Make Arduino show message when mic is live

I'm working at a radio station and we are looking to replace our current mic live system with an Arduino which displays "Mic live" on an LED message board when the mic is active. The mic live signal is sent via a mixer. Firstly, would this be achievable using the arduino, and if so, can someone explain how I could make the Arduino do something when it gets a signal from the mixer?

We need to know what form this signal is in.

You might have to do something to it to allow the digital system to read it safely.
Then it is a simple matter of using an if statement to determine what message you want to display.

If you can, post back the model of sound board you are using.

I think its a 3v control signal. I usually work on more the computer maintenance and web side of things at the radio station so I'm not 100%. We use an AIRLAB MK-2 if that helps.

You need to measure it and not just guess.
If it is in the order of 3V then you can simply connect it to an analogue input and read it.
Then compare it to a threshold, a number half way between an on and off reading to trigger your message. You also need to connect the ground of the arduino tot he ground of your mixer.

Manuals are wonderful things!

http://www.d-r.nl/Downloads/Airlab-Manual%20V2.03.pdf

You have two different options. Each mic line has its own "on air" indicator. This is on the same TRS connector as the cough/com button. It would appear to be a 6mA current driver. It is meant to drive the coil of a relay or drive an LED directly (no resistor). The open circuit voltage is not specified, but 3.0 V would not be unreasonable. If you wanted to you could place a red LED from tip to sleeve and read the voltage on the high side, and you should be guaranteed about 1.8V by the LED. You could also use a resistor around 860 ohms in place of the LED. If you wanted to be really safe you could probably directly drive an optocoupler with the on air signal and feed its output to your Arduino. Alternatively, the open circuit voltage might be very predicable and fine to feed in directly, its limited to 6mA, so you can't do too much harm.

Option two is to use the main on air outputs (pg 35) that are for everything instead of individual channels. These are rated at 24V open circuit voltage and 50mA short circuit current. This will cook an LED, but a simple resistor voltage divider will get that down to Arduino friendly territory easily.

There are of course many other ways you could choose. You could use a relay since these channels are made to drive relays, but it seems a bit excessive for this application.