Kit for building a popular transistor tester

Today I'll try to tell you about one of the most popular DIY measuring instruments. Or rather, not just the instrument itself, but a kit for building one.
I'll say right away that you can find it cheaper already assembled, but what could replace the fun of building an instrument yourself?
Anyway, if you're interested, come on in :)

There's a good reason this instrument is considered one of the most popular multifunction testers.
It earned that reputation thanks to its simple assembly, wide range of functions, and pretty decent specifications.
It first appeared quite a while ago. It was designed by the German engineer Markus Frejek, but at some point he stopped developing this project, and another German, Karl-Heinz Kübbeler, took it over.
Since it doesn't contain many components, various radio amateurs and DIY enthusiasts soon started building their own versions and improving it.
About a year ago, I posted a couple of versions that you can build yourself.
The first added a lithium battery for standalone operation and a charger for it.
The second got a few more modifications. The main differences were a slightly revised encoder connection, a redesigned control circuit for the step-up converter used to test Zener diodes, and a software change so you don't have to hold down a button while testing them. The battery converter and charger were also moved onto this board.
At the time I published it, the second version was almost fully featured; the only thing it really lacked was a graphical display.

In this review, I'll tell you about a simpler, but more user-friendly version of the instrument (thanks to its graphical display), which is quite accessible even to a beginner electronics hobbyist who wants to build one.

As always, I'll start the review with the packaging.
The kit arrived in a small cardboard box. That's already better than last time, but I'd still like to see nicer packaging for kits like this, with color printing and thicker cardboard.
Inside the box was the kit in an antistatic bag.


The whole kit is sealed in an antistatic bag with a zip closure, so it might come in handy for something later :)


After unpacking, it looked, let's say, like a bit of a jumble. But it's worth noting that the display was placed face-down against the circuit board, so it would be pretty hard to damage it—although the postal service sometimes manages to make the impossible possible.


Today's review will be a little more streamlined than my previous kit reviews, since I don't have anything particularly new to say about assembly, and I don't want to repeat myself. But I'll spend a little more time on the components that weren't included in the previous kits I reviewed.

The circuit board measures 75 × 63 mm.
The build quality is good, and assembling and soldering it was a thoroughly enjoyable experience.




As with the DDS generator board, the components are clearly labeled, and there's no schematic included.
As on the DDS generator board, the manufacturer used the same trick with double-layer vias. Though for some reason, they left a small trace stub in one spot.


The device’s “brain” is an Atmega328 microcontroller made by Atmel. It’s far from the most powerful microcontroller used in this tester. I used an Atmega644; there are also versions based on the ATmega1284, I think.
The issue isn’t really the microcontroller’s “power,” but the amount of flash memory available for storing the program. The device gradually gains new features, and the program grows in size, so more “brainy” controllers are used.
After testing the device and its capabilities, I can say that the microcontroller seems to be used to its full potential here. At the same time, the higher-end version probably wouldn’t add anything new, since there’s no way to improve things without redesigning the board.


The device uses a 128×64 graphical display.
The original version of the tester used a display with 2 lines of 16 characters, just like my first version.
The next expansion of the project involved switching to a display with 4 lines of 20 characters, since the smaller display often couldn’t fit all the information.
After that, to make the device more convenient to use, the developer decided to switch to a graphical display. The key difference is that a graphical display can show a graphic representation of the component being tested.


And here’s the complete kit.


Of course, I’ll include the device’s schematic :)
I originally started tracing the schematic from the board, but then decided to look for it online and found it. The schematic I found did have one small inaccuracy, even though it was for this very kit: it was missing two resistors and a capacitor used for the frequency measurement input.


I’ll go over the key sections of the schematic separately.
The most critical section is highlighted in red: a network of six resistors. These need to be selected with particular care, because the accuracy of the readings depends on their precision. They must also be installed in the correct positions; if you mix them up, the device will still work, but the readings will be nonsensical.
The reference voltage circuit is highlighted in green. It’s just as important, but easier to reproduce, since an adjustable TL431 shunt regulator is much easier to find than precision resistors.
The power control circuit is shown in blue.
The circuit is designed so that pressing the button powers up the microcontroller. It then keeps the power on itself and can switch it off when needed.

