Yesterday I was experimenting around a little with the Darlington Driver 50V / 500mA Transistor array chip model number ULN2803A. I wired up a simple circuit on my Bread Board with 2 mini Songle 12V 10 Amp relays, some resistors, micro tactile buttons, wires, a single red LED and the project 2 LED flasher circuit that came with the starter kit I got.
Now this is really simple setup using the Darlington Transistorarray chip in that my micro buttons (in this case) are switching on a stepped down 9V current through the resistors to simulate a low current input to the Darlington array chip. This is ideal since I intend on using an Arduino to control input signals to the chip, and Aduino uses a low 5V output, same as a USB device. On the other side (Outputs) of the Darlington array chip I have the necessary voltage needed to trigger the magnetic coils of the mini Songle 12V Relays and as you no doubt already know that on the switch side of the relay I can have anything I want to activate up to 10 Amps, which is a lot of current. I can use higher rated Relays if I need to power anything that requires more than 10 Amps.