Using late 2009 iMac stock power supply with Raspberry Pi

I have a late 2009 model A1312 iMac 27" with a bad mainboard. I have removed the board and am going to install a Raspberry Pi with a LCD to HMDI controller board. Rather than introduce a power supply, I would like to use the stock iMac power supply. I have confirmed that when plugged into a voltage source I get 12V on pin 4. What is required on pin 12 to "turn on" the iMac power supply? I know that pin 12 is the power on request signal from the mainboard, but I don't know what voltage that signal is. Is it a ground, a certain voltage?