The other sections are fairly standard and not particularly interesting: the crystal resonator, display connection, and 5 V power regulator.

As I mentioned above, the schematic became popular thanks to its simplicity. The original version didn’t include the encoder connection circuit (resistors R17, 18, 20, 21) or the frequency counter input circuit (R11, 13 and C6).
The core of the device is really the algorithm that cycles through different combinations of output switching, with the outputs connected to a resistor matrix, and measures the resulting voltages.
That’s what Markus Frejek came up with, laying the groundwork for work on this very interesting device.
The schematic started gaining all its extra features mainly after Karl-Heinz Kübbeler took it up. I may be a little off, but as far as I know, the device later “learned” to measure frequency, work as a frequency generator, measure capacitor ESR, and test crystal resonators and Zener diodes, among other things.
As the project developed, Chinese manufacturers took an interest in the device and released a kit based on one of the versions. They also sell fully assembled versions.


As I mentioned above, the key parts of the circuit are several resistors that need to be highly accurate.
This kit comes with resistors rated at 0.1% accuracy, indicated by the final violet band. Many thanks to the manufacturer for that.
In the table for identifying resistor values above, the highest accuracy listed is only 0.05%.
Finding precision resistors can often be a problem when assembling a device like this.


After installing these resistors on the board, I recommend moving on to the 10k resistors, since there are more of those than any other value. It will then be easier to find the rest.


The kit also included resistors with other values. To make assembly easier, here are their markings:
2 pcs. 1k
2 pcs. 3.3k
2 pcs. 27k
1 pc. 220 Ω
1 pc. 2.2k
1 pc. 33k
1 pc. 100k


Once all the resistors are installed, the board should look something like this.


There shouldn't be any questions about installing the capacitors and crystal resonator. I explained the markings in one of my previous reviews; just be careful and you'll be fine.
The only things to watch out for are the 10 nF capacitor (marked 103) and the polarity of the electrolytic capacitors.


The circuit board after installing the capacitors.


The kit included three transistors, a 7550 voltage regulator, and a TL431 adjustable shunt regulator.
Install them on the board according to the markings. Both the component positions and their orientation are indicated.


Almost all the main components are in place.


Make sure the socket for the microcontroller is installed the right way around. A socket installed incorrectly can cause no end of frustration later.


And that's the main component installation done. At this point, we can move on to soldering.
I'm often asked what I use for soldering.
I use solder from an unknown manufacturer. I bought it by chance, but in a large quantity. The quality is excellent, but I can't tell you where to buy it because I don't know; it was quite a while ago.
The solder contains flux, so I don't use any additional flux on boards like this.
My soldering iron is a basic Solomon, connected to a miniature soldering station—or rather, a power supply—with temperature control. The iron runs on 24 V.

The board was a pleasure to solder. There wasn't a single spot where I needed to add flux or clean anything up.


The small components are soldered in, so we can move on to the larger ones:
14-pin ZIF socket
Encoder
Display connector socket
LED.


I'll briefly describe a couple of the new components.
First, the encoder.


I found an image on Wikipedia that helps explain how an encoder works.


To put it simply, in a nutshell:
An encoder (the one in the photo) has two contacts that close as you turn the knob.
But they close in a clever sequence: turning it one way closes the first contact, then the second; after that, the first opens, followed by the second.
Turning the knob the other way reverses the sequence completely.
The microcontroller determines which way the knob is turning based on the order in which the contacts close. The encoder knob rotates through 360 degrees and has no stop, unlike a potentiometer.
They are used for various purposes, one of which is as a control for adjusting different electronic devices.
Sometimes they also have a pushbutton, whose contacts close when you press the knob. This kit uses that type.

Encoders come in different types: mechanical-contact, optical, Hall-effect, and so on.
They are also classified by how they work.
This kit uses an incremental encoder, which simply outputs pulses as it turns. There are other types too, such as absolute encoders, which let you determine the knob's angle at any given moment. These are used in angle sensors.
For the more curious, here's a link to the article on Wikipedia.


