Don:
It is not really necessary for you to go to extreme measures to isolate a relay from your microcontroller. I would think that for a relay with a 200 mW coil that it really would not be necessary to go to the extent of using a SSR. It is often possible to get good isolation for a relay by simply using a separate power & ground connection for the relay and buffering the microcontroller output pin with a transistor or two. Since 8051 type parts like to have an active low type output to provide drive and have the pin be high in the inactive state (kind of thrust on you by default at power up) here is an idea for a relay with a 12V coil you can use. The most important things to consider are:

1) Make sure to select a PNP transistor that can supply sufficient current for the relay coil.

2) Do not forget the clamp diode that shunts the flyback voltage from the relay coil. Best location is as close to relay coil as possible.

3) Avoid making a ground loop out of the return path of the relay coil negative coonection that makes the current produce a voltage drop in the ground connections of other components of the microcontroller.



Michael Karas