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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:


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:


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:


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.


Project Steps

Cut the PC Board

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!)

Prepare the PC Board Components

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.

Solder the Component Leads

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!

Connect the Battery Clip Leads and Mount the PCB

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!

Test a Transistor!

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).

More on the Transistor

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:

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