I am puzzled from the start. Your title says "Open-collector Arduino...", yet you show the base of the transistor taken to ground via the 10K resistor. Surely, with an open-collector output, you need a pull-UP resistor on the base which the Arduino output pulls DOWN when activated. Could this be the cause of your problem - unstable input voltages on the transistor base?
Correct me if I am wrong, but I thought the Arduino had Totem Pole Output?Ardiuno gives + 5V output so there is a pull down resistor
Correct me if I am wrong, but I thought the Arduino had Totem Pole Output?
Active Pull-Up to (+) Bus and Active Pull-Down to Ground
You need to measure the Voltages at the Input Pin of the PLC
when the Arduino Output Pin is ON, and then OFF
to discover what is wrong ...
I am also interested in how the Arduino Ground and the PLC Ground are tied together?
"Common Point" where ?
OK, I see your post regarding Max Reverse Voltage on LED.
So, you were operating the PLC Input in the "Gray Area" ...
That is "better", BUT read this ...
Then it's not an open-collector output! By definition, "open-collector" means that the collector of the (NPN) transistor is left open. Thus in order for it to do something, it needs to be connected to a load which is tied to the +5V rail. Then, when the transistor operates, it pulls the lower side of the load to ground and current flows. If you connect nothing to the collector except a resistor to ground, nothing will happen - you will always have 0V on your output!Ardiuno gives + 5V output so there is a pull down resistor
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