Need advice on microcontroller to switch relay on/off
V+ goes through the relay, into the collector of the darlington pair inside the driver, then down to ground.  A second V+ connection to pin 10 acts as a 'flywheel' diode to stop back-emf.

Connect GND to ground (anywhere), one of the outputs to the relay, and the other side of the relay to +12V.

The logic '1' signal is enough to power the base of the darlington pair to turn it on/off.  It needs no power supply of its own.

Here is the circuit built on breadboard.  The connectors along the top are:


Black: Common ground
Yellow: +5V
Blue: +3.3V
Green: +12V
Red: -12V