plzzzzzzzzz can any one tell me how to control a relay using 8051 uc. i need circuit description i.e wether to use a transistor????????? plz help me im just beginner of uc.
Any old npn transistors will do the job if you just want to control one or two relays.
Connect base to output port via 1k resistor.
Connect relay between collector and V+ wirh parallel diode (cathode to V+).
Connect emitter to 0V (ground)
ULN2803 is convenient if you want to control lots of relays or other stuff that takes more than a few mA.