I'm new to arduino, but I want to replace the crummy Trex Timer with an arduino. My plam is to build in motion and light detection. The LED's are on a 12V 5amp circuit. Is there a type of switch that the arduino can control to open and close a 12v 5amp circuit?
If you intend to only have your lights be on and off I would use a Relay.
This one Relay SPST-NO Sealed - 30A - COM-00101 - SparkFun Electronics is rated for 30A and works at a switching voltage of 5V, which a standard Arduino can provide.
Kate, you rock!
Jchaza:
Kate, you rock!
Aaaaaaaaaaaaaa? No sorry not a woman.......
No big deal just thought I would point that out.
Keep in mind an Arduino is a controller, not a power driver.
A relay may use little power, but in most cases it's far too much to handle for a micro-controller.
Arduino Playground - HomePage shows a schematic how to control a relay safely.
I do wonder what the circuitry of the Led system looks like now. A relay as switch is a good safe choice, but indeed gives no possibility to dim the light if you can't already. Dimming may... also be possible using Mosfets/the right transistors instead.
By the way, since I "tease" some of the small rascals in the streets with the wrong sex occasionally, that goatee in my avatar has grown a lot by now, I don't mind you calling me bearded lady 
For switching the LEDs I'd use a MOSFET (e.g. STP40NF10L). The gate of the STP40NF10L can be driven directly from an arduino running on 5v, and it has a very low on-resistance, so it should run cool. You could also use a Darlington driver (e.g. TIP102), but it needs to be driven from the arduino via a 1K series resistor, and the device drops up to 2 volts, so it's going to get warm and will need a heatsink.
It is best to drive the gate of a FET through a 100R resistor so that the peak current out of the pin is kept below the damaging 40mA level. When switching on gates look like a discharged capacitor and those initially look like a short circuit.