The kit also includes a socket. Unlike the previous one, you don't need to press the component's leads against the contacts when inserting it.
The socket has two positions, as shown in the photo:
1. Socket open; you can insert a component
2. Socket closed; the contacts are pressed against the component leads.
By the way, it's best to install and solder the socket while it's open, since its contacts shift slightly depending on the lever position.


A few words about installing the LED.
Sometimes you need to raise an LED above the circuit board. You can position it by hand, or make the process a little easier and more precise.
I use insulation from a stranded cable for this.
First, determine the required height, then cut a piece to the right length and slide it over the leads.
The rest is straightforward: put the LED in place and solder it. This method is especially useful when mounting several LEDs at the same height; just cut the required number of tubes to the same length.
An added bonus: the LED is harder to bend to one side.


Once these components are installed and soldered, we can move on to the final step: installing the display.
An attentive reader will notice that I made a small mistake, which I only caught during testing.
I soldered the power wires incorrectly. Out of habit, I connected the positive lead to the square pad and the negative lead to the round one. This kit is wired the other way around, as indicated by the markings. Follow the markings on the circuit board when soldering.
Fortunately, nothing happened; the device simply didn't turn on. So we can count reverse-battery protection among its advantages.


First, install and screw in the mounting standoffs. Be sure to install them on the main board first.
Then insert the male connector into the female connector.


The display has a lot of contacts, but only some of them are used, so it has to be installed in this order.
Install the display in its original position.


The mounting holes should line up.
If the display is straight, the contacts will line up by themselves.
Before soldering, remember to cover the front of the display with something.


Everything is assembled, but there's one component left. Don't worry, we haven't forgotten to solder anything, and the manufacturer didn't include it by accident.
It's not an extra part—in fact, it's very much needed.


The kit includes a 0.22 µF capacitor.
This capacitor is needed when calibrating the device. I think the manufacturer was right to include it in the kit, since it lets you calibrate the device without having to hunt down extra components.


All done. Connect the battery and... nothing happens :)
That's normal. The circuit doesn't have an obvious power switch, but it does have one.
To turn the device on, press the encoder knob. This powers up the processor, which then sends a command to the power control circuit and keeps it switched on.


It's on, but it clearly isn't happy about something—look at all that text on the screen.
Let's see what's wrong.


First, the device displays the battery voltage and tries to switch to component-testing mode.
Since nothing is connected, it reports that the component is missing or damaged.
But the device hasn't been calibrated, so it displays the corresponding message:
Not calibrated!
To calibrate it, short all three contacts on the panel (in our case, the middle contact and one contact from each of the left and right groups of three), then turn the device on. Actually, there's another way to do this, which I'll describe later.


After the “isolate probe” message, remove the jumper and leave the contacts open.
Then, when prompted, connect the capacitor included in the kit to terminals 1 and 3.


Well, let's try calibrating it.
1. To do this, I simply went into the menu by holding the power button for a couple of seconds, then selected Selftest.
To enter the menu, press and hold the encoder button.
Navigate the menu by turning the encoder
Select an option or mode with a short press of the encoder button

2. The device displays a message asking you to short the contacts. You can use a piece of wire or a jumper—anything will do, as long as all three contacts are connected together.
3, 4. The device measures the resistance of the jumper, the traces leading to the socket, and so on.


1, 2. Then it takes some more measurements I don't understand, and finally tells you to remove the jumper.


I lift the lever and remove the jumper, but the device keeps measuring something.


1. At this stage, connect the capacitor supplied with the kit to terminals 1 and 3 (you can use a different one, but the supplied capacitor is the easiest option).
2. After the capacitor is connected, the device continues measuring. You don't need to press the encoder button at any point during calibration; everything happens automatically.


That's it—the calibration is complete. The device is ready to use.
If necessary, you can repeat the calibration. Just select the corresponding menu item again and go through all the steps above.


Let's take a quick look at the menu options and see what the device can do.
Transistor — measures semiconductor parameters and resistor values
Frequency — measures the frequency of a signal connected to the GND and F-IN contacts on the board. They're at the top right, above the display.
F-generator — generates square-wave pulses at various frequencies.
10bit PWM, — outputs square-wave pulses with adjustable duty cycle.
C+ESR — I didn't quite understand this menu item, since selecting it just displays this text on the screen, and that's it.
rotary encoder — tests encoders.
Selftest — we've already used this one; it starts self-calibration
Contrast — adjusts the display contrast
Show data — I'll show you this one a little later.
Switch off — forcibly switches off the device. The device does have an auto-off function, but it isn't active in every mode.

