I have two micro processor in which I need to connect both of them to the same single pole switch which is grounded on one side. One of the micro processors is sitting at 12v while the other is sitting at 5v. I just cant connect these two inputs together and than connect them to the grounded switch...can I?
connecting them together may cause damage or premature triggering. What circuit should go between them? They are just inputs so there should only be a few ma of current. I was considering a diode between the inputs but thought the voltage drop across the diode might cause issues.

The current circuit is very similar to the one attached.

 

That arrangement will cause a bit more current to flow from the 12V source than from the 5V source. You could increase the 10K resistor to reduce the current. Otherwise I don't see a problem, but I must admit I don't quite understand why the micro is sourcing current at its input pin?

 

I think diodes would be better.

 

What is the "logic low" level?

 

I think diodes would be better.

Do you mean something like this:

 

What is the "logic low" level?

Im a bit confused...when the switch closes the level is 0v?

 

No, just the diodes.

With the resistors, and switch "off", the 5 volt input will see 12 volts. Maybe that's OK.

A diode on the 5v is all you need, as it would be biased off with switch open.

 

OK so like this:

 

Im a bit confused...when the switch closes the level is 0v?

I mean will it be considered "low" at .6 volts. TTL was 0 to .8 volts.

Your input specs may be different.

 

I mean will it be considered "low" at .6 volts. TTL was 0 to .8 volts.

Your input specs may be different.

The 5v microprocessor is an Arduino Mega...I'd need to look at the specs for it

 

looks like a low is 0-1.5 and high is 2-5

 

OK so like this:

Except no resistors.
But it only works if .6 volts will be seen as "logic low".

If not use a diode with very low forward drop.

 

looks like a low is 0-1.5 and high is 2-5

Good to go then.
One diode on the 5v leg.
Another on the 12v can't hurt.
Also, the diodes must have some "keep alive" current thru them.
Are there pull-ups?