What type of relay do I need?

I wish to drive a relay with the Arduino. Looks like I should use a transister to do this to protect the device but I'm not sure how to spec out what relay I should purchase.

The application is 18V DC that will be powering electric slot cars. This is not a high current application as these little cars don't pull a lot. I was thinking solid state but beyond that I have no idea what to look for. Advice?

As you said, you drive a relay with a transistor because the Arduino can't supply enough current to activate the coil in the relay. Relays are spec'ed based on current, switching voltage and driving voltage, so depends on your circuit. If the driving side is 5V (the side that triggers the coil) then get a 5V relay. The switching side typically can take high voltage 120V or more and it is spec'ed by the current it can pass. If it is less than 1Amp, then any small relay will work. Check this out: http://www.jaycar.com.au/images_uploaded/relaydrv.pdf

Edit: relays are also normally open (apply signal to open switch) and normally closed (apply signal to close switch) and "bi-stable" (forget the name) but you apply signal to open and then apply opposite signal to close

“bi-stable” (forget the name)

They are called latching relays

A more elegent solution for simple switching of DC voltages today is to just use a logic level MOSFET transistor, rather then a switching transistor and a relay. It's gate can be directly driven by an Arduino output pin. If you search around this forum you should find some examples of logic level MOSFET transistor switching examples.

Lefty

Thanks I like the MOSFET idea.