Transistors are really useful and basic components and have a wide application uses. They are used for Gates, amplifiers, microchips etc. Though, I think for hobbyists they have one usability problem. You cannot visually recognize whether a transistor is NPN or PNP. You can of course look up their datasheets, but it takes time. Once I even went to a store to buy PNP transistors but, by mistake bought NPN. So, in this instructable I'm going to show you how to make a device from which you can quickly determine whether a transistor is NPN or PNP.

Step 1: Components and Tools Needed

Components :

  • 1* Bulb (that works well on 8-9 V supply)* (as an indicator bulb)
  • Stack headers
  • 1* DPDT rocker switch (switch with six pins)
  • Perf board
  • 1* Tic Tac box (for enclosing the finished product)
  • 1* 9 V battery with battery snap
  • Transistors (for testing)


  • Soldering equipment
  • Cutter

* I'm using a bulb because it is non-polarized (i.e. it will work no matter which direction the voltage is applied).

Step 2: Basics of a Transistor

The basic property of the transistor I am going to use for this circuit is:

  • For NPN transistor, if you connect VCC to base and GND to the collector or emitter, it would conduct electrically.
  • For PNP transistor, if you connect GND to base and VCC to the collector or emitter, it would conduct electrically.

In case you want to know more about the basics of transistor, watch the two videos given below. They offer an excellent explanation. I got to know about the basics of the transistors and how to use them by watching these videos.

Videos by Mr Ben Eater.

Step 3: Working of the Circuit

In this circuit, we want to switch the direction of the voltage and see when the circuit becomes conductive. To do so, we will use a rocker switch. The rocker switch circuit is a simple example of a H-bridge circuit (it can switch direction of the voltage).

So, now by using the rocker switch you can pull either the base or the emitter/collector to VCC by switching the voltage direction. The bulb is connected in series with rocker switch and the Transistor. So, if the Transistor is NPN and base is pulled to VCC, the bulb would glow. Similarly, if the Transistor is PNP and base is pulled to GND the bulb would glow. Please refer to the diagram given above.

Step 4: Make the Circuit !

Now for making the circuit. Cut the perf board to the size of the Tic Tac box. Cut a 3 piece socket of the stack headers. Solder the circuit onto the perf board as shown in the schematic given above. J1, J2, J3 are the stack headers. The six pin component is the rocker switch

I decided to put the circuit board into a Tic Tac box to give it a nice and handy finish. Note: when you are soldering the 9V battery snap make sure you pass the two wires through Tic Tac lid's opening first; and then solder.

Step 5: Test Your Circuit

Now test the circuit. Put a NPN transistor through the stack headers and set the switch to NPN mode (The side of the switch which pulls the transistor's base to VCC). If the bulb glows the circuit probably works. Now put a PNP transistor and set the switch to PNP mode (the side of the switch that pulls the transistor's base to GND). If the bulb glows then your circuit works.Please refer to the images given above.

Step 6: Enclose It !

Now make holes for the switch and the stack headers using the cutter. Put the circuit board in the box. Make sure the top part of the switch comes out of its hole slightly so that you can access the switch. Scrape out the sticker of the Tic Tac box so you can see the bulb glow properly. Close the lid and mark which side of the switch tests for NPN and PNP. Glue the perf board to the Tic Tac box so that when you throw the rocker switch to either side the whole circuit won't move. That's it!

If you have any suggestions please tell me and check out my other instructables.

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About This Instructable



More by Tanmay Deuskar:DIY H-Bridge Shield for Arduino With Transistors The Tic Tac Adjustable Power Supply The Transistor Testenator 
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