Simple Transistor Tester Makes Sorting Easy


Hacker [Dino Segovis] is back with yet another installment of his Hack a Week series, and it’s looking like he isn’t too worse for wear after hunkering down to face hurricane Irene.

This week, it seems that [Dino] is having some problems separating his PNP transistors from his NPNs. After Albert Einstein proves to be less than useful when it comes to sorting electronic components, [Dino] decided to build a simple transistor tester to help him tell his PNPs and NPNs apart without having to resort to looking up product data sheets.

The tester itself is relatively simple to build. As you can see in the video below, it consists of a power supply, an LED, a few resistors, a pair of known transistors, and not much else. When everything is hooked together, the NPN/PNP pair causes the LED to light up, but the circuit is broken whenever one of the transistors is removed. Inserting a new transistor into the empty spot on the breadboard immediately lets you know which sort of transistor you have inserted.

Sure you can tell transistors apart with a multimeter, but if you have a whole drawer full of loose components, this is a far more efficient option.


26 thoughts on “Simple Transistor Tester Makes Sorting Easy

    1. Isn’t that kind of the point of a transistor, that it have the same pinout?

      This might be pretty useful for all the parts I plan on scavenging in the near future. Good to get some basic sorting done, then move on to defining a specific use for each one (power handling, etc.).

      1. Uh, no.. different transistors do have different pinouts.

        They all have a collector, a base, and an emitter, but which one of those is ‘the middle pin of a TO92 package’ can change from one part to the next.

        If the flat side of the TO92 is facing you, the pins of a 3904 (from left to right) are: emitter, base, collector (EBC). For a BC317, the order is CBE. For a BC167, it’s ECB.

        To find the collector and emitter of an unknown transistor, put a diode tester across the leads until you find two that have no connection in either orientation. The remaining pin is the base.

        To find the type, see which direction current flows between the base and the other two terminals. If it goes from base to emitter/collector, you have an N-type device. If it goes from emitter/collector to base, you have a P-type device.

        To decide which is the collector and which is the emitter, make a simple inverter (one C-or-E pin connected to ground for an NPN, to Vcc for a PNP, a 10k resistor between the other C/E pin and the opposite rail, and a 1k resistor going to the base). Hook the free end of the 1k resistor to the wiper of a pot, and watch the voltage at the node where the 10kR meets the transistor. When the resistor is connected to the collector, you’ll get a lot of gain. When it’s connected to the emitter, you’ll only get a little bit of gain.

    1. Exactly what I was going to say..

      I’ve evolved a whole color code: BJTs get a dab of white paint, mosfets get grey, FETs get purple. N-type devices get a dab of red, P-type get blue. Then I have a whole range of other colors to distinguish 3904s, 4401s, 8050s, etc.

      The red/blue, white/grey/purple thing is all I remember off the top of my head. The category colors are just there to help me spot that one 8050 I dropped in the 3904 bin.

  1. Now I’m tempted to make a project using one of those $2 uC’s to classify the part by type, polarity, and HFE. The hardest part is probably learning the new architecture from scratch. :)


    OK I’ll do it. When I get done, I’ll post it. The sockets are probably going to be some spare ZIF ones I have laying around but the posted project will use the cheaper kind. Back in a week(end)!

    From a more practical point of view, I think someone should have a coproject on sorted parts bins. Everyone can compare their favorites/cheapest/cleverest/etc. solutions.

  2. It’s always helpful to go by the actual part number printed on the transistor, I have had a few where part of the number rubs off in my parts drawer but I can usually still read enough to know what I’m dealing with.
    Props for making something usefull with scrap parts, looking good, just needs a slick tiny case and it’s set…

  3. My favorite tool for this (and one of my favorite electronic tools, pediod) is the Semiconductor Tester from M3 Electronix ( It tests a whole variety of different things from transistors to diodes to fets, etc. And give you all the characteristics. Very easy to use, very useful, and a fun kit to put together.

    1. Nice as far as it goes, but the comments of others point out the limitations of the project. I really don’t know of a circuit that gets around them. Ideally a circuit that use one socket, and can both ID it as a NPN or PNP, and ID the leads as a bonus. In the meantime I have my BK Precision in circuit transistor test that does that. A nice estate auction buy.

  4. On the page u main maintion of if one of the transistor is removed then the circuit will be broken, does it mean that the circuit is only design to test two transistor at the same time? And i apreciat the circuit you design!

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