hi my project is like this .I want to to implement it in a circuit.I am a noob

 

I'm confused.
Please be more elaborate.

 

I have a solenoid which must be triggered by the voltage of the led.12v is the source and the voltage to activate the solenoid

 

What are the specs of solenoid?

 

input=12v
current 1A

 

No problem. Just use a magic switch.

 

3V is coming from microcontroller?

Read this article:
http://www.pcbheaven.com/userpages/basic_transistor_circuits/

 

sorry friend for the lack of details.The 3v is a led positive voltage for another device.when the device reaches a certain temperature a led is on which I want to use as a trigger for the powering of the solenoid valve .The 12 v source is already ground so as the output to the solenoid is also grounded

 

Diodes drop voltage so you don't really have 3V at the LED you got like 1V. Since typical red LED has Vf of 2V.
3V-2V=1V
Or you took the multimeter and confirmed that you got 3V after the LED?

 

Consider this.
Anywhere near correct?

 

Consider this.
Anywhere near correct?

#12, are you trying to blow up the gate of the NFET?

 

I think the OP needs a high-side switch if one leg of the solenoid is grounded and he want LED lit = solenoid energised?

 

#12, are you trying to blow up the gate of the NFET?

Wrong symbol? I thought I put a p-fet in there.

Arrgh! AFU. The solenoid is on the wrong side of the mosfet.

 

Better now?

 

Yes, but Wild has to validate that he can, in fact, tie the negative end of the 12V supply to the LED cathode.

 

sorry friend for the lack of details.The 3v is a led positive voltage for another device.when the device reaches a certain temperature a led is on which I want to use as a trigger for the powering of the solenoid valve .The 12 v source is already ground so as the output to the solenoid is also grounded

Get a DC SCR to act as a switch which uses a low (3 volt) control voltage and can drive a 1 amp inductive load at 12 volts to handle your solenoid. That is the simplest turn key one component solution I can think of. DC SCR should get some Google hits for you. The SCR goes in parallel with your LED assuming you have a On/Off 3 volt signal to work with. That or remove the LED and replace with the SCR and place another LED across the relay coil.

Ron