2025 Component Abuse Challenge: Reverse Biasing An NPN BJT

November 6, 2025 by John Elliot V 10 Comments

For the Component Abuse Challenge our hacker [Tim Williams] observes that N-P-N reads the same way forwards and backwards, so… what happens if we reverse bias one? (Note: this remark about N-P-N reading the same forward and backward is a lighthearted joke; in fact the level of doping in the emitter and collector is different so those Ns are not fungible and will exhibit different properties and have different characteristics.)

In the margin you can see how the question was originally posed by Bob Pease back in March 18, 1996.

In his article [Tim] mentions that some transistors are specifically designed to operate when reverse biased, which [Tim] calls “inverted mode”, whereas most transistors are not designed to work in this fashion and that’s the sort of abuse that could damage the component and lead it to malfunction.

But what is V_out? [Tim] reports that he measured approximately -0.4 volts using his high-impedance meter. We tried this experiment in the lab ourselves but we were not able to duplicate [Tim]’s result; however there is a long list of potential reasons for such an outcome. If you do this experiment yourself we would love to hear about your results in the comments section!

If you’re still learning about transistors you might like to check out our five part series on transistors as amplifiers, starting here: Won’t Somebody, Please, Think Of The Transistors!

Thanks to [Tim] for his submission, we wish him the best of luck in the competition!

2022 Sci-Fi Contest: Schrödinger’s Trigger Is Trained On Electrons, Not Cats

April 18, 2022 by Kristina Panos 20 Comments

While it’s true that Hackaday scribes and their families are sadly unable to compete in our contests, Hackaday alum are more than welcome to throw their hat in the ring. [Legionlabs] even made a game of it — they used only parts from the scrap heap, and even played beat the clock to build a real, science-fictiony, working thing in eight hours or less.

Okay, cool, but what does it do? Well, put simply, a rising edge on the input drives one of two outputs, lighting one of two drool-worthy flanged LEDs. Which output will [alight] is unknowable until observed, thus the Schrodinger’s aspect. In practice, the output is determined by sampling. In this case, the sampling is of the time difference between three electron-tunneling events.

Stage one of Schrodinger’s Trigger is a pair of inputs — one variable 10-15 VDC input and 5 VDC input. Then comes the electron-tunneling event generator. [Legionlabs] is reverse-biasing a semiconductor junction (a 2N551 transistor). What does that mean? If we consider the junction as a diode and apply voltage in the wrong direction, what happens? At best, nothing; at worst, the smoke monster appears to admonish us.

But with a semiconductor acting as a diode, some electrons are bound to jump across the junction. This is known as tunneling, and is a useful phenomenon as it is purely random.

Stage three consists of amplifying the signal from these rebel electrons via hex inverters. Why not op-amps? The CD4069s were cheaper and within reach. Finally, the amplified signals are sampled with an ATtiny12, and some assembly logic figures out which LED to light.

It’s nice to see an entry that leans more toward the science side of things while winning aesthetically. We dig the nice ABS enclosure, and are totally envious of [Legionlabs]’ access to flanged LEDs and those glass table top mounting point discs in the corners.

Play To The Beat Of This Robotic Drummer In A Box

June 8, 2019 by Dan Maloney 1 Comment

No drummer? No problem! With a little ingenuity, you can stuff the essentials of a drum kit into a box, and automate your rhythm section.

Mind you, [Franco Molina]’s “DrumCube” doesn’t quite have the flash of a human drummer, but it does keep a steady beat and has a charm of its own. The drum machine is mostly mechanical, reminding us somewhat of the Wintergatan Marble Machine which is as captivating to watch as it is to hear. The DrumCube has a snare drum played by two servo-controlled sticks, a kick drum using foam waggled back and forth between two piezo transducers hooked to a low-pass filter, and a reverse-biased transistor white noise generator used for the hi-hat. Sadly, the large gear appears to be just for show. An Arduino runs everything and makes sure the mechanical drum hits are synced to the electronic cymbals, which was no mean feat.

The video below shows it in action accompanying [Franco] on his guitar, and it looks as good as it sounds. Prefer a more compact, all-electronic drum kit? Here’s one that fits in your pocket.

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