Sorry if this was unclear,
I want to turn ON/OFF the whole board, not individual relays (which are in my case controlled by 2x 595 shift registers)
This is the relay board (on which there is already 2x ULN2803, Magician
And I want to put a switch controlled by the Arduino, between the power supply and that screw connector (in the front of the picture)
pito, I have read that I shouldn't use PNP transistors because:
for a PNP transistor, active high becomes active low, meaning the transistor will switch on when you apply 0V from your Arduino, but it won't switch off when you apply 5V from the Arduino. You'll need to apply 12V to the base of the transistor to switch off
Is the BC517
a good choice?
Firstly you need to answer my second question:
"If the former then how are the individual relays switched?"
The point is if you remove power to the relay board and something is still driving its individual inputs, you may damage
the board (depending on how its inputs are designed). Alas just providing a photo doesn't give useful information - a link to the
datasheet is what is needed.
I'll assume you can arrange to remove individual drive to its inputs before powering it down for now.
You'll need to do high-side switching of the board (since grounds need to be common), which means
a PNP transistor or a p-channel MOSFET. You'll also need a NPN small signal transistor to level-convert
upto the 12V rail.
Look at the diagram in posting #24 of this thread: http://forum.arduino.cc/index.php?topic=130384.15
Choose a p-channel MOSFET with an Rds(on) of 0.05 ohm or less and you'll cope with several amps no problem. The NPN
can be anything small in that circuit.