If it really is necessary to drive a relay with an 8051 type processor when the processor is on a +5V supply and the relay is to be powered from a higher voltage such as +12V, as in your case, and there is a desire to utilize low cost bipolar transistors for the the circuit elements then the following type of the circuit is a good choice.



This technique takes advantage of the 8051's ability to pull an output low to GND to fully turn on the first stage PNP. Using three stages also permits translation of the 0->5V swing from the microcontroller to the drive level for the final stage PNP. This circuit also keeps the relay from clicking for a short instant at power on time. The circuit is specific to a design wherein it is convenient to connect one side of the relay coil to GND.

Michael Karas