noise

48 Articles

DIY laser microphone on cutting mat

Spy Tech: Build Your Own Laser Eavesdropper

March 18, 2025 by 33 Comments

Laser microphones have been around since the Cold War. Back in those days, they were a favorite tool of the KGB – allowing spies to listen in on what was being said in a room from a safe distance. This project by [SomethingAbtScience] resurrects that concept with a DIY build that any hacker worth their soldering iron can whip up on a modest budget. And let’s face it, few things are cooler than turning a distant window into a microphone.

At its core this hack shines a laser on a window, detects the reflected light, and picks up subtle vibrations caused by conversations inside the room. [SomethingAbtScience] uses an ordinary red laser (visible, because YouTube rules) and repurposes an amplifier circuit ripped from an old mic, swapping the capsule for a photodiode. The build is elegant in its simplicity, but what really makes it shine is the attention to detail: adding a polarizing filter to cut ambient noise and 3D printing a stabilized sensor mount. The output is still a bit noisy, but with some fine tuning – and perhaps a second sensor for differential analysis – there’s potential for crystal-clear audio reconstruction. Just don’t expect it to pass MI6 quality control.

While you probably won’t be spying on diplomats anytime soon, this project is a fascinating glimpse into a bygone era of physical surveillance. It’s also a reminder of how much can be accomplished with a laser pointer, some ingenuity, and the curiosity to see how far a signal can travel.

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Long-tail pair waves

Current Mirrors Tame Common Mode Noise

March 17, 2025 by 12 Comments

If you’re the sort who finds beauty in symmetry – and I’m not talking about your latest PCB layout – then you’ll appreciate this clever take on the long-tailed pair. [Kevin]’s video on this topic explores boosting common mode rejection by swapping out the old-school tail resistor for a current mirror. Yes, the humble current mirror – long underestimated in DIY analog circles – steps up here, giving his differential amplifier a much-needed backbone.

So why does this matter? Well, in Kevin’s bench tests, this hack more than doubles the common mode rejection, leaping from a decent 35 dB to a noise-crushing 93 dB. That’s not just tweaking for tweaking’s sake; that’s taking a breadboard standard and making it ready for sensitive, low-level signal work. Instead of wrestling with mismatched transistors or praying to the gods of temperature stability, he opts for a practical approach. A couple of matched NPNs, a pair of emitter resistors, and a back-of-the-envelope resistor calculation – and boom, clean differential gain without the common mode muck.

If you want the nitty-gritty details, schematics of the demo circuits are on his project GitHub. Kevin’s explanation is equal parts history lesson and practical engineering, and it’s worth the watch. Keep tinkering, and do share your thoughts on this.

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Turning Down The Noise On SMPS

March 17, 2025 by 13 Comments

On paper, electricity behaves in easy-to-understand, predictable ways. That’s mostly because the wires on the page have zero resistance and the switching times are actually zero, whereas in real life neither of these things are true. That’s what makes things like switch-mode power supplies (SMPS) difficult to build and troubleshoot. Switching inductors and capacitors tens or hundreds of thousands of times a second (or more) causes some these difficulties to arise when these devices are built in the real world. [FesZ Electronis] takes a deep dive into some of the reasons these difficulties come up in this video.

The first piece of electronics that can generate noise in an SMPS are the rectifier diodes. These have a certain amount of non-ideal capacitance as well as which causes a phenomenon called reverse current, but this can be managed by proper component choice to somewhat to limit noise.

The other major piece of silicon in power supplies like this that drives noise are the switching transistors. Since the noise is generally caused by the switching itself, there is a lot that can be done here to help limit it. One thing is to slow down the amount of time it takes to transition between states, limiting the transients that form as a result of making and breaking connections rapidly. The other, similar to selecting diodes, is to select transistors that have properties (specifically relating to inherent capacitances) that will limit noise generation in applications like this.

Of course there is a lot more information as well as charts and graphs in [FesZ]’s video. He’s become well-known for deep dives into practical electrical engineering topics like these for a while now. We especially like his videos about impedance matching as well as a more recent video where he models a photovoltaic solar panel in SPICE.

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Open-Source Random Numbers

February 20, 2025 by 26 Comments

Whether it’s a game of D&D or encrypting top-secret information, a wide array of methods are available for generating the needed random numbers with high enough entropy for their use case. For a tabletop game this might be a single die but for more sensitive applications a more robust method of generating random numbers is needed. Programmers might reach for a rand() function of some sort, but these pseudorandom numbers don’t cut the mustard for encryption. For that you’ll need a true random number generator (RNG), and this open-source hardware RNG uses one of the better methods we’ve seen.

