It’s often said that if something is worth doing it’s worth doing right, or maybe even worth overdoing. This is clearly a concept that [ANTALIFE] takes very seriously, as made abundantly clear by projects like the solar powered “beating” heart he made as a gift for his wife. What for most of us would have ended up being a junk bin build becomes a considerable engineering project in his hands, with a level of research and fine tuning that’s frankly staggering.
But [ANTALIFE] didn’t put this much thought into the device just for fun. He wants it to remain functional for as long as 30 years, and hopes he and the missus can still look on it fondly in their retirement years. Keeping an electronic device up and running for decades straight means you need to look carefully at each component and try to steer clear of any potential pitfalls.
The biggest one was the battery. More specifically, the fact he couldn’t use one. The lifetime of most rechargeable batteries is measured in hundreds of cycles, which for a device which will be charged by solar every day, means the battery is going to start showing its age in only 4 to 5 years. That simply wasn’t going to cut it.
[ANTALIFE] did some digging and realized that the solution was to use a supercapacitor, specifically the AVX SCMS22C255PRBA0. This is little wonder is rated for a staggering half million cycles, which in theory means that even with daily use it should still take a charge in the year 3300. In practice of course there are a lot of variables which will reduce that lifetime such as temperature fluctuations and the Earth being conquered by apes; but no matter what caveats you put on the figure it should still make 30 years without breaking a sweat.
Similar thought was given to choosing a solar cell with a suitably long lifetime, and he did plenty of testing and experimentation with his charging circuit, including some very nice graphs showing efficiency over time, to make sure it was up to snuff. Finally he walks the reader though his light-sensitive ring oscillator circuit which gives the device its pleasing “breathing” effect once the lights go down.
We’d love to bring you an update on this device in 30 years to see how close [ANTALIFE] got, but as we’re still trying to work the kinks out of the mobile version of the site we can’t make any guarantees about what the direct-brain interface version of HaD might look like. In the meantime though, you can read up on the long term battle between supercapacitors and traditional batteries.
The principle is well understood: use a motor in reverse and you get a generator. Using this bit of knowledge back in 2001 is what kick-started [Ted Yapo]’s Hackaday Prize entry. At the time, [Ted] was searching for a small flashlight for astronomy, but didn’t like dealing with dead batteries. He quickly cobbled together a makeshift solution out of some supercapacitors and a servo-as-a-generator, hacked for continuous rotation.
A testament to the supercapacitors, 17 years later it’s still going strong – leading [Ted] to document the project and also improve it. The original circuit was as simple as a servo, protection diode, some supercapacitors, and a LED with accompanying resistor; but now greater things are afoot.
A DC-DC boost converter enables constant power through the LED, regardless of the capacitor voltage. This is achieved by connecting the feedback pin of an MCP1624 switcher to an INA199 current-shunt monitor. The MCP1624 kicks in at 0.65V and stays active down to 0.35V. This is all possible due to the supercapacitors, which happily keep increasing current as voltage drops – all the way to 0.35V. Batteries are less ideal in this situation, as their internal resistance increases as voltage drops, as well as increasing with age.
When testing the new design, [Ted] found that the gears on his servos kept stripping when he was using them to charge capacitors. Though at first he attributed it to the fact that the gears were plastic, he realized that his original prototype from 2001 had been plastic as well. Eventually, he discovered the cause: modern supercapacitors are too good! The ones he’d been using in 2001 were significantly less advanced and had a much higher ESR, limiting the charging current. The only solution is to use metal gear servos
Last time, we covered storing and charging a 3000 Farad supercapacitor to build a solar-powered, portable spot welder. Since then, I’ve made some improvements to the charging circuit and gotten it running. To recap, the charger uses a DC-DC buck converter to convert a range of DC voltages down to 2.6 V. It can supply a maximum of 5 A though, and the supercapacitor will draw more than that if allowed to.
After some failed attempts, I had solved that by passing the buck converter output through a salvaged power MOSFET. A spare NodeMCU module provided pulse width modulated output that switched the MOSFET on for controlled periods of time to limit the charging current. That was fine, but a constant-voltage charger really isn’t the right way to load up a capacitor. Because the capacitor plates build up a voltage as it charges, the current output from a constant-voltage charger is high initially, but drops to a very low rate in the end.
Before Lunar New Year, I had ordered two 3000 F, 2.7 V supercapacitors from China for about $4 each. I don’t actually remember why, but they arrived (unexpectedly) just before the holiday.
Supercapacitors (often called ultracapacitors) fill a niche somewhere between rechargeable lithium cells and ordinary capacitors. Ordinary capacitors have a low energy density, but a high power density: they can store and release energy very quickly. Lithium cells store a lot of energy, but charge and discharge at a comparatively low rate. By weight, supercapacitors store on the order of ten times less energy than lithium cells, and can deliver something like ten times lower power than capacitors.
[Mike Rigsby] has moved a train with a coin cell. A CR2477 cell to be exact, which is to say one of the slightly more chunky examples, and the train in question isn’t the full size variety but a model railroad surrounding a Christmas tree, but nevertheless, the train moved.
A coin cell on its own will not move a model locomotive designed to run on twelve volts. So [Mark] used a boost converter to turn three volts into twelve. The coin cell has a high internal resistance, though, so first the coin cell was discharged into a couple of supercapacitors which would feed the boost converter. As his supercaps were charging, he meticulously logged the voltage over time, and found that the first one took 18 hours to charge while the second required 51 hours.
This is important and useful data for entrants to our Coin Cell Challenge, several of whom are also going for a supercap approach to provide a one-off power boost. We suspect though that he might have drawn a little more from the cell, had he selected a dedicated supercap charger circuit.
Clearly a believer in the old adage, “Go Big or Go Home”, [Ted Yapo] has decided to do something that seems impossible at first glance: starting his car with a CR2477 battery. He’s done the math and it looks promising, though it’s yet to be seen if the real world will be as accommodating. At the very least, [Ted] found a video by [ElectroBOOM] claiming to have started a car with a super capacitor, so it isn’t completely without precedent.
Doing some research, [Ted] found it takes approximately 2,000 W to 3,000 W at 14 V to start the average car engine. This is obviously far in excess of what a coin cell can put out instantaneously, but the key is in the surprising amount of potential energy stored in one of these batteries. If the cell is rated for 1000 mAh at 3 V, [Ted] shows the math to find the stored energy in Joules:
According to the video by [ElectroBOOM], he was able to start his car with only 6,527 J, and [Ted] calculates it should only take about 9,000 J on the high side from his research. So as long as he can come up with a boost converter that can charge a capacitor with high enough efficiency, this one should be in the bag.
[Ted] has started putting together some early hardware, and has even posted the source code he’s using on a PIC12LF1571 to drive the converter. He notes the current charge efficiency is around half of what’s needed according to his calculations, but he does mention it was an early test and improvements can be made. Will it start? If it does, this is some awesome Heavy Lifting.
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.