Introduction: Transistor Tester

Picture of Transistor Tester

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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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:

Picture of

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.


memo0s made it! (author)2016-09-12

Thanks , I've been looking for this circuit for long time , I just started the Hobby so I hope u don't mind, Can you explain to me why we used this values of the resistors at those specific locations

Thanks Again

LeumasYrrep (author)memo0s2016-09-18

Thanks for checking out my Instructable and welcome to the electronics hobby.

I would love to say that I painstakingly calculated each resistor value for the optimum performance but honestly I made this project when I was just getting started and most likely copied the resistor values from another of my projects or a transistor schematic I had found online and didn't understand myself what they were doing.

Looking at my terrible schematics (I have gotten better) I see that I used a 470ohm resistor as the current limiter for the LED. To avoid damaging the LED we need to limit the maximum current. The same is true for the base of the transistor. The 10K resistor limits the maximum current that will be allowed to flow through the transistor.

If you have extra transistors and LEDs I suggest you try different values to see what happens, the worst that can happen is you burn up a few cents worth of transistors and LEDs and the best is that you learn what values of resistors are required.

Also if you haven't already, check out how a transistor works from the best teacher on YouTube: Dave Jones.

Don't worry about if you don't understand all of it, it is not necessary but still interesting and a good look into transistors. Check out his other videos, I have learned so much from watching random videos of his.

Have fun!

memo0s made it! (author)LeumasYrrep2016-09-21

Thanks for the fast replying I really appreciate it ...

I already did what you suggested about trying different values ,because I thought from the first sight 10kR might be too much , then when I made it , the LED emitting was dim . so after some time I figured the optimum value for that resistor was 1k . more than 1k will cause dimming of the LED,

lower than 470R will cause LED flickering or Fail of LED Emitting with some transistors , dont know why acully

anyway the project was handy and fun to make, so here is my last version of it with SMD components

and thanks again for the link , helped alot.

Mojtaba Sultani (author)2016-01-04


opeyemi.seun2 (author)2015-09-17

Schematics please!

check out the intro step.

MROHM (author)2015-02-01

Simple But So Useful..Many Thanks!!!!!

hakuna matata (author)2015-01-08

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????

dudes (author)2014-06-19

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

LeumasYrrep (author)dudes2014-06-29

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

mettaurlover (author)2012-12-11

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.

elfarandulero (author)2012-07-15

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

floxin (author)elfarandulero2012-08-25

Hope this helps and that the sketh goes through

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.

What is the "into step"? and where is the link for it?

*intro step. It is the very first page, second picture.

floxin (author)2012-08-22

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.

floxin (author)2012-08-22

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.

offtherails2010 (author)2012-05-03

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 !

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 !!!


offtherails2010 (author)2012-05-01

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" !!!

slangtom (author)2012-03-28

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.

LeumasYrrep (author)slangtom2012-03-29

It was my original hope that I could use a micro-controller, the Arduino, to be able to do what you are describing. I was going to use a LCD display that would tell me the pin layout and type of transistor but I ran into problems. First I wanted all transistors to be tested in a single socket but in placing all the circuitry "overlapping" to that socket I was getting false readings even before placing the test transistor. Also the only was for the micro controller to detect the test transistor I would need transistors to connect into the Arduino and strange things started to happen whey you are testing one transistor but have a few more further into the test circuit.

Long story short I was never able to figure out how to make this method work. I am not sure if it is even possible to do but my simple understanding of circuity prevented me from going any further.

jensenr30 (author)2012-03-03

Hi. Ryan here. I have to say that this instructable is great!
It's well written and very easy to follow.

I made a transistor tester that uses four socket pins to test both EBC and ECB.
the leftmost three pins (see pic) test transistors to see if they are EBC. To test your transistors to see if they are ECB, simply rotate the transistor 180 degrees and move the transistor to the rightmost pins (see pic).
I would have used a battery almost identical to yours, but I didn't have any button cells that weren't dead.

That is my version of your transistor tester!  XD


jensenr30 (author)jensenr302012-03-03

OH! And I almost forgot. I put feet on the bottom. They are made of superglue (ghetto as hell) and they work really nicely. see pic


LeumasYrrep (author)jensenr302012-03-04

Very nice. I like how you have reduced the number of components by doubling up the test sockets. Thanks for the pictures.

jensenr30 (author)LeumasYrrep2012-03-07

no prob. I like sharing my better ideas (and hiding the bad ones! lol!)

mayureshjoshi (author)2012-03-07

please give me this video

mrmerino (author)2012-03-02

I wonder if you could "float" the back of the board on some sort of liquid latex, so you'll have a nice nonconductive place to put your hand. Or just screw it to a piece of wood.

profpat (author)2012-02-25

this a great instructable! will build one for myself! thanks for sharing!

LeumasYrrep (author)profpat2012-02-25


HOPKIR01 (author)LeumasYrrep2012-02-28

how much it will cost all together??????? & COULD U GIVE A Brief description about it. like in which store i ll get the meterial etc.i like it its really nice!!!!

project_builder (author)2012-02-25

Nice instructable I will build one myself. But I also have a suggestion, try making a project box for it.

pfred2 (author)project_builder2012-02-26

A box is a little extreme for a project like this. Nailing it to a scrap piece of wood might be nice though.

I had initially considered a project box. I love wood working so I was going to design my own enclosure but I desided aginst any kind of box.

First the circut would have to be altered, the IC sockets would need to be on a separate smaller board so that I could place then on the underside of the box lid while having them stick out of cut outs through the top. I have tactile switches with longer buttons so I could leave them as they are and build some wooden pegs on the box lid to press them. Also the LEDs would need to be extended.

By skiping an enclosure I was able to simplify the circuit and keep the tester in as small a package as I could solder. I have these flat plastic organizers that I place all my components in and as an added bonus this tester is just small enough to fit in one of the compartments next to my transistors.

Dr.Bill (author)LeumasYrrep2012-02-26

Do a nice box with a lid. Open the lid shows the top of the board. Board being recessed in the bottom half of the box, close the lid. Might be very nice kind of box that looks good enough to place on display.

This is honestly a project that I think goes better without a box... A project box'd just get in the way whenever you want to swap out the transistors.

That being said, I'd like to see one for Flame Emitting *ahem* ... I mean, Field Effect Transistors... slip of the metaphorical tongue...

GonzoDuke (author)2012-02-26

Thanks for the project! Did mine on a piece of strip board I had on hand.

LeumasYrrep (author)GonzoDuke2012-02-26

Great! Thanks for posting a picture.

mrmerino (author)2012-02-25

Boy, I wish I'd had this when I built the Tillman preamplifier. Wait, does this work on fets?

profpat (author)mrmerino2012-02-25

i dont think this will work for fets, fets has a different type of biasing!

Brosiman (author)2012-02-25

Is it me or did you make a mistake with your acronyms? :

"- ECB stands for Emitter Collector Base. 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.

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

But a great project!

LeumasYrrep (author)Brosiman2012-02-25

Great catch! I must have switch the acronyms while typing. Thanks for pointing it out I will fix that right away.

Michael Chen (author)Brosiman2012-02-25

Yep, it is backwards

codyg102 (author)2012-02-25

Great job! Well documented, lots of pics. Thanks for sharing it.

About This Instructable




Bio: Full time Mechanical Engineer
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