Make: Projects

Battery Clip Transistor Tester

Build this simple tool for testing TO-92 package NPN transistors.

When it comes to transistors, I’m a fan of NPNs (versus PNPs) for their ubiquity and reliability, and because I’ve memorized many of their specifications over time. IMG_20140930_104431_detailTransistors come in many types and packages, and I find myself most-often working with TO-92 package bipolar junction transistors – or BJTs. These are the most-common type of transistors, and TO-92 is short for Transistor Outline Package, Case Style 92. This specifies the three-lead design with the component elements encased in plastic or epoxy; one side is curved and the other flat, usually with some information imprinted on the flat side.

I recently prototyped numerous single-, double-, and triple-transistor FM transmitter circuits. However during the prototyping stage – building circuits on breadboards – I encountered numerous circuits that were acting errantly.

Since I was only prototyping the circuits for fun, a number of the components were not bought new, and instead sourced from my junk drawer. I didn’t plan on building more than one of each of the designs, and was merely building several different designs to compare and contrast features of each circuit.

Some quick multimeter probing lead me to suspect the transistors in my circuits. I attempted to use my multimeter’s mini-hooks to attach to the transistor leads inserted in the breadboard, but some leads were obstructed by other components, and I didn’t want to redesign my layouts simply to test the transistors. I wanted a quicker, more-reliable solution. (Plus not all multimeters have ohmmeters for testing transistors, therefore I wanted something that wasn’t multimeter-dependent.)

I needed a standalone transistor tester, a simple tool where I could quickly insert an NPN (or PNP, with an easy redesign) transistor, push a switch, and get confirmation that the transistor’s Collector-Base junction was operating correctly.

Flipping through Charles Platt’s book Make: Electronics – as I often do for inspiration – I stumbled upon Experiment #10, specifically the Fingertip Switching sub-section. The experiment is accompanied by this image:

finger-tip_switch

I thought, “Aha! Of course.” The finger bridges the Collector-Base junction, allowing electricity to flow through the circuit, from the Collector to the Emitter. In this sense the finger is a lot like a switch, or as illustrated on the following page of the book, a finger pushing a momentary button:

base-b-hand

1024px-bjt_npn_symbol_caseIn the layout above it’s quite easy to see the breadboard design, but here the finger is seen pushing a button, accompanied by a few arrows, letters, and a circle. The hand-finger-button is a stand-in for the transistor’s Base, and the whole illustration is a stand-in for the symbol used to represent NPN transistors in a circuit, seen on the left.

A mnemonic for the NPN transistor’s symbol is the arrow is Not Pointing iN; again as opposed to PNP transistors whose symbol arrow Points iN Proudly.

Now back to Platt’s experimental circuit. The fingertip switch experiment had a few things I didn’t want, and was missing a few attributes I did want. It used 12VDC and if I was leaning towards a tool for my workbench I’d prefer a 9V design. It also included no feedback, no indicator light; in other words it worked, but there was no way to know when it was working. I wanted to add an LED to the design.

I got ahead of myself and didn’t realize p.75 of Platt’s book also contains a design for a fingertip switch using an LED, but by then I had sifted through a few resources and sketched a few simple designs. Many are similar, and this is the one I settled on:

fingertip-switch-circuit

You can breadboard this circuit in only a few minutes:

Touch both of the green leads with one finger and the LED will light up.

Touch both of the green leads with one finger and the LED will light up.

