I have an existing circuit design needs a simple indicator LED which is then going ON when the output of the ULN2803 has output respect to the +VS.
Initially I ran a test just hooking up a LED with a current limiting resistor at the output of the ULN2803. It seems to be OK. It turns ON and OFF as per the input from the optocoupler. But, I don't have a test load with me so, I am not sure what concerns could happen if I connect one. The load is essentially won't be more than a few hundreds mA.
Here is the schematic piece;
My main concern is the load's self. Maybe it is not in my case but I have to think about it anyway. I have two concerns come to my mind (you can add more if you see more);
Concern #1. How can I prevent a current leakage when the load is connected? If this happens then the LED will lid at least a bit so it would be annoying. If so, what would be the solution for that? Perhaps, increasing the resistor value according to the load's effect? or placing the LED somewhere else, maybe at the input of the ULN2803?
Concern #2. Should I worry about a possible impact paralleling two resistors (LED circuit and load)? If so, again, I should maybe move the LED somewhere else, but, it will be then the same effect goes at the new place like the input of the ULN2803, won't it?
+VS = +24V LED: 0402 smd package, 20mA, 2V Forward voltage
Any input greatly appreciated.