We all know that it’s easy to get caught out by fake electronic components these days, with everything from microcontrollers to specialized ASICs being fair game. More recently, retro components that were considered obsolete decades ago are now becoming increasingly popular, with the unijunction transistor (UJT) a surprising example of this. The [En Clave de Retro] YouTube channel released a video (Spanish, with English dub) documenting fake UJTs bought off AliExpress.
These AliExpress UJTs were discovered after comments to an earlier video on real UJTs said that these obsolete transistors are still being manufactured and can be bought everywhere, meaning mostly on AliExpress and Amazon. Of course, this had to be investigated, as why would anyone still manufacture UJTs today, and did some Chinese semiconductor factory really spin up a new production line for them?
Perhaps unsurprisingly, some tests later and after a quick decapping of the metal can, the inside revealed a bipolar transistor (BJT) die (see top image on the left). Specifically, a PNP BJT transistor die, packaged up inside a vintage-style metal can with fake markings claiming it is a 2N2646 UJT.
The video suggests that scams like these might be because people want to get vintage parts for cheap, and that’s created a new market for people who would rather get scammed than deal with the sticker shock of paying for genuine new-old-stock or salvaged components. For example, while programmable unijunction transistors (PUTs) like the 2N6028 are still being manufactured, they cost a few dollars a pop in low quantities. UJTs used to be common in timer circuits, but now we have the 555.
In my first real job after leaving Uni an engineer told me that there was no real reason to specify a UJT when a combination of a PNP and an NPN transistor could do the same job, and be more repeatable. I have used this circuit several times since.
See https://www.instructables.com/Circuit-Collection-of-the-Programmable-Unijunction/ for a discussion.
+3.
…an extremely valuable piece of component-knowledge and circuit-design and which most people have forgotten—if they ever knew it (and, yes: there are very many situations where a UJT and / or PUT is the only correct, elegantly simple solution).
Many thanks for the refresher.
Three ‘thumbs-up’.
Yaho.com? Really?
Why would anyone want to buy a UJT? What is is good for? Do people need them to fix old gear or do they build new stuff with them? That’s the part that’s missing from the article, IMHO.
(I bought a bunch of germanium diodes off AliExpress recently, so I’m genuinely interested ;-))
Why would anyone want to buy vacuum tubes (valves), antiques, or classic cars? Old stuff makes for many good hobbies for certain people.
i googled it and found that a UJT has a negative response — becomes less conductive as the base current increases — over a part of its range. not sure i interpretted it correctly but it seems to add to the dizzying array of basic types of transistor. seems like it would have unique implications for oscillators.
i try not to think too much about exotic transistors. i know if you’re building a power supply there’s things like triacs that can be really relevant. but i think a lot of the diversity represents an ancient obsession with minimizing parts count…these days, throwing 3 or 3,000,000 transistors at an oscillator doesn’t seem like a big deal so these exotic transistors aren’t as interesting. anyways i resent that they take up space in my brain, when truly i struggle to properly understand even a BJT :)
A unijunction transistor can be used to make a simple oscillator whose frequency can be set by a single resistor (or capacitor). So we have many 1970s circuits for electronic organs using a unijunction, a capacitor, and some resistors. The original Stylophone worked that way, I think. Add another bipolar transistor in a fixed-frequency phase-shift oscillator configuration for vibrato. I built an organ like this myself, probably from a schematic in a Babani book.
For anyone wishing to explore these fun circuits, Babani books can be found at the World Radio History website.
Now that I think about it, Everyday Electronics magazine published an electronic bagpipes project using unijunction transistors.
May 1974 issue of Everyday Electronics: https://www.worldradiohistory.com/UK/Everyday-Electronics/70s/Everyday-Electronics-1974-05.pdf
I have just about finished a transistor-only Nixie clock, which uses a KT117A UJT in a 32.768kHz crystal oscillator. After applying power, it takes several seconds for the oscillations to build up, fed by the UJT’s characteristic negative resistance. Along with frequency dividers and counters, the whole thing took almost 200 transistors, but it is a joy to see in operation :)
I get it now. Negative resistance, oscillators and electronic organs. Thank you all for your responses!
PSA: This video is in Spanish, not Robot American. Once I realised this I changed it back to the original audio track and enabled subtitles instead.
There is a browser extension that forces original audio and original subtitles:
https://addons.mozilla.org/en-US/firefox/addon/youtube-no-translation/
Control your life, don’t let the tech bros ruin everything.
The desktop website of YT allows changing audio tracks (incl. original).
If you’re on a mobile browser, choose “desktop website”.
That is if you store cookies forever and never sign out. The addon automates it so you can have your way every time on a clean browser session. raises hands apologetically
Original Stylophone pocket synthesizer used a UJT for the oscillator. Another transistor for driving the speaker, and another for vibrato. THREE transistors total. Beat that haha.
Also, a UJT osc has a spikey waveform, rich in harmonics, unique sound.
I’ve only used UJT’s as pulsed gate drive for big SCR’s, or xenon strobe lights, or the Jana mosquito repeller.