Of course, I also wasn’t going to keep the circuit permanently mounted on a breadboard. I prefer having a tool that I can quickly reach for whenever I need to test TO-92 package NPN transistors. I sketched a few designs, looked at a few available PC board layouts, and settled on what you see below for this Battery Clip Transistor Tester. I designed it to be small enough to mount directly onto a heavy-duty 9V battery clip (as opposed to the “insulated” clips whose wires are encased in a soft nylon pouch). I hope you enjoy building this super simple tool – if you use a lot of NPN transistors in breadboard or through-hole circuits this tool may save you a lot of frustration.

step5x0

Steps

Step #1: Cut the PC Board

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  • I settled on this general-purpose PC board because its layout slightly mimicked the layout of a standard breadboard, while simplifying the connections between components. Also with this PC board I could use as few as four rows, which was nearly identical in width to the heavy-duty snap connector!
  • Begin by using a rotary tool with a cut-off wheel. Use a pair of helping hands or vice grip to hold the PC board steady while you cut. Such small cuts require a steady hand and patience. You'll notice I cut through the bottom row of pads on the PC board. It doesn't really matter where you cut, so long as you end up with two pieces, similar to the pieces seen in image 3.
  • The smaller pad is from the outer rows of the PC board, trimmed closely around the copper pads. (I had fun cutting this part to size!)

Step #2: Prepare the PC Board Components

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  • Get a 3-pin header, cut and strip the wires. I stripped the wires so the LED would be slightly below the bottom of the 3-pin header (as seen in image 2). If you're a junk collector like me you might have plenty of these things laying around from the era of personal computer desktop motherboards. Any header pins will suffice. Schmartboard's female jumpers (RadioShack #2760144) is another satisfactory part.
  • TIP: if the wires of the 3-pin header are stranded - not solid - I recommend tinning them with solder prior to inserting them in the PC board.
  • Cut and strip three tiny lengths of white, solid-core wire. You'll use two to jump the fingertip switch pad to the Collector and Base of the transistor; and one to jump the Emitter to ground.
  • Mount the resistor, 3-pin header, LED, and fingertip switch pad wires & pad to the PC board; you can see a simplified layout in image 3, or go here for a full-size version of the same diagram. Bend the component leads so you can flip over the PC board and solder the connections.
  • Note: In the next Step's first image my fingertip switch pad is not seen; the pad fell off and the wires fell out, but you can see the pad and solder points in the second image. This part is very small and a bit finicky. Be patient and you'll get it to mount in place properly.
  • Also note: I cut two tiny lengths of blue and yellow heat-shrink tubing for the LED's leads. This is entirely optional. I did this because heat-shrink tubing helps make the leads feel a bit more secure and therefore less likely to bend accidentally.

Step #3: Solder the Component Leads

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  • Exercise more patience, and solder the components in place. I wanted all the components as snug and close to the PC board as possible; feel free to check and double-check each component before soldering.
  • When you're done, clip all the leads and you'll have a pretty little circuit board!

Step #4: Connect the Battery Clip Leads and Mount the PCB

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  • Curl the battery's snap connector's wires to get an idea of where to cut and strip the wires. I wanted a nice, aesthetic curl, but you could trim these even shorter for a straight run to the PC board.
  • Solder the battery clip leads to the PC board's rails.
  • Apply a small dab of hot glue to the top of the battery clip and firmly press the circuit onto the glue. Hold this in place for a half-minute until the glue dries. You've now assembled a tiny tool for your electronics toolbox!

Step #5: Test a Transistor!

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  • First thing is to test a transistor to ensure the tool works properly. Insert an NPN transistor with the TO-92's package imprint facing you; in other words you should be able to read "2N3094" - or whatever type of NPN transistor you are testing - when the transistor is inserted.
  • With the transistor's Collector, Base, and Emitter pins correctly inserted in the 3-pin header, tap the fingertip switch pad and you should see the red LED light up.
  • Not satisfied with having to remember which header pin corresponded with which transistor lead, I printed the symbol for an NPN transistor accompanied by the words "NPN ONLY" and mounted this to the front of the 3-pin header (see image 2).

Step #6: More on the Transistor

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Battery Clip Transistor Tester

OK now you've made a tool for testing NPN transistors. But you're probably still wondering what's going on inside that little plastic package? I recommend watching the following two videos, which include some "Aha!" learning moments and are even funny at times, while being extremely informative about what transistors are made of and how they operate:

Nick Normal

Nick Normal

I'm an artist & maker. A lifelong biblioholic, and advocate for all-things geekathon. Home is Long Island City, Queens, which I consider the greatest place on Earth. 5-year former Resident of Flux Factory, co-organizer for World Maker Faire (NYC), and blogger all over the net. Howdy!


