Hi All,

Hoping that someone can put me out of my misery. I have been searching the internet for the past 5days for an answer to this question and feel like I am starting to lose my mind. Can anyone explain why the attached schematic has a 1N4148 on each of the SDI_5V, SCLK_5V and CS_5V lines? The only thing I can think of is that it must have something to do with level shifting between the 3.3v Max31865 chip and the 5v Arduino that it is meant to connect to.

Full disclosure I am planning on having boards made as I wanted to try out SMD assembly and thought this would be a fun project. I originally started looking into this to assist in selecting the right 1n4148 component however now also want to know what they are doing in this design and how/if/why i would incorporate them into future SPI module designs.

 

Sorry team, I finally found the answer I was looking for on the adafruit forums. For anyone looking for the same information this link explains the function of the diodes.

https://forums.adafruit.com/viewtopic.php?f=22&t=172165&p=840379&hilit=max31865#p840379

 

Here’s how it works

 

Here’s how it works

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Thanks Wolframore, that really helps make it even clearer.

What difference does the forward voltage on the diode make in this schematic? Intuitively it feels like I should go for as low as possible but does it really matter?

 

1. as long as we get TTL low it doesn’t matter in this case.
2. I had to find a version I can read to get closer... your low will be closer to 0.4v. With 10k pull-up your current will be closer to couple hundred uA. It’s below the chart that datasheet gives.

I like the design because of low part count.

 

1. as long as we get TTL low it doesn’t matter in this case.
2. I had to find a version I can read to get closer... your low will be closer to 0.4v. With 10k pull-up your current will be closer to couple hundred uA. It’s below the chart that datasheet gives.

I like the design because of low part count.
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Thanks again Wolframore. Sorry for being a bit of a noob here but we are right at the edge of my knowledge here. Which datasheet is this info on? I think I am getting a bit confused when I look up 1N4148 on Digikey or RS-components due to the huge number of variations. I am not sure I understand how to differentiate between a variation that will/won't work.

 

The data is from Vishay: Vishay 1N4148. There are many manufactuers who make this part, it’s a standard. Unless you have some special requirement that demands a careful study of all the variations I would order the least expensive.