johnwasser:
The datasheet seems to indicate a 2.7 kHz square wave appears on the 'buzzer' pin when a card is present. It is not clear if the signal continues until the card is removed. If it does, you could use that signal. Is there a LED on the board that lights up (and stays lit) when a card is present? You might be able to tap into the signal that drives the LED.
There is an onboard LED, but it only flashes once a card is first detected.
Grumpy_Mike:
I would say not. This is not a normal function of a reader, it could be done but is hardly ever implemented. I once had a customer request to do this, and I did it but it requires custom software inside the reader. As I was at a company that designed and made readers this was not so hard. But in my experance this is not normally what is wanted.
And what about a while loop with some sort of delay? say every 5 seconds The loop tests to see if that card is still "available" and records a time stamp of sorts.