how do i trigger a 12v lamp with 2v?

Hi, I'm new here and found the site searching via google. 

Car:  1987 Buick Grand National

Alarm:  Viper 791XV

Issue:  I have an aftermarket instrument cluster.  In it, there is a 'security' lamp that I would like to have illuminated when the Viper alarm is activated.  Currently, there is a small LED coming off of the 'brain' of the Viper that illuminates when the alarm is activated.  I called DEI and they told me that there is 2 volts going to that LED when the alarm is activated.

I would like to know if it is possible to use that 2V to trigger a relay into supplying 12V to illuminate the 'security' lamp?  Or, would it be better to find another LED and housing that would fit into the slot where the 12V lamp is on the cluster?  The 12V lamp housing is about a 3/8" diameter. 

Thanks, I tried the search but the closest thing I could find was the low voltage trigger post and that really didn't seem to apply to me.

I believe the DEI alarm switches the (-) side of the LED, so you would attach the base of a 2906 PNP transistor (or similar) to the (-) LED output.

Connect the Emitter to 12vdc and connect the collector, through a resistor of some sort (depends on current, but a 1/2 watt 470ohm resistor would be a good starting point) to the light.  Depending on the resistance of the indicator light (assuming it is incandescent) you may be able to drastically change the resistor value or even eliminate it.  The point of the resistor is to prevent more then 200mA of current to pass through the transistor.  However, since you are just driving an small load with a short duty cycle you can get away with a little more abuse. 

Emitter being the 'security' lamp I want to illuminate?

Collector?

I looked in the glossary of terms on the left, but I didn't see any of those explained.  I am not an expert by any stretch of the imagination so layman terms please. 

[quote]Depending on the resistance of the indicator light (assuming it is incandescent) you may be able to drastically change the resistor value or even eliminate it.  The point of the resistor is to prevent more then 200mA of current to pass through the transistor.  However, since you are just driving an small load with a short duty cycle you can get away with a little more abuse. [/QUOTE]

I guess I'm having a hard time visualizing how this all gets wired together.  Is there a diagram of what this should look like somewhere? 

I appreciate the answer, I asked a couple of alarm shops if they could do this; one said flat out said no and the other said he could but it would probably take him a few days and cost me a days worth of labor which seems excessive to me.

I would be afraid that the output is current limited and most likely won't have enough juice to power a tradional relay.  You could go with a reed relay that will use significantly less current, but at that point you might as well just get a transistor and be done with it.

A basic transistor (like the 2N3906) has three pins on it - a base (trigger input) an emitter (switch in) and a  collector (switch out).  Assuming the (-) LED output is current limited then you can connect it directly to the base pin of the transistor.  Then, connect the emitter pin directly to 12vdc.  Now, you'll have one pin left on the transistor - the collector - this is your output.  Connect it to the lamp, through a 470 ohm 1/2 watt resistor. 

Like I said, you could most likely get away without the resistor if you wanted to - but I would make sure the light in your dash is incandescent and not an LED.  If it is incandescent it will act as a current limiter.

I appologize for not being more clear!

It's a standard automotive lamp, not an LED.  On the lamp itself, the collector would be supplying the 12V to the lamp?  It sounds pretty easy...just gotta locate the parts now. 

You should be able to get all the parts at Radioshack.

You are correct, the collector will output 12vdc when a ground is applied to the base.

Alright, I'll check out Radio Shack this weekend and hopefully be able to collect all the parts to wire this thing up.

Thanks for the help and I'll let you know how it goes.