS rod to be a snug fit. Actually I drilled half way through with one drill and then finished drilling with one number size drill smaller.The plastic ABS rod that I used came from the plunger of a disposible insulin syringe. My cat (RIP) was diabetic so I had a few new ones left, they are great for dispensing solvent glue or contact cleaner.The plunger was 0.113 in diameter and I used a drill close to that drilling halfway through and I followed that with the next drill size under to finish the drilling. I used a razor blade to trim any plastic burrs flush with the light pipe surface.Once I had the hole drilled I inserted the ABS rod and adjusted it so that it barely protruded though the light pipe on the switch side.Once you are at that point you need to check and see if the switch activates properly and adjust the rod accordingly so that the switch is fully activated when the surface of the light pipe is flush with the surface of the switch or a few thousandths sooner. You don't want to have it too long as when you put the PCB and the light pipe back into the enclosure the switch will be activated all the time.When you have determined that the protrusion is correct take a utility knife with a sharp blade and cut the ABS rod flush with the top side of the light pipe (opposite from the switch side of course!).Retest and make sure all is good then take some solvent type cement (Ambroid Pro-Weld or Micro-Mark Same stuff or equiv.) and a fine brush and wick the cement around the circumference of the ABS rod. When the cement dries verify that there is no protrusion or burrs on the top side of the light pipe as anything there will limit the button travel when the unit is assembled.Now I wonder if Microchip will modify the mold for that light pipe. It won't take much. A small bump wold have done it.I did not measure mine but a few thousandths (probably less than 50) would be all it takes. It could be that their mechanical stack up is so close that a switch with the next length actuator would have been too long. In the end they should have skipped the fancy stuff and should have used the tried and true method used on the PICKit 2,I thought about just gluing a protrusion on the light pipe but the hole method makes it easy to get the adjustment correct and stabilzes the rod for the final trim if needed.I would have used a 4-40 Nylon screw and then drilled a hole and threaded it, or perhaps used some really short nylon setscrews but I did not have those handy but that probably would have been my first approach as that would allow a really fine adjustment.It takes a little time to do this but the result will be worth it as the button is very easy to push and will work every time. Now if I could just get the PTG download to work every time.I hope this helps other EEVBlog members as the problem sure was a PITA for me!SamW3OHM