Transistor Tester




About: Full time Mechanical Engineer

The purpose of this circuit is to test NPN and PNP transistors and to identify their pin layouts, ie ECB, EBC. I find myself testing a lot of transistors to determine their pin layout and type and as such find that building the test circuit on a breadboard has become a hassle so I wanted an easy circuit that would be more permanent and would allow me to test transistors.

The transistor type NPN/PNP determines the polarity of the connections. The orientation of the transistor is the pin layout. I define the pin layout when looking at the top of the transistor with the flat side facing toward you. That is probably not the official way to classify them but it makes more sense to me when I am placing them in a circuit.

- EBC stands for Emitter Base Collector. When I use this acronym I am saying that the pin to the left is the emitter, the middle pin is the base, and the right pin is the collector.

- ECB similar to the above acronym stands for Emitter Collector Base. The emitter pin is again on the left but the middle pin is the collector and the right pin is the base.

I have gone through about 100 transistors that were pulled from old electronics and have never found any other orientations so i have found that there is no need to test for anything but these two orientations.

Step 1: Parts

This circuit requires few parts and it should be easy to obtain many of the required parts from your own supplies or pick them up locally. I had all the parts on hand that were leftover from previous projects or where recovered from old electronics. If you need to buy these parts I suggest ordering from a supplier on eBay. Only the circuit board you will have to find at Radio Shack if there is one locally or use a different prototype board. While you are ordering parts pick up more then you need, these parts are useful to have on hand.

You will need:
 - 1x Radio Shack Printed Prototype Board 276-150
 - 2x 8 pin IC Socket
 - 4x tactile switch
 - 4x 10k ohm resistor
 - 4x 470 ohm resistor
 - 4x 3mm LEDs
 - Solid core wire
 - CR2032 battery and holder
 - NPN and PNP transistors to test

Step 2: Place Battery

We will start off by placing the battery holder on the prototype board and soldering it in. Place the circuit board in a landscape position. You want the positive battery connection to solder onto the top voltage rail while the bottom voltage rail receives the negative connection. You will have to twist the battery holder so that both connections are made. Place the holder to the right as far as possible so you will have room to build the test circuit on the left.

Step 3: Place the 8 Pin IC Holders

Two 8 pin IC sockets are used to hold the transistors as we test them. Place the first socket one space from the left of the prototype board. The other socket is place two spaces next to the first socket. This spacing will allow us to run wire later so the we can provide power to the testing circuits.

Solder the IC sockets in place. While soldering the sockets in place bridge the gap between the following pins and the nearest voltage rail. Starting from the left, pins 1 and pins 6 top and bottom. See the pictures for a better explanation.

To bridge the gap solder the IC socket in and then place a bit of solder on the voltage rail closest to the pin. Now touch both the IC socket pin and the voltage rail with the soldering iron tip and apply more solder until the connection is made. Go slow so that you do not bridge pins that you would not want to connect. Remember you are only connecting 4 pins!

NOTE: I would show a picture of the pins bridged but I did not bridge them until I was building each test circuit. Take a look at the final solder layout in step 6 of this Instructable if you are still having trouble. 

Step 4: Place the Resistors

Lets work on one test circuit at a time. We will start with the NPN transistor with a pin layout of EBC; Emitter, Base, Collector.

Place the 10K ohm resistor in a horizontal orientation so that it connects to pin 2 on the IC socket and extends to the last pin on the left of the board. Place the 470 ohm resistor on the 3rd pin of the IC socket in a vertical orientation. Solder the resistors in, if you are using new resistors or have resistors with long leads do not trim them yet. We will use these leads for future connections.

Step 5: Place the Switch and LED

Now to place the switch and LED in the circuit.

The switch will provide positive power to the 10K ohm resistor. Place the switch with one pin farthest to the left so that it aligns vertically with the resistor. If you have a switch with 4 pins make sure you place it so that the connection will be broken between the left and right set of pins. Solder the switch in place. Now you will need to bridge the gap between the 10K resistor and the switch. If you had long legs on your resistor bend one now so that it touches the switch connection. Solder and then trim this connection. If you did not have long legs on you resistor you will need to bridge the gap in a similar way that we did with the IC sockets and the voltage rails.

Now place the LED in the circuit between the 470 ohm resistor and the right pin of the switch. Place the LED so that the anode, +, connection is connected to the pin on the switch. Solder the LED in place. Using the LED leads bend the anode so that it touches the switch pin to create a connection between the two. This is where the positive power will split off. Bend the cathode of the LED so that it connects with the 470 ohm resistor. Solder these two connections.