The device, called RAVA, is based on a property found in many electronic devices called avalanche breakdown. Avalanche breakdown occurs when a high voltage (in this case approximately 25V) is applied in the reverse bias direction, with this device using a pair of Zener diodes. When this high voltage is applied, an “avalanche” of electrons occurs which allows the diodes conduct in the opposite direction that they would when they are forward biased. This isn’t a constant current flow, though; there are slight variations over time which can be amplified and used as the random number generator. The noise is amplified over a series of op amps and then fed to an ATmega32U4 microcontroller which can provide the user with 136.0 Kbit/s of random data.

Unlike other random number generators, this device is based on a method generally accepted to be truly random. Not only that, but since it’s based on discrete hardware it can be accessed directly for monitoring and replacement in case of faults, unlike other methods which are more “black boxes” and are more opaque in their processes which are thus harder to audit. We also appreciate it’s open-source nature as well, and for some more information on it be sure to check out the paper on it in IEEE. If you’re looking for something to generate random numbers but will also bring some extra flair to the next game night, take a look at this radioactive dice replacement.

Film Capacitors Can Go In The Wrong Way Round? Who Knew!

January 27, 2025 by 33 Comments

You can work with a part for many decades, and still learn something new about it. At least we can, and we don’t mind admitting it. Take film capacitors — we all know they aren’t a polarized part like an electrolytic capacitor is, but as [TheDannVal] points out, that doesn’t mean both their leads are the same.

This might sound counterintuitive, but if you consider for a moment their construction it makes sense. A film capacitor is made from two strips of foil with a strip of plastic film between then, rolled up tightly into a cylinder. One of the pieces of foil that forms one side of the capacitor ends up on the outside of the cylinder, and thus forms the shield for the other. Thus if that side isn’t connected to the lower impedance side of whichever circuitry it resides in, it can pick up noise, while the inside strip of foil can not. It’s so obvious when demonstrated, but we have to admit to never having considered it before. Some film capacitors have a line marked on them to denote the connection forming the shield, for those that don’t he provides a couple of methods for detecting it.

The full video is below the break, and maybe you too can now pay attention to your capacitors for lower noise audio circuitry.

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A solar-powered decibel meter the size of a business card.

2024 Business Card Challenge: NoiseCard Judges The Sound Around You

June 22, 2024 by 6 Comments

Let’s face it: even with the rise of the electric car, the world is a noisy place. And it seems like it has only gotten worse in recent years. But how can we easily quantify the noise around us and know whether it is considered an unhealthy decibel level?

That is where the NoiseCard comes in. This solar-powered solution can go anywhere from the regrettable open office plan to the busy street, thanks to a couple of 330 µF capacitors. It’s based on the low-power STM32G031J6 and uses a MEMS microphone to pick up sound from the back of the card, which the code is optimized for. Meanwhile, the LEDs on the front indicate the ambient noise level, ranging from a quiet 40 dB and under to an ear-splitting 105 dB or greater.

When it comes to building something the size of a business card, every component is under scrutiny for size and usefulness. So even the LEDs are optimized for brightness and low power consumption. Be sure to check it out in action after the break in various environments.

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AI System Drops A Dime On Noisy Neighbors

April 25, 2024 by 4 Comments

“There goes the neighborhood” isn’t a phrase to be thrown about lightly, but when they build a police station next door to your house, you know things are about to get noisy. Just how bad it’ll be is perhaps a bit subjective, with pleas for relief likely to fall on deaf ears unless you’ve got firm documentation like that provided by this automated noise detection system.

OK, let’s face it — even with objective proof there’s likely nothing that [Christopher Cooper] is going to do about the new crop of sirens going off in his neighborhood. Emergencies require a speedy response, after all, and sirens are perhaps just the price that we pay to live close to each other. That doesn’t mean there’s no reason to monitor the neighborhood noise, though, so [Christopher] got to work. The system uses an Arduino BLE Sense module to detect neighborhood noises and Edge Impulse to classify the sounds. An ESP32 does most of the heavy lifting, including running the UI on a nice little TFT touchscreen.

When a siren-like sound is detected, the sensor records the event and tries to classify the type of siren — fire, police, or ambulance. You can also manually classify sounds the system fails to understand, and export a summary of events to an SD card. If your neighborhood noise problems tend more to barking dogs or early-morning leaf blowers, no problem — you can easily train different models.

While we can’t say that this will help keep the peace in his neighborhood, we really like the way this one came out. We’ve seen the BLE Sense and Edge Impulse team up before, too, for everything from tuning a bike suspension to calming a nervous dog. Continue reading “AI System Drops A Dime On Noisy Neighbors” →