and not even have gone down this path!

This type of problem has been discussed several times on this forum, and, as you can see, others are aware of that fact.

The obvious answer to his intial question, " Can I drive a 12 Volts 400 Ohms relay (like LEONE SC5-S-DC12V OR GOODSKY RW-SH-112D)by a '51 microcontroller working at 5 Volts? " would be a resounding "NO!"

He certainly needs buffering, but then he's shown that he did figure that much out.

The previous occasions on which this has come up, some sort of relay driver was often recommended, and, if he has only one such relay to drive, then one of the little TI or NSC relay driver ic's in an 8-pin package would have sufficed, wouldn't it? They even offer high-side drivers that can still be driven from a 5-volt MCU. All that would have appeared in a simple search. Unless he's going into a production situation where he can save $2US per 10K units by using discretes, I would not recommend such a solution. Discretes are for people who know how to use them, and, in today's market, don't offer much saving.

A search would have shown him a wide range of open-collector or open-drain solutions that would have reduced the problem to a $0.25US device and a pullup resistor.

I guess my view is gradually becoming jaded as I see, more and more, the posts from people who won't even do the most basic things to look for a solution.

That may, in fact, not be the case here, but I certainly don't see any evidence that a search was attempted, as there should, at least, have been a discussion of why a relay driver was inappropriate.

RE