relay overkill

Hi, I am working on a little project and need some advice.
I am using an arduino (probably a regular one, but possibly the fio) and the plan was to use a relay to close a circuit on another piece of electronics. The electronics are 12v, and obviously its important to keep the 2 circuits separated. The circuit in the electronic has little or no load. It's very similar to the button sketch for arduino, except 12 volts.
Originally I was going to use a relay, but as I started to lay out the schematics I realized it was over kill. What do you suggest in its place?
Darlington, opto, something else?

Thanks,
Jim

and obviously its important to keep the 2 circuits separated.

Why?
If you have to keep them totally separate use an opto isolator.
However if you can have the grounds common then you can use a transistor or a FET.

Ok, Opto-Isolater it is.

The arduino part of the project must be portable and separate from the main piece of electronics, that I own and stays with me. The main piece of electronics is rented from various venders and is quite expensive, it has a port that my project connects to. So yeah I want to keep them separated.
Thanks Grumpy Mike.

Jim

The main piece of electronics is rented from various venders and is quite expensive, it has a port that my project connects to. So yeah I want to keep them separated.

If the rental device shows that you just need a switch (with no pull-up resistors or anything like that), then a relay is probably best because a relay is just an electrically activated switch. You may not need the isolation, but you would like to replicate a switch.

The fact that it’s an expensive rental is not an indication that you need ground isolation. i.e. If you rent a P.A. system, you can plug-in your own microphone and there is no isolation.

And no matter what you do, there is still a hard-connection between your circuit and the rented equipment… If you use an opto or a relay, one side of the opto/relay is still connected to their equipment.