simulate this circuit – Schematic created using CircuitLab

I'm at a complete loss over here. What I'm trying to do is switch 12V voltage with a NPN transistor (the switched signal later goes to a P-CH-MOSFET). My problem is that nothing works. I've tried a simple circuit as this with a 1K resistor, tried adding a pull-down resistor (tried 1K and 200 ohm values), tried using different transistors (TIP122, BC337, PN2222a), tried using different controllers (Arduino, Intel Edison) and nothing works.

To elaborate, the problem is that I add the load to the collector (whether it's a LED strip, a single LED with a resistor, multimeter) it constantly stays on. Also I do have common ground, I have the power supply and controler's grounds connected to the same rail on the breadboard.

What are the possible problems? Is there hope to solve this or are somehow all of my components are fried? Appreciate all the help.

EDIT: added one of the schematics that I tried. For load I'm actually using a RGB LED strip, so the load in this circuit is just an example.

EDIT2: adding a picture of the circuit on the breadboard. enter image description here

EDIT3: small update, forgot to mention that my transistor is BC337-40, the difference is that it has hfe of 250.

  • 1
    \$\begingroup\$ post a diagram for the complete circuit, added load as well. Use the schematic editor you get with the resistr/capcitor/diode/pencil button in the editor, when you edit your post. \$\endgroup\$
    – Neil_UK
    Jan 21, 2017 at 13:08
  • \$\begingroup\$ Added an edit with the circuit. \$\endgroup\$
    – Marius
    Jan 21, 2017 at 13:20
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    \$\begingroup\$ Sure the NPN is not damaged yet? Check with open base. \$\endgroup\$
    – user76844
    Jan 21, 2017 at 13:42
  • \$\begingroup\$ maybe you can show us a picture of your real circuit \$\endgroup\$
    – dirac16
    Jan 21, 2017 at 13:46
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    \$\begingroup\$ EXACT markings on the transistor ... TO92s come in different pinouts, some of them quite surprising ... sometimes the same transistor is available in more than one pinout (suffix K or L may indicate a stranger... You can't easily identify collector or emitter with a multimeter but you can definitively identify the base. \$\endgroup\$
    – user16324
    Jan 21, 2017 at 15:34

1 Answer 1


The circuit you should should definitely shut off the LED current when V1 is switched from 5 V to 0 V.

Most likely the problem is that the transistor is connected incorrectly. Look carefully at the datasheet to see which leads are emitter, base, and collector, then make sure your circuit is really connected that way.

It's hard to tell from your picture how exactly the transistor is hooked up.

Also note that while many transistors have leads E-B-C from left to right when looking at the flat, that is certainly not universally true. You have to read the datasheet for your specific transistor, then double-check that it is connected correctly.

  • \$\begingroup\$ I've looked at all the datasheets I found and it's clearly left to right CBE. \$\endgroup\$
    – Marius
    Jan 21, 2017 at 15:53
  • \$\begingroup\$ I even tried flipping the transistor, got the same result. I should probably assume that the transistors are faulty, right? \$\endgroup\$
    – Marius
    Jan 21, 2017 at 18:04
  • \$\begingroup\$ @Marius: It would be odd of a bunch of different transistors all had the same failure. I suspect something else going on. A decent picture of your breadboard wiring might help. \$\endgroup\$ Jan 21, 2017 at 20:01
  • \$\begingroup\$ @OlinLathrop Here's the circuit as clear as I can get it. I disconnected the power so the LEDs wouldn't ruin the visibility, but otherwise the LEDs are constantly on. \$\endgroup\$
    – Marius
    Jan 21, 2017 at 20:33
  • \$\begingroup\$ Just to make it more clear, the gray wire from collector goes to the load resistor. Emmiter is also connected behind the transistor with a short gray wire to the ground rail. \$\endgroup\$
    – Marius
    Jan 21, 2017 at 20:38

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