Now lets provide positive power. Run a wire from the positive voltage rail to the second pin from the bottom. Now run a wire from the wire we just placed to the connection between the LED and switch. Solder all connections.

Now place a NPN transistor in the socket as show and push the button. If the transistor works the LED should light up. Not working? Do you have the correct Transistor? Is your circuit connections correct?

Step 6: Finish It Up

Now for the other three types. The NPN circuit to test the pin orientation of ECB is a mirror image of the previous circuit.

The PNP circuits are mirror images of their similar NPN circuits. Just remember to connect the switch and LED to negative voltage. Also remember to place the LED correctly so the the Anode connects with the resistor.

To finish it up add some labels so you never forget where the different type of transistors go.

Step 7: How to Use It

Now that you have built the tester it is time to find out how to use it. There are two main types of situations you will encounter.

Situation 1: You know the transistor type and orientation.

If you know the transistor type and orientation place it in the correct testing circuit. Hold the tester in the landscape position with     the battery on the right side. Place the transistor in the correct circuit for its type and orientation with the flat side facing you. With the transistor firmly in the socket press the tactile button. If the LED lights up the transistor passes the test.

Situation 2: The transistor type and orientation is unknown.

Start with the tester in a landscape position with the battery to the right. Place the transistor in the bottom left circuit, the circuit that tests if it is an NPN with an EBC orientation. Make sure to place the transistor in the socket with the flat side facing you. With the transistor in the socket observer the LEDs status without pressing the button, don't touch it just yet.

 - Is the LED on? If so then the transistor is a NPN type but does not have the EBC orientation. Remove it from the socket and place it in the socket to the right to test if it has the orientation of ECB.

 - Is the LED off? Press the button and observe the LED status. If the LED turns on they you have found out three things about this unknown transistor. One it is of type NPN. Two if has an orientation of EBC and three it works.

If the LED remains off with or without pressing the button place it in the PNP testing circuit and try to determine the orientation.

The main thing to take away from this situation is that if a transistor is placed in the correct transistor type testing circuit but the wrong orientation the LED will illuminate. This holds true for both NPN and PNP test circuits. By observing this you will know if you have guessed the correct transistor type but chose the wrong orientation. Knowing this can save you a lot of time when you are testing a pile of unknown transistors.

NOTE: In handling this tester you may have noticed something odd happen if you touch the back side of the circuit, specifically bridge the switch with your finger. If you do this with the correct transistor in the socket the LED will light up without pressing the button. Don't worry nothing is wrong with your circuit or the transistor, in fact this is good. Your finger is conducting a small amount of current which is picked up by the transistor. The transistor, doing one of the things it is designed to do, picks up this small current and amplifies it by letting current pass through to the LED. So while handling the tester try to keep your fingers from brushing the back of the tester to prevent false readings.

Step 8:

Thank you for reading my Instrucable. I have done my best to present a well written and visual Instructable so that you can easily build this circuit. Having said that if you find any step unclear or have questions feel free to ask and I will try to answer them as best I can and as fast as possible. If that fails I know that our lovely and extremely helpful community here at Instructables will be more then willing to help.

2 People Made This Project!


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40 Discussions

hakuna matata

4 years ago

IC 1 is for PNP transistor and to work out the format the left side of the IC is to determine if it is EBC while the right side is to determine ECB am I correct please let me know same for IC 2 for NPN left side is EBC and the right side is ECB correct????


5 years ago

could u provide a full schematic? I'm having trouble understanding where everything goes. thanks

1 reply

Reply 4 years ago on Introduction

Here you go! Let me know if there are any errors, I just threw it together today.

Transistor Tester.png

6 years ago on Introduction

A note- it's bad habit, electronically speaking, to call ground negative and high positive, since high can be at 0V or less and ground can actually be at a negative voltage, though in most cases it's irrelevant, but it's still a good thing to remember.


6 years ago on Step 8

Nice circuit but without the schematic diagram, it will be hard to follow on the instructions, if you have the schematic diagram I will be grateful if you publish it.Nothing like the schematic diagram for electronic; I learned electronics many years ago and always used schematic for any electronics projects

4 replies

The into step has the schematic. There are only two circuits that are very basic but the layout of the pins is what creates 4 different circuits for this tester.


