Relays, Darlingtons, and Diodes

I am using this Darlington array to drive some relays

http://www.digikey.com/product-detail/en/ULN2003AN/296-1979-5-ND/277624

and was wondering if I still need to use diodes to block the back EMF or if I can safely leave them out.

If you have the relay power connected to the COM pin, you don't need them (though there's no harm in including them) - the ULN2003/2803 has integrated clamping diodes for that purpose between each output and the COM pin. If your relay is particularly large, or you're paranoid, go ahead and include a separate one.

Thanks!

Also, as far as I can tell, I can put 12V to the com pin and still control that with a 5V Arduino, correct? No extra hardware needed?

Yup.

Oh, and 2003's are one of those items that digikey is kinda pricey on. Ebay sellers are cheaper on super-common parts like this. (I'm seeing $8 for 50, $7 for 50 of the SMD version)

Thanks for your help!

I'd recommend always stocking ULN2803's not ULN2003's since sometimes 7 is not a useful number of drivers (say you are driving things via '595 octal shift registers. As far as I can see the ULN2803 was produced because they realised the ULN2003 was a mistake :)

Hi, and make sure that the E terminal is connected to the arduino gnd.

Tom.... :)