Energy Harvesting Wristwatch Uses a Versatile Photodiode

There’s some interesting technology bundled into this energy harvesting wristwatch. While energy harvesting timepieces (called automatic watches) have been around for nearly 240 years, [bobricius] has used parts and methods that are more easily transferable to other projects.

Unlike early mechanical systems, this design uses the versatile BPW34 PIN photodiode (PDF warning). PIN photodiodes differ from ordinary PN diodes in that there’s a layer of undoped ‘intrinsic’ silicon separating the P and N doped layers. This reduces the utility of the diode as a rectifier, while allowing for higher quantum efficiency and switching speed.

They are typically used in the telecommunications industry, but have a number of interesting ‘off label’ applications. For example, the BPW34 can be used as a solid-state particle detector (although for detecting alpha particles you’re better off with something in a TO-5 package such as the Hamamatsu S1223-01). The fast response speed means you can send data with lasers or ambient light at high frequencies – a fun use for an LED lighting system or scrap DVD-RW laser.

Some common solar panels are essentially large PIN photodiodes. These are the brownish panels that you’ll find in a solar-powered calculator, or one of those eternally waving golden plastic neko shrines. They specifically offer excellent low-light performance, which is the basis of the energy harvesting used in this project.

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Hackaday Prize Entry: Self Sustained Low Power Nodes

Consider for a second the Internet of Things. A vast network of connected devices, programmable matter, and wearable electronics can only mean one thing: there’s going to be a ton of batteries. While changing the battery in a smoke detector may seem tolerable, changing the batteries in a thousand sensor nodes is untenable. The solution to this problem is self-contained sensor nodes, and right now the best power source for mobile devices is probably solar.

For his Hackaday Prize entry, [Shantam Raj] is building a self-contained sensor node. It’s a Bluetooth device for the Internet side of this Thing, but the real trick to this device is solar energy harvesting and low power capabilities through optimized firmware.

Basically, this system is a low-power SoC with Bluetooth. The power from this device comes from a small solar cell coupled with a very efficient power supply and some new, interesting supercapacitors from Murata. These supercaps are extremely small, have high storage capacity, low ESR, and fast charging and discharging. The test board (seen in the video below) provides a proof of concept, but this device has a problem: there’s a single ‘sanity check’/power LED on the board that consumes 4 mA. The microcontroller, when running the optimized firmware, only consumes 1 mA. Yes, the LED thrown into the prototype that only serves as an indication the device is on is the biggest power sink in the entire system.

This project is great, and it’s exactly what we’re looking for in The Hackaday Prize. If the Internet of Things ever happens as it was envisioned, we’re going to be buried under a mountain of coin cell lithium batteries. Some sort of energy harvesting scheme is the only way around this, and we’re happy to see someone is working on the problem.

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Powering A Laptop With Supercapacitors

What do you do when you find a small horde of supercapacitors? The correct answer is a spectrum of dangerous devices ranging from gauss guns to quarter shrinkers. [Rinoa] had a less destructive idea: she’s replaced the battery in a laptop with a bank of supercapacitors.

The supercaps in question are 2.7 Volt, 500 Farad caps arranged in banks six for a total of about 3 watt-hours in each bank. The laptop used for this experiment is an IBM Thinkpad from around 1998. The stock battery in this laptop is sufficiently less advanced than today’s laptop batteries. Instead of using a microcontroller and SMBus in the battery, the only connections between the battery and laptop are power, ground, and connections for a thermocouple. This is standard for laptops of the mid-90s, and common in low-end laptops of the early 2000s. It also makes hacking these batteries very easy as there’s no associated microprocessors to futz around with.

With all the capacitor banks charged, the laptop works. It should – there isn’t a lot of intelligence in this battery. With one bank of six supercaps, [Rinoa] is getting a few minutes of power on her laptop. With a stack of supercaps that take up about the same volume as this already think Thickpad, [Rinoa] can play a few turns of her favorite late-90s turn-based strategy game. It’s not much, but it does work.

Check out [Rinoa]’s video below.

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Will Supercapacitors Ever Replace Batteries?

