Circuit diagram for Oak Level Shifter Shield Kit

[werner]

I received the Oak Level Shifter shield kit as part of my KickStarter rewards.   But I cannot seem to find the circuit diagrams and/or build instructions anymore of the digistump website.  Are these available yet?
         

[exeng]

Warning: "THIS TUTORIAL HAS NOT YET BEEN REVIEWED, PLEASE DO NOT USE UNTIL IT HAS".
The build instruction for the level shifter is here http://digistump.com/wiki/oak/tutorials/levelshift but it is the initial draft and subject to review and final blessing from Erik. Also, while photos have been provided, they are not link yet. The DokuWiki for this has the disclaimer at the top that says "THIS TUTORIAL HAS NOT YET BEEN REVIEWED, PLEASE DO NOT USE UNTIL IT HAS".

I suspect the Erik is working on more pressing things than reviewing and releasing shield tutorials at this time. You are welcome to review the tutorial but if you use, it would be at your own risk. I built two Level Shifter Shields and I can say that it is a little more involved than some of the simpler shields. Your comfort with your soldering skills and patience (ability to proceed methodically) should be a guide for proceeding without the approved instructions. Part placement top or bottom, part orientation, multiple resistors - multiple values, thermal considerations (transistors), order of placement, decisions about header types if you plan to deviate from what is provided, some tight quarter soldering (fine tip required) are some of the things you will run into. There are also some things to be aware of when powering the level shifter (i.e. the 74HCT245 voltage specs).

As for the schematics, I haven't seen one. I ohm'd out a few connections that I was curious about before building the board. If you do review the tutorial, I would love to get feedback.
exeng

[werner]

Thank you very much.   By the way, there is something wrong with the images on that page... none are showing.  I realize  Erik is probably swamped by a huge backlog to catch up with and he has my sympathy.  But its probably worth fixing that as it will probably affect many people keen to get started.

[exeng]

The images are not linked yet. Once Erik (Digistump) reviews the doc and blesses it the images will be visible. Yep, I'm sure Erik is working on higher priority things right now.

[PeterF]

Hi exeng,

Tutorial looks ok. Two points - since the 74HCT245 is not an automatic bi-directional shifter, I'm presuming the DIR pin (pin 1) is bound to one of the Oak's pins... so would you be able to check which pin it is and document that? I'm also about to add a note that to use bi-directional protocols such as OneWire, you'll need to use the I2C SCL & SDA pins, as they are run through a bi-directional mosfet shifter. However, someone needs to test if this actually works on that board (which I can't as I don't have one ) So that may be a bit 'iffy'

Pete

[exeng]

Yep, was made aware of the non-bidirectional aspect of the Level Shifter by Erik when I had arbitrarily selected P5 (convenient screw terminal) to connect a OneWire device to and couldn't get it to work. Sure be noted in the tutorial.

I don't have a schematic for the level shifter but Ohm'ing it out reveals that DIR (Direction Control) is connected to VIN (also to VCC on the 74HCT245). So it's important to note the 5V recommendation (max limits of the 74HCT245) when powering the level shifter. The 74HCT245 is configured DIR -> VIN, QE (active low) -> GND which means An is [input] and Bn is [B=A] (see 74HCT245 datasheets). BTW, all of the 5V labelled pins on the level shifter are connected to VIN.

Sometime today or this evening I will try my DS18B20 device again using the level shifter pins that you suggest are bi-directional. Stay tuned...

Steve (exeng)

[exeng]

OK, Ran my D18S20 test sketch with the level shifter on both pin 2 (SCL) and pin 0 (SDA) successfully. So these I2C pins must be bi-directional and will support OneWire devices if a level shifter is being used. That would only be necessary if you needed to attach other 5V devices as the DS18B20 can operate at 3.3v

Attached the particle.io log for each test.

[PeterF]

Thanks for that Steve! I can remove the 'has not been tested bit'

I expected it to work as I've seen mosfet shifters before, and if i2c works, little reason to expect OneWire not to... but not good to commit to the helper docs unless you are sure!

Yes, the voltage limits have to be made clear in big bold flashing print... and I think you pointed it out pretty clearly in the wiki page... I might just rephrase that a bit as I got a bit lost reading it...

Pete