I don't know why, but from a distance this photo reminded me of the good old VC.


A little about the Show data menu item, which I don't understand.
I couldn't figure out what it's for when using the device, since in this mode the screen displays what can be shown on the screen.
It also displays the auto-calibration parameters in this mode.




The fonts displayed on the screen are also shown in this mode. I think this is more of a diagnostic feature, just for checking what gets displayed and how, nothing more.
The last photo shows the contrast adjustment mode.
The default setting is 40. I tried adjusting it, but the original setting seemed to be the best.


That’s it for the inspection; now we can move on to testing.
Since the device is quite versatile, I’ll test a range of components—not necessarily with precise values, but enough to get an idea of what it can do.
If you’d like me to test a particular type of component, let me know and I’ll add it.
1. 0.39025 µF capacitor, 1%
2. 7850 pF capacitor, 0.5%
3. Some kind of Jamicon 1000 µF, 25 V
4. Capxon 680 µF, 35 V, low-impedance


Capxon 10000 µF, 25 V


1. 75 Ω resistor, 1%
2. 47 kΩ resistor, 0.25%
3. 1N4937 diode
4. 25CTQ035 diode assembly


1. BC547B bipolar transistor
2. IRFZ44N MOSFET


1, 2 — 22 µH inductor
3, 4 — 100 µH inductors of different types


1. Relay coil
2. Sounder with built-in oscillator.


Let’s test the device in generator mode.
10 kHz
100 kHz
In my opinion, the waveform is quite acceptable even at 100 kHz.


The generator’s maximum frequency is 2 MHz. Of course, the waveform looks worse here, but the oscilloscope probe was set to 1:1, and the oscilloscope itself isn’t particularly high-frequency.
The next setting is 1000.000 mHz—don’t confuse that with MHz. That’s just how it labels a 1 Hz signal :)


Output mode with adjustable duty cycle.
Frequency: 8 kHz


Now let’s take a look at the built-in frequency counter.
The oscilloscope’s built-in generator was used as the signal source.
1. 10 Hz square wave
2. 20 kHz sine wave
3. 200 kHz square wave
4. 2 MHz square wave


But at 4 MHz, the frequency counter gave up. The maximum measurable frequency is 3.925 MHz, which is actually quite good for a multifunction instrument.
Unfortunately, checking the frequency measurement accuracy is quite difficult, since few people have a good calibrated signal generator. But for most hobbyist applications, this accuracy is more than adequate.


And finally, a group photo.
Two instruments from previous reviews, together with their new “sibling.”


Summary.
Pros
Well-made circuit board.
Complete kit for building a working instrument, plus a calibration capacitor
0.1% resistors included
Very easy and enjoyable to assemble; suitable even for complete beginners
The finished instrument has good specifications.
I happened to discover that the instrument has reverse-polarity protection :)

Cons
The kit comes in very basic packaging
Battery-powered; a rechargeable battery would have been a much better choice

My take: I think this is a very good kit. I would definitely recommend it as a gift for a beginner electronics hobbyist. It could use an enclosure and rechargeable battery; the battery won't last long, and replacements are quite expensive.
I was pleasantly surprised that the kit included the “right” resistors and a calibration capacitor. The former improves accuracy, while the latter makes things more convenient—you don't have to hunt down a capacitor for calibration. You can calibrate it and start using it as soon as it's assembled.
Of course, this kit costs more than buying the same thing already assembled. But how do you put a price on the experience of assembling it yourself, the skills you gain, and the little bit of hands-on experience?

That's about it. I hope you found the review interesting and useful. I'm happy to answer questions and hear suggestions for additions to the review.
Coming up, I'll be reviewing another small instrument that I hope will be interesting. I haven't found the original version yet, but the tests will show what it's all about.

Update — link to download the assembly instructions (in English)

The product was provided by the store for review.