ULN2803A Driver IC
This is a 50 V, 500 mA Darlington transistor array. The device consists of eight NPN Darlington pairs that feature high-voltage outputs with common-cathode clamp diodes for switching inductive loads. The collector-current rating of each Darlington pair is 500 mA. The Darlington pairs may be connected in parallel for higher current capability.
ULN2803A Applications include relay drivers, hammer drivers, lamp drivers, display drivers (LED and gas discharge), line drivers, and logic buffers. It's perfect for driving relays and can be driven directly from a microcontroller's digital pins, it has 8 outputs so you can easily drive 8 relays with this chip. Of cause it is not limited to relays, any device using less than 50V and 500mA can be driven by this IC.
The ULN2803A device has a 2.7-kΩ series base resistor for each Darlington pair for operation directly with TTL or 5-V CMOS devices.