Recharging your mobile phone or your electric vehicle in a few minutes sure sounds appealing. Supercapacitor technology has the potential to deliver that kind of performance that batteries currently can’t, and while batteries are constantly improving, the pace of development is not very fast. Just remember your old Nokia mobile with Ni-Cad batteries and several days of usage before a recharge was needed. Today we have Lithium-Ion batteries and we have to charge our phones every single day. A better energy storage option is clearly needed, and supercapacitors seem to be the only technology that is close to replace the battery.

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Hand Cranked Generator Charges Supercaps, Starts Car

Pity the lowly lead-acid battery. A century of use as the go-to method for storing enough electrons to spin the starter motor of a car engine has endeared it to few.  Will newer technology supplant that heavy, toxic, and corrosive black box under your hood? If this supercapacitor boost box is any indication, then we’d say lead-acid’s days are numbered.

To be fair, we’ll bet that number is still pretty big. It takes a lot to displace a tried and true technology, especially for something as optimized as the lead-acid battery. But [lasersaber]’s build shows just how far capacitive storage has come from the days when supercaps were relegated to keeping your PC’s clock running. With six commercial 400F caps and a custom-built balance board, the bank takes a charge from a cheap 24V hand generator. The output is either to a heavy-duty lighter socket or some automotive-style lugs, and the whole thing is housed in a simple box partially constructed using energy stored in the bank. Can the supercaps start a car? Stay tuned after the break for the answer.

Although we’ve seen supercaps replace a motorcycle battery before, we’re a little disappointed that the caps used here only have a 1500-hour life – lead-acid wins that fight hands down. But this one gives us lots of ideas for future builds, and we’re heartened by the fact that the supercaps for this build ring up to less than $70.

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The Comic Book World Of Capacitor Marketing

The Economist is an interesting publication, a British weekly newspaper that looks for all the world like a magazine, and contains pithy insights into world politics and economic movements. It’s one of those rare print news publications that manages to deliver fresh insights even to hardened web news junkies despite its weekly publication date.

It was typical then of their wide-ranging coverage of world industries to publish a piece recently on the world of supercapacitors, with particular focus on Estonia’s Skeleton Technologies. This is an exciting field in which the products are inching their way towards energy density parity with conventional batteries, and news of new manufacturing facilities coming online should be of interest to many Hackaday readers.

Exciting though it may be it was not the news of a new capacitor plant in Germany that provided the impetus for this piece. Instead it was the language used by the Economist writer delicately skirting the distinction between the words “Supercapacitor” and “Ultracapacitor”. Images of flying crimefighters in brightly coloured capes spring instantly to mind, as Captain Ultra and Superman battle an arch-villain who is no doubt idly bouncing a piece of burning Kryptonite against the wall in readiness for the final denouement.

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History Of The Capacitor – The Modern Era

The pioneering years in the history of capacitors was a time when capacitors were used primarily for gaining an early understanding of electricity, predating the discovery even of the electron. It was also a time for doing parlor demonstrations, such as having a line of people holding hands and discharging a capacitor through them. The modern era of capacitors begins in the late 1800s with the dawning of the age of the practical application of electricity, requiring reliable capacitors with specific properties.

Leyden Jars

Marconi with transmitting apparatus
Marconi with transmitting apparatus, Published on LIFE [Public domain], via Wikimedia Commons
One such practical use was in Marconi’s wireless spark-gap transmitters starting just before 1900 and into the first and second decade. The transmitters built up a high voltage for discharging across a spark gap and so used porcelain capacitors to withstand that voltage. High frequency was also required. These were basically Leyden jars and to get the required capacitances took a lot of space.


In 1909, William Dubilier invented smaller mica capacitors which were then used on the receiving side for the resonant circuits in wireless hardware.

Early mica capacitors were basically layers of mica and copper foils clamped together as what were called “clamped mica capacitors”. These capacitors weren’t very reliable though. Being just mica sheets pressed against metal foils, there were air gaps between the mica and foils. Those gap allowed for oxidation and corrosion, and meant that the distance between plates was subject to change, altering the capacitance.

In the 1920s silver mica capacitors were developed, ones where the mica is coated on both sides with the metal, eliminating the air gaps. With a thin metal coating instead of thicker foils, the capacitors could also be made smaller. These were very reliable. Of course we didn’t stop there. The modern era of capacitors has been marked by one breakthrough after another for a fascinating story. Let’s take a look.

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