  • wilsondavis101

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  • HapPy VeRma

    iam confused in ebc just want to ask that ebc is fixed in pins or any pin can be made collector, emitter or base by soldering please help and please tell that if i connect only base to to +ve terminal of battery so the current will flow from emitter and what about collector then does current will flow from there also actually i got these confusions while watching transistor tester video and emergency light one video please help

    • nicknormal

      If I follow your comment correctly, you can’t connect Base to +Ve, only Collector, per this circuit; the Base is simply a switch.

      • HapPy VeRma

        um actually sir iam an beginner iam the student of 10 and very much interested in making diy projects don’t know much about npns and pnps and also mosfet transisters just want to know that the base is only a medium to supply current from it to emitter and also why it can’t be connected to +ve terminal of power supplies directly

        • nicknormal

          Think of Base in NPNs as being like a block, or wall; current *wants* to go from Collector->Emitter, but it can’t, because the Base is blocking current. Once the Base is switched, it allows current to go from C->E. Watch the second video in the final step to see if that clarifies what I’m talking about, and how that works.

          • HapPy VeRma

            oh okay before i was thinking that collector is nothing but base is only the thing which pass the current through emitter but now as per as your explanation i got it about base collector and emitter and what’s your second video name and want to ask we can connect base in +ve or -ve terminal of power supply as per as the requirement of project huh please tell me more about base and the thing i got is the base is an blocker only activates the current to flow through emitter when activated

            well thanks for previous help
            and thanks for not to ignore my questions your feed backs are appreciated please help me in these more question given above

  • HapPy VeRma

    please also tell me that if we connect base and emitter together then what will happen and collector is connected to +ve terminal of battery and and another connection of emitter to -ve terminal of the battery whal will happen please tell fast

  • Guest

    jkii . true that Patricia `s report is impossible… on wednesday I bought Saab 99 Turbo since I been making $8569 thiss month and also ten/k this past month

    . it’s actualy my favourite-work I’ve had . I began this three months/ago and pretty much straight away was earning more than $75… p/h . you could try here MORE DETAIL HERE

  • HapPy VeRma

    okay nick thankss pal by watching battery clip transistor tester video and emergency lighting system video i understood about transistors very well it according to you that i got understood is that when base is not activated current cannot flow from collector to emitter once it gets activated current flows from collector to emitter now the last thing that i want you to tell me that we can acitvate base by flowing current only so i want to know that did some current also go from base to emitter or not and is flowing conventional current is only the way to get base activated

    • HapPy VeRma

      i got the video only the answer to the questions are reamaining please tell those

  • HapPy VeRma

    oh thanks to the video i got it thanks to you nick pal you’ve helped very much again thanks pal

  • HapPy VeRma

    hey pal last thing i want to know that is some of the current also flow from base to emitter while base is activated it flows or not please tell and thanks in advance

  • Guest

    fhjy . true that Patricia `s report is impossible… on wednesday I bought Saab 99 Turbo since I been making $8569 thiss month and also ten/k this past month

    . it’s actualy my favourite-work I’ve had . I began this three months/ago and pretty much straight away was earning more than $75… p/h . you could try here MORE DETAIL HERE

  • Guest

    dgx. I see what you mean… Clarence `s blurb is terrific, on friday I bought a great new Dodge after having made $4460 this past 4 weeks and-just over, 10k this past month . without a doubt its the most-comfortable work Ive had .

    I started this seven months/ago and almost straight away began to bring home at least $70, per/hr . over at this website HERE’S MORE DETAIL

  • http://ananasblau.com/ Thomas R. Koll

    I love the base-b-hand.jpg image. Had to laugh.