capacitor

135 Articles

Dynamic RAM From First Principles

June 22, 2026 by 26 Comments

Before the past year, many of us took computer memory for granted. It was one of the lower-cost parts of a PC build and was usually available in whatever quantity one desired. As its cost has skyrocketed, a lot of PC builders and other users of computers in general are taking a deeper look at memory, how much is really needed, and what its functions truly are. [Igor] is working on a drum sequencer project which needs a small amount of memory, and has built this dynamic RAM from discrete components.

The first video goes into the construction of the memory array and how its addressed. It’s only eight bytes total, and using fairly large electrolytic capacitors to store data means that a gigabyte of this memory would take up well over a thousand acres, but it’s still enough memory for [Igor]’s needs. In addition to the capacitor, each bit uses a pair of diodes to determine if a read or write is occuring, and a set of transistors on the read and write busses to perform those actions. Worth noting here is that dynamic RAM like this needs to be refreshed because the capacitors lose charge over time, but these large capacitors can hold charge sometimes overnight, as [Igor] has confirmed experimentally.

There’s a followup video to the construction of these modules as well, where [Igor] demonstrates a number of ways this module can be used, from controlling LED arrays, 7-segment displays, and then installs it into his drum machine. With 64 bits available it’s capable of creating up to eight beats with eight samples available per beat. Although there are complete machines available for all of this, we appreciate his goal of not buying any pre-manufactured hardware and instead constructing it all from the ground up. There are analog drum machine options available in this same style as well.

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This Alarm Clock Has The Capacity To Wake You

June 15, 2026 by 12 Comments

Every now and then a project comes into the Hackaday feed that has so many levels of wrong about it that you really shouldn’t do it at home, but is amusing enough to feature anyway with a warning. So it is with [ArcaEge]’s Capacitor Alarm Clock, which wakes up its unfortunate owner by blowing up electrolytic capacitors with reverse voltage. If you survive, you’ll certainly be awake!

It’s inspired unsurprisingly by an [ElectroBoom] video, and the premise is simple enough. An ESP32 serves as the clock, and triggers a relay for the alarm, which in turn overloads a suitably low-voltage electrolytic capacitor in a socket. The resulting explosion which appears in a video we’ve placed below the break, wakes the slumberer.

We don’t have to tell you that this is not the safest of hacks, and is presented here only for your entertainment. But it does provide a few points of interest, for example in identifying the difference between capacitors with a vent, and those without.

This isn’t the first time we’ve seen a project based around exploding capacitors, and that one maybe was a don’t-do-this-at-home too.

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2026 Green Powered Challenge: Supercapacitor Enables High-Power IoT

April 28, 2026 by 3 Comments

With all the battery technologies and modern low-current sleep modes in most microcontrollers, running a sensor and microcontroller combo off-grid and far away from any infrastructure is usually not too difficult a task. Often these sorts of systems can go years without maintenance or interaction. But for something that still has to be off-grid but needs to do some amount of work every now and then like actuating a solenoid or quickly turning a servo, these battery-based systems can quickly run out of juice. To solve that problem, [Nelectra] has come up with this high-power capacitor-based IoT system.

Although supercapacitors don’t tend to have the energy density of batteries, they’re perfectly capable of powering short tasks in off-grid situations like this. They’re also typically able to tolerate lower voltages, extreme temperatures, and shock better than most batteries as well. A small solar cell on the top of this device keeps it topped up, and when running in deep sleep mode can hold a charge for up to six days. In more real-world applications supporting sensors, relays, or other actuators, [Nelectra] has found that it can hold a charge for around three days. When a quick burst of power is needed, it can deliver 1.5 A at 9 V or 500 mA at 24 V.

[Nelectra]’s stated goal for this build is to bridge low-power energy harvesting and practical field actuation, enabling maintenance-free systems such as irrigation control and remote switching without batteries, going beyond simple sensor applications while not relying on always-on power from somewhere else. Something like this would work really well in applications like this automated farm, which has already provided some unique solutions to intermittent power and microcontroller applications that need very high reliability.

2026 Hackaday Green Powered Challenge

The Cutting Truth About Variable Capacitors

January 1, 2026 by 10 Comments

If you’ve seen a big air-variable capacitor, you may have noticed that some of the plates may have slots cut into them. Why? [Mr Carlson] has the answer in the video below. The short answer: you can bend the tabs formed by the slots to increase or decrease the capacitance by tiny amounts for the purpose of tuning.

