How do you interface a relay to a microprocessor type 89C51?

Answer 1

you cannot directly interface a relay to a microcontroller, you need a buffer circuit in between the two, you can use a NPN transistor BC 547, connect the controller pin directly to the base of the transistor (since there is an internal 10k pull up resistor inside the controller no need of any base resistor). Connect the transistors emitter to the gnd & the collector to the relay coil, the another end of the relay coil goes to the supply, check this link for complete information http://www.dnatechindia.com/index.php/Tutorials/8051-Tutorial/Relay-Interfacing.html