We want to use the Rockwell Photosensors 42JT-F5LET1-A2 for the '24 game. In looking at the datasheet , it seems they can be configured for the output to be only a Low Side Drive ( NPN) instead of the Push-Pull that is typical. A low side drive would allow us to use the internal pull up in the RoboRio digital I/O without anything like a logic level shifter (from 12V to 5V ) on the sensor output. Yet no where in the datasheet can we see how to accomplish this low side drive configuration. Going to Rockwell’s website for technical support seems to complicated as they want a company name and other things not germane to a robotics team. Can anyone offer advice on this sensor?
From what I gather, this sensor is indeed a push/pull sensor, and there is no option to put it into an NPN open collector only configuration as you so desire.
That said, my recommendation for using this sensor with a Roborio would be to use an opto-isolator, and power the sensor from 12V on a VRM, and power the output side of your opto-isolator from 5V from a VRM, and you should be all set. You might also want to give the opto-isolator a ground reference directly from the Roborio to prevent signal noise.
Don’t use the same VRM as powering your radio, if you power your radio from a VRM.
This opto-isolator board should work nicely. https://www.amazon.com/NOYITO-4-Channel-Optocoupler-Photoelectric-Converter/dp/B07TCVTD2K/
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A simpler, but arguably more risky, solution would be to just use a resistor divider. We’ve used many industrial sensors over the years where we need to level-shift like this and just solder a few resistors in the cable. You do have some risk of pushing 12V into the roboRIO if you lose ground on the divider, but once you start taking about failure modes, no solution looks that great.
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Thanks. We had used voltage divider resistors with Banner Sensors in previous years , but those were either NPN or PNP outputs. With the Rockwell sensors being push-pull, we needed a different solution. I think what we will try is using a small signal transistor (like the 2N3904) with a voltage divider into the base from the sensor output. If we lose ground, at best we always have a low input into the RoboRio, and at worst we burn out the transistor.
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