I'm working with an Arduino Due and four 5V TTL pressure sensors. I'm using this logic level converter to convert from the 3.3V Due to the 5V sensors. I've been having problems using all four at a time, so I broke out the oscilloscope and noticed a voltage level problem. When I have one device plugged in, logical low is ground. When I have two devices, it creeps up a bit. Three devices, it creeps up to around 2 volts. When I have all four devices plugged in, logical low is around 2.5 V, which is above the logical low threshold for the Due, and thus, nothing works. Does anybody have any ideas on how to fix this?
EDIT: forgot to mention, when I say "logical low," I mean on the TX line. The RX line is fine.
EDIT 2: Added link to data sheet in the post instead of in the comment, and added more detailed schematic, if it helps. All sensors are hooked up the same way as the in the detailed schematic.
Here is a cartoon-y schematic:
Slightly more detailed schematic:
Best Answer
If I understand your problem statement correctly, you are saying that when you try to transmit a logical low from at lest one device to the Due, the level converter only manages to pull the Due RX wire down to 2 Volts?
What does the RX wire look like on the other end (5V end) of the converter?
I would expect something like this to happen if the sensors are using too strong pull-ups, or if they are not intended for fan-in, and thus all try to transmit a logical high when one of them is trying to transmit a logical low.
If that's the problem, I would attach each of the sensors with a diode, Cathode towards the sensors, such that the sensors can only pull the line down, not up. Then I would use a pull-up on the common end (anodes) of the diodes, which you connect to the level translator input. Perhaps a 2.2 kOhm pull-up would be appropriate (giving about 2.5 mA current through the diode when pulled low.)
Also, did you read the top paragraph of the description you linked to? The part that says: