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.

  • \$\begingroup\$ How is power applied to the 3.5 volt LED and does the 12V LED share any common power supply rails? \$\endgroup\$
    – Andy aka
    Commented Dec 19, 2014 at 18:02
  • \$\begingroup\$ 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? \$\endgroup\$
    – EM Fields
    Commented Dec 19, 2014 at 18:43
  • \$\begingroup\$ Yes that is correct. Just edited the original question with more details. \$\endgroup\$
    – rockrazz
    Commented Dec 19, 2014 at 23:43

1 Answer 1


To select a switching transistor, you'll need to know how much current will flow through your 12V LED. Choose an NPN transistor that can pass that current, allowing a good margin of safety. You're looking for the Ic(max) parameter in a data sheet.

  • 1
    \$\begingroup\$ Also note that using a transistor means that there will be no isolation between circuits as there would be with a relay. \$\endgroup\$ Commented Dec 19, 2014 at 15:48
  • \$\begingroup\$ @IgnacioVazquez-Abrams Good point! \$\endgroup\$ Commented Dec 19, 2014 at 15:58
  • \$\begingroup\$ There must be some isolation if this is possible. If 3.5v is going in and 12v is going in, they share the common? \$\endgroup\$
    – rockrazz
    Commented Dec 19, 2014 at 23:45
  • \$\begingroup\$ not sure in my mind how the switch works on a transistor, i guess. In the relay, flow in/out to activate switch, other side in/out for the switch. On the switch transistor, are the 3 solder legs doing similar as well as the tab on the top of the transistor to make the 4 in/outs? \$\endgroup\$
    – rockrazz
    Commented Dec 19, 2014 at 23:48
  • \$\begingroup\$ A NPN transistor in common emitter configuration (which is how you do low-side switching) shares the negative rail between trigger and target. \$\endgroup\$ Commented Dec 20, 2014 at 15:06

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