For example, if you have a radio receiver with a dial, you can adjust the capacitor to make certain spots on the dial have an exact frequency. Obviously, you can only adjust in bands depending on how many slots are in the capacitor. Sometimes the adjustments aren’t setting the oscillator’s frequency. For example, the Delco radio he shows uses the capacitor to peak the tuning at the specified frequency.

You usually only find the slots on the end plates and, as you can see in the video, not all capacitors have the slots. Of course, bending the plates with or without slots will make things change. Just don’t bend enough to short to an adjacent plate or the fixed plates when the capacitor meshes.

Of course, not all variable capacitors have this same design. We’ve seen a lot of strange set ups.

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Dark lab setup with scientific looking drink dispenser

Scared For A Drink?

October 31, 2025 by 1 Comment

Halloween is about tricks and treats, but who wouldn’t fancy a bit to drink with that? [John Sutley] decided to complete his Halloween party with a drink dispenser looking as though it was dumped by a backstreet laboratory. It’s not only an impressive looking separating funnel, it even runs on an Arduino. The setup combines lab glassware, servo motors, and an industrial control panel straight from a process plant.

The power management appeared the most challenging part. The three servos drew more current than one Arduino could handle. [John] overcame voltage sag, brownouts, and ghostly resets. A healthy 1000 µF capacitor across the 5-volt rail fixed it. With a bit of PWM control and some C++, [John] managed to finish up his interactive bar system where guests could seal their own doom by pressing simple buttons.

This combines the thrill of Halloween with ‘the ghost in the machine’. Going past the question whether you should ever drink from a test tube – what color would you pick? Lingonberry juice or aqua regia, who could tell? From this video, we wouldn’t trust the bartender on it – but build it yourself and see what it brings you!

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2025 Component Abuse Challenge: An LED As A Light Dependent Capacitor

October 16, 2025 by 15 Comments

The function of an LED is to emit light when the device is forward biased within its operating range, and it’s known by most people that an LED can also operate as a photodiode. Perhaps some readers are also aware that a reverse biased LED also has a significant capacitance, to the extent that they can be used in some RF circuits in the place of a varicap diode. But how do those two unintentional properties of an LED collide? As it turns out, an LED can also behave as a light dependent capacitor. [Bornach] has done just that, and created a light dependent sawtooth oscillator.

The idea is simple enough, there is a capacitance between the two sides of the depletion zone in a reverse biased diode, and since an LED is designed such that its junction is exposed to the external light, any photons which hit it will change the charge on the junction. Since the size of the depletion zone and thus the capacitance is dependent on the voltage and thus the charge, incoming light can thus change the capacitance.

The circuit is a straightforward enough sawtooth oscillator using an op-amp with a diode in its feedback loop, but where we might expect to find a capacitor to ground on the input, we find our reverse biased LED. The video below the break shows it in operation, and it certainly works. There’s an interesting point here in that and LED in this mode is suggested as an alternative to a cadmium sulphide LDR, and it’s certainly quicker responding. We feel duty bound to remind readers that using the LED as a photodiode instead is likely to be a bit simpler.

This project is part of the Hackaday Component Abuse Challenge, in which competitors take humble parts and push them into applications they were never intended for. You still have time to submit your own work, so give it a go!

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It’s A Variable Capacitor, But Not As We Know It

September 17, 2025 by 22 Comments

Radio experimenters often need a variable capacitor to tune their circuits, as the saying goes, for maximum smoke. In decades past these were readily available from almost any scrap radio, but the varicap diode and then the PLL have removed the need for them in consumer electronics. There have been various attempts at building variable capacitors, and here’s [radiofun232] with a novel approach.

A traditional tuning capacitor has a set of meshed semicircular plates that have more of their surface facing each other depending on how far their shaft is turned. The capacitor presented in the first video below has two plates joined by a hinge in a similar manner to the covers of a book. It’s made of tinplate, and the plates can be opened or closed by means of a screw.

The result is a capacitor with a range from 50 to 150 picofarads, and in the second video we can see it used with a simple transistor oscillator to make a variable frequency oscillator. This can form the basis of a simple direct conversion receiver.

We like this device, it’s simple and a bit rough and ready, but it’s a very effective. If you’d like to see another unusual take on a variable capacitor, take a look at this one using drinks cans.

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