I want to operate a relay with the data pin of serial port but when the relay is attached to the pin the voltage drops from 11 volts to 2 volts.The relay does not work.How can I manage the voltage drop?

 

The output of the serial port is a logic level voltage, and is not intended to drive a load like a relay. You would need to construct a circuit that will sense the level shift and drive the relay. An LM311 voltage comparator might be able to do this. It's output will sink 50 ma.

 

http://www.edn.com/archives/1996/102496/di_02.htm

This can drive motors, relays, etc up to the limits of the mosfet and power supply

 

i want to design my own circuit. idont want to use any of the available chips.
so please provide some logic that i can understand.
thanks