6 years ago on Introduction

There is a wide variety of simple bipolar transistors. When I go to buy some, the salesperson usually asks: plastic or metal even for such an elementary device as the "workhorse" 2N2222.
Google "Mouser Electronics", a well known electronics distributor. Go left to "Product Finder". Click on "Discrete Semiconductors" and then click on "Transistors Bipolar" and "Show Products". Select page 4 and look at left images 13 and 14 from top. The 6th column from left says "Data Sheet" (click to open)and you will see a TO 92 package (plastic) and TO 39 package (metal) respectively. Kind of confusing. The article deals only with plastic packages. When it comes to metal (TO39), my rule of thumb is that the lead physically closest to the metal tab is the Emmiter but I am not sure that this is always true.


6 years ago on Introduction

it is easy to add stuff when others have done the hard work. I respectfully submit the following comments (in several messages).
1) There is a standard numbering system for IC sockets that avoids any possible confusion. It is based on the half circle notch (indent) in the plastic frame seen pointing downwards on the parts picture (shown under Step 1). (A) Place the socket in front of you so that the slots will be in two horizontal parallel rows. (B) Orient the socket so that the half circle notch is at your left. (C).Starting from the left on the lower row, the slots are 1, 2, 3, ...x. When you reach the end of the lower row, jump counter clock wise to the top row and continue numbering x+1, x+2,.. to the end. If you have to insert an IC, look at its top surface and you will see a tiny embossed circle (ususally) or some other mark. That mark must point the same way as the half circle notch.

Hi !

Okay, ive stumbled upon a problem, i've only just started learning electronics and finding it extremely difficult !

But i'm tring to follow the exact specs as this transistor tester as to avoid any mistakes but im cnow confused to heck and back !

Problem 1:
- Very Early-on, the diagram showing the simple schematic of the NPN Transistor tester circuit shows the BASE of the NPN being connected to the 10K resistor but in all the photo's the BASE of the NPN transistor (bottom-right test circuit on prototype-board looking at it from non-solder-side!) is connected to the 470 OHM Resistor.
PLEASE please please can anyone confirm why this is ?

Problem 2:
On the same circuit (bottom-right test circuit on prototype-board looking at it from non-solder-side!) the LED's 'Notch' {meaning the Cathode} is facing upwards... No matter how i orient the LED on my setup {using the same components, no substitutes!} instructions say 'Make sure the Anode is connected to the resistor' but how can this be possible for the LED's Notch facing up ? I really cant make head-nor-tail of this...

Please help as this is an excellent learner project for me and i have many transistors to test, lol, but go easy on me, Learner-Here, lol x2!

Many thanks in advance for the help with what will seem like child's-play to you folk, but rocket-science to me !

2 replies

I think I have a solution for you:

Problem 1:
From the simple test schematic I have the base of all the transistors are connected to the 10K resistor. In this schematic the base pin is always drawn as the middle of the three transistor pins. No when we go to real life components the pin layout is not the same. For the bottom right transistor the pin layout from left to right is; Emitter, Collector, Base. So the base moves from the middle pin to the right pin.

Problem 2:
The picture in step 6 appears to be playing tricks on my eyes. It looks like the bottom right LED has a flat facing up. I am sure this is not the case. Just make sure to place you LED with the correct polarity. I like to use a button cell to quickly check the polarity of LEDs before placing them, I never trust the makings on any of the LEDs I pull from old electronics.

Many many thanks for the extremely appreciated help as i really need to make this tester circuit especially as it uses very simple components for a complete noob like me, i am so very grateful for the extra help!!!

Obviously your circuit works as others have made it too and even made some changes but because ive only just started learning electronics this year its playing havoc with my old brain trying to learn new tricks, lol !

I'll be resuming this project over the weekend and am quite determined to make an enclosure for it as well because of the great practical use of this very important circuit !!!

Once again many thanks for your kind patience with assisting a complete noob and making it a few good steps closer for me to make this awesome circuit !!!


Many many Thanks for a truly amazing set of instructions - nice high res pics and simple enough for me to start making as ive got a bunch of transistors that i dont know if they work or not !!!

Also i have a bunch of rechargeable LIR 2032 Coin Cells to finally start making use of them !!

Thanks so much and Keep making your instructables with extremely outstanding high-res pictures !!! (2mb per pic - nicely done !!!)

!!! "A Simple Yet VERY Effective Piece of Work" !!!


7 years ago on Introduction

thank u for an insightful upload. May i know how can I use a PIC micro controller to test a BJT Transistor. This is a project i am currently working on in School, but i am finding it hard to understand how the PIC will be able to detect the state of the transistor (ie if its NPN or PNP). Also what mechanism will it employ in the test and using what theory.