A relay is an electro-magnetic switch which is useful if you want to use a low voltage circuit to switch on and off a light bulb (or anything else) connected to the 220v mains supply.
The ULN2803 is a small integrated circuit that contains 8 transistor driver channels. Each channel has an input to a resistor connected to the base of a transistor and a 1 amp open collector output...
The ULN 2003 and the ULN 2803 are similar devices. The 2803 was the more popular choice because it required fewer external components. The ULN2803 is a small integrated circuit that contains 8...