Hi Guys.
I am going to make an emergency LED system using Arduino. I have a lot of L7805cv 5v and AMS 1117 3v regulators. My question is, Can I use them to make power on off switch to a LED of 4v, 3 watt? (Can they work as switching regulator?
They don't have some kind of "enable" pin, so the can not be used to switch on and off. Use a mosfet, or a transistor, or use a programmable RGB led strip (like the NeoPixel or other).
Can they work as switching regulator?
No, they are linear regulators. But i think what you are trying to do has nothing to do with regulation. You need a switch. Use a transistor...
Oh dear! Yet again. 
Don't do this!
Give us Web links to the LEDs you want to use, how many of them, and what battery you propose to use, and how you propose to activate your emergency lighting. Then we can explain the best way to implement it.
Thank you so much for your kind support. I am going to use relay for this purpose now.
It could be done with a negative voltage but that would require
a supply and a couple more transistors as level shifters.
Dwight
vickeyhort:
Thank you so much for your kind support. I am going to use relay for this purpose now.
Actually, since the relay only draws power from the mains and does not present a load to the batteries, that is a very sensible way to do it.