hi i have a gsm alarm that has outputs it says. to wire a antiparallel diode with relay coil to them.
it says when switched to high it is 5v and low is 0v when i checked the volt's on high it was only 4.7v
all i want to do is use it to switch on a 12v light

 

That diode is called a swamping diode. It is meant to keep the flyback (aka, kick back) from the coil from damaging the rest of the sensitive solid state electronics. Without a schematic it is hard to be possitive, but that sounds what it is like.

 

Did the alarm specification tell you how much current it can switch at 5 volts? You must have that information if you want to use a simple 5 volt relay to energize your 12 volt lamp. This current spec must be higher than the relay's coil current.

Sounds like when they specified the 5 volt output value they forgot to mention 'nominal'.

If you can use a relay to turn your 12v light on, make certain that the relay contacts are rated for 4 times the normal operating current of the lamp. This is the inrush current of a lamp when first turned on. If you fuse this circuit, use a slow blow fuse.

Use a common 1N4001 diode across the relay coil.

 

This current spec must be higher than the relay's coil current.

I think Jaguarjoe is referring to TWO specifications: 1) The ability of the alarm circuit to power the coil of the relay, and 2) The capacity of the relay, how big a load it can safely switch. Failing to consider either of these could lead to a problem.