What type of transistor would be required?

I am trying to turn on a 12V LED, whenever power is applied to a 3.5V LED. I want to use a switch transistor instead of a relay. With the relay, 3.5V+/- on the switch activates 12V passthru; what switch transistor could I use to achieve this?

Great with old school (relay) but need the help of the pros here to use a transistor instead. Not sure that this will work, but I'm assuming it can. Any help or advice would be a great help.

The 12v LED uses 20mA (.24w ~ 1/4Watt) of power to run thru the transistor. Be a Newbie in the transistor world, what am I requiring for a transistor. Can 2 different voltages run thru a transistor, the same way it would with a solid state relay?

I was trying to light the 12v LED on a circuit that has an LED on it. When I connected it, I was extremely dim. Upon metering the circuit, I realized it was 3.5V. I have to run this 12v LED, so I'm trying to do this with a relay style of switch. I want to use a transistor, because I have been reading it's quicker switching and doesn't use much of the 3.5 draw to remain open, which the relay would do.

Help! not sure what to use or how to arrange this with a transistor.

• How is power applied to the 3.5 volt LED and does the 12V LED share any common power supply rails? Commented Dec 19, 2014 at 18:02
• Would you please explain in greater detail what you mean by: "I am trying to turn on a 12V LED, whenever power is applied to a 3.5V LED" ? I take that to mean that the comma shouldn't be there and that you already have an LED circuit where an LED turns ON when 3.5 volts is sent to the circuit, and that you now have a 12 volt LED that you want to turn ON, using a transistor, when the other LED turns on. Is that right? Commented Dec 19, 2014 at 18:43
• Yes that is correct. Just edited the original question with more details. Commented Dec 19, 2014 at 23:43