Switching LEDs off of CANbus Signal

Thread Starter

scienceofdiagnostics

Joined May 13, 2019
3
I am working on building a j1939 breakout box and want to add LED indicator lights to Can H and Can L lines. I am trying to figure out how to fully switch a 12v LED on/off based off the 2.5v-3.5v signal change or 1.5v-2.5v change.

I believe I could use a transistor to accomplish this but cant quite wrap my head around the actual circuit design.

Any thoughts or advice would be greatly appreciated.
 

bwilliams60

Joined Nov 18, 2012
1,343
Just out of curiosity, what would the purpose of this circuit be? Active communication takes place at 500,000 bps and not sure what relevance an LED would have? Would a scope not be a better visual indicator of what is going on? Just curious.
 

Thread Starter

scienceofdiagnostics

Joined May 13, 2019
3
Just out of curiosity, what would the purpose of this circuit be? Active communication takes place at 500,000 bps and not sure what relevance an LED would have? Would a scope not be a better visual indicator of what is going on? Just curious.
So a breakout box would just go between whatever scan tool/data link you are using and the machine. It allows easy access to the pins via banana plug with no fear of accidental damage to the j1939 connector pins. This makes troubleshooting a no communication or network issue quick and simple as you can leave your scope and diagnostic tool hooked up at the same time.

The LEDs are just a super quick visual indication that there is some bus activity, not so much whether or it is healthy. Say you are wiggling wires down the machine, it's easy to see the LEDs from a distance to see if they are stop blinking indicating an open for example.
 

Thread Starter

scienceofdiagnostics

Joined May 13, 2019
3
So you are looking to build something like this:

OR this
https://www.amazon.ca/OTC-3415-CAN-Test-Box/dp/B00A6AS3XU
That is correct. I actually have the Pico box, the OTC, and the AESwave SmartBob: they all have their specific pros and cons per job. They are all for j1962 though.

I think I have it figured out at this point though. I was originally going to use NPN transistors but I ordered some TL431 to use for voltage comparator circuit to switch the LEDs accordingly.
 
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