The Ib is around 5mA and the pi zero draws about 120 mA during bootup and settles down to about 80mA steady state. So, when I send a Hi through the arduino digital pin, the Pi starts booting - and all goes fine. However, the moment the boot up process is complete and the current draw from collector falls, the transistor switches off. Have confirmed the same by increasing the steady state power draw of the Pi by attaching it to a HDMI display - and some USB peripherals - and the pi remains up - no problem.
I am a beginner to using BJTs - and gather I am doing something stupid to land up in this problem. Any help in helping me understand what I am doing wrong will be of huge help.