i need to take what is the output for a led on a comemrcially purchased board, disconnect the led and feed that into an arduino pin, so that i can put a state indicator on an LCD and thus monitor the state of varios boards at the same time through the LCD.
no output on either board will go over 5v
Unless you share a common power source between the Arduino and the commercial board, I doubt this would work, since the current would need to flow in both boards.
As AWOL suggested what you want is called an opto-coupler or opto-isolator. One way is to take the LED out of the board and connect the LED power and ground to an opto-coupler on one side, and the Arduino pin and ground on the other. Here is a tutorial: http://arduino-info.wikispaces.com/Popular-ICs
The other way is to make your own opto-isolator as AWOL suggests. An opto-isolator basically has a LED inside of the chip, and a light sensor. Since you have a light already, you would just need to attach a photo sensor to the led, and then cover it up with tape so the only light that the photo sensor sees is the LED.
my question is two fold,
1 can i use digital read to get the state of each input. and then use an if statement show the indicator based on the digital input being over 50 for example,
A digital input only returns 0 or 1. It does not return a voltage like 50. You could use an analog input if you needed that.
With digital inputs, you need to decide whether to add a resistor to your circuit for the normal pull down inputs, or switch to use pull up switches (which flips the sense of the digital input, it would return 1 if there is no connection).
It occurs to me that you may need to sample the light several times, since the commercial board may be turning it on/off quickly to get a desired brightness.
2, what should i do to protect the arduino input pins against over voltage?