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[Reply #15 on:]

Dave,

Here is my theory about use of the 125.  I modified an Adafruit kit using a 4050.  The 4050 is in a 16 pin DIP with two NC pins. I got it to "fit" by soldering wires to the pins and letting it float upside down above the board.  I convinced Limor at Adafruit that the mod solved SD reliability problems.

The 125 is in a 14 pin package and it just fits in the space where the resistor level shifter was so the rest of the board needs no mods.  I suspect that was at least part of the reason.  Only Limor knows why she used the 125.

It had to be a DIP since it is a kit for beginners.  Only three level shifters are needed but I couldn't find a simple part like the 4050 with fewer than 16 pins.  Do you know of one?

Thank you for this most useful info. I will try to use the 4050 in my further designs and see how it turns out. The things is that I am in a poor third world country where certain digital ic's are hard to come by, hence my advocacy of resistor based shifters. I will order some 4050's from ebay which i can use for my future designs. I also think that keeping the wires short to a few inches (3-4 inches max) might improve the stability of resistor based shifters for ppl who need to use them. It's important to know that the SPI bus frequency is 8mhz and any extra length of wire can lead to stray cpacitance!

[Reply #16 on:]

...I got it to "fit" by soldering wires to the pins and letting it float upside down above the board...

The official technical term for that is the "dead bug" technique.  I use a tiny spot of hot-melt glue to stabilize it.  (Hot-melt glue is the electronics assembly equivalent of duct tape.)

....simple part like the 4050 with fewer than 16 pins.  Do you know of one?

I don't know of any.

Note that I don't actually object to the use of a 3.3 Volt part that is specified as "5 Volt tolerant," and I think that there is always a reason that people do things.  I was just wondering...


Regards,

Dave