I'm looking to use an arduino to control some LED solar garden lights. I want to keep the lights as they are, just replace the rocker switch so they can be controlled by the arduino board. I want to keep the circuits separate so no voltage from the arduino is used for the lights as they already run on their own circuit and it would make my design more adaptable. Is there a type of switch that i can use to do this? I think the best way would be a relay circuit or somehow controlling the resistance, so when the light is to be off the resistance is set to high, when the light is on it is set to low.
Is there a type of switch that i can use to do this?
Leave the switch in place. Add an opto-isolator in line with it.Turn the switch on. The Arduino can then turn the opto-isolator on and off.
If there a way of using a relay? looks the best option to me, ive seen it used in similar applications but instead of for little 1.2v lights like mine, for a full home lighting system.
if anyone has suggestions what relay i can use for my little 1.2v lights it would be greatly appreciated or a schematic for my own circuit. I've found these but I'd imagine they're a bit overkill for my lights, but would it still work?
http://www.ebay.co.uk/itm/1-Channel-5V-Relay-Module-Shield-Board-f-Arduino-ARM-PIC-AVR-DSP-ARM-Electronic-/111252767598?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item19e72e836e
My current design consists of an arduino mega 2560 with a sensor shield, PIR sensors and a light sensor. Just got the lights to connect and switch using the results of processing my sensor inputs.
You realise of course, that the Arduino will consume more power than all of the solar lights? 
And a relay, more power still?
Yeah Ive found i can use transistors to replace the switch.
Hi, what level of University are you? What course?
If you can use a transistor then you can use an opto coupler and still have isolation.
If you can program and assemble, I can't see why you can't do a parametric search on E14, Farnell or RS. for your relays.
Tom....... 
opto isolator is the way to go.
histo:
opto isolator is the way to go.
Well, no for all the replies referring to opto-isolators, because there is actually no requirement for isolation as the project was posed, and the current to power the LED in the opto-isolator is an unnecessary waste.
{The OP was indicating a desire to use separate power sources for the two parts, but there is no reason the grounds cannot be commoned.}