using 4v to switch12v ign live

Right MrI ive checked out which side switches on the LED-its the 4v positive,the ground remains constant.when the Soundlinx is in the changer position the voltage at the LED drops to .1 of a volt.when in the car kit position it goes to the 4volt mentioned-hope this helps.

Actually you can use a 12 volt relay.  LED positive wire to a 1,000 ohm resistor.  Other end of that resistor to the base of a TIP3055 transistor.  Radio Shack part number 276-2020  collector of the transistor to terminal 85 of the relay.  Emitter of the transistor is connected to ground.    You must insulate the tab of the transisitor somehow.  You should not need a heatsink.  But you should monitor it closely at first to see what kind of heat buildup you get.

Left leg of transistor = base.  Center leg = collector.  right leg = emitter. 

apply power to terminal 86 of the relay.

If you already purchased the 5 volt relay wire it up as follows.  LED positive to a 100 ohm resistor.  Other side of the resistor to the base of the transistor.  Collector of the transistor to 12v.  Emitter of the transistor to the coil of the 5 volt relay.  Ground the other coil connection of the relay.

Hi Mr i-got the components well sort of-was wondering if I can use a reed relay for this?also with the transistor which way round do u view it for the poles-its flat with a metal backing to 1 side (heat sink?) and finally from what ive been reading on the net will I need a diode in there somewhere to stop the relay coil damaging the transistor.thanks for your help uve been brilliant-sorry for asking more question but I just want to learn.

With the lettering of the transistor facing you, and the legs pointing down, the left leg is the base, center leg is the collector, and the right leg is the emitter.

If the transistor was a smaller one, you may need the diode.  The transistor you are using is able to handle the relay backlash without any problem.

How much current does your device draw?  What is the current capacity of the reed relay?

Yep have an Ohm meter ( if I can remember how to use it-lol) The relay has 4 connections 3 gouped at one end in a triangle and the 4th at the other,imagine a dot to dot arrow-the two outer poles of the triangle are the coil the point of the triangle and the 4th at the other end are the switch- hope that makes some sense.