One of the biggest challenges for wireless sensor networks is that of power. Solar panels usually produce less power than you hoped, especially small ones, and designing super low power circuits is tricky. [Strange.rand] has dropped into the low-power rabbit hole, and is designing a low-cost wireless sensor node that runs on solar power and a supercapacitor.
The main components of the sensor node is an ATMega 328P microcontroller running at 4Mhz, RFM69 radio transceiver, I2C temperature/humidity sensor, 1F supercapacitor, and a small solar panel. The radio, MCU, and sensor all run on 1.5-3.6V, but the supercap and solar panel combination can go up to 5.5V. To regulate the power to lower voltage components a low-drop voltage regulator might seem like the simplest solution, but [strange.rand] found that the 3.3V regulator was consuming an additional 20uA or more when the voltage dropped below 3.3V. Instead, he opted to eliminate the LDO, and limit the charging voltage of the capacitor to 3.6V with a comparator-based overvoltage protection circuit. Using this configuration, the circuit was able to run for 42 hours on a single charge, transmitting data once per minute while above 2.7V, and once every three minutes below that.
Another challenge was undervoltage protection. [strange.rand] discovered that the ATmega consumes an undocumented 3-5 mA when it goes into brown-out below 1.8V. The small solar panel only produces 1 mA, so the MCU would prevent the supercapacitor from charging again. He solved this with another comparator circuit to cut power to the other components.
We see challenges like these a lot with environmental sensors and weather stations with smaller solar panels. For communication, low power consumption of a sub-Ghz radio is probably your best bet, but if you want to use WiFi, you can get the power usage down with a few tricks.
We’ve all committed the sin of making a little arduino robot and running it off AA batteries. Little Flash is better than that and runs off three 350 F capacitors.
In fact, that’s the entire mission of the robot. [Mike Rigsby] wants people to know there’s a better way. What’s really cool is that 10 A for 40 seconds lets the robot run for over 25 minutes!
The robot itself is really simple. The case is 3D printed with an eye towards simplicity. The brains are an Arduino nano and the primary input is a bump sensor. The robot runs around randomly, but avoids getting stuck with the classic reverse-and-turn on collision.
It’s cool to see how far these capacitors have come. We remember people wondering about these high priced specialty parts when they first dropped on the hobby scene, but they’re becoming more and more prevalent compared to other solutions such as coin-cells and solder tab lithium batteries for PCB power solutions.
[Barry] sent us a tip about a video from [electronupdate], describing an experimental cell phone charger. It’s a familiar issue: Your cell phone battery is low, and you aren’t in a position to plug it in for hours to charge. Some phones, including the one in his video, have swappable batteries, but that isn’t always an option either. As he explains in the video, a wall outlet can deliver the joule capacity of a high-end battery in a matter of seconds, but it is impossible to charge a battery that quickly. Capacitors, on the other hand, charge near-instantly.
[electronupdate] decided to look at the possibility of using super capacitors to power a typical usb plug. It would allow you to charge a secondary power supply in a short period of time, and then get on your way, letting your phone charge slowly from the device.
His experiment wasn’t entirely successful, possibly because he used 2.7V capacitors, which required a boost regulator and limited the useful voltage range. We think he might have had better success using 120V capacitors and a switching power supply, but it would be nice to see the various options compared.
Oh, [electronupdate] describes using this circuit as you are rushing to your airplane. We aren’t convinced carrying a couple super capacitors through a TSA checkpoint would be the best idea… YMMV.
Continue reading “Supercap-Based Cell Phone Charger”
[Caleb] was given a tiny LED flashlight which has a crank used to charge it. Unfortunately it wasn’t holding a charge, and constant cranking didn’t work very well either. He cracked it open to find a single lithium button cell. Instead of using a drop-in replacement he soldered in his own super capacitor.
The stock device is remarkably simple. It uses a standard DC motor as the generator. It’s connected to the crank using a set of gears, with the two red wires seen above connecting it to the control board. Four diodes make up a bridge rectified and apparently feed directly into the battery. No wonder that cell went kaput!
But this orientation isn’t bad for using capacitors. They can be charged directly and the switch which attaches the LEDs to voltage doesn’t interfere with their operation. The last problem was making room for them in the case. [Caleb] considered a few different approaches, but ended up just heating the plastic enclosure until it could be deformed to make room for the additional parts.
A group of researchers have figured out how to produce graphene using a DVD drive. This discovery helps clear the path for mass production of the substance, which was discovered in the late 1980’s. More recently, the 2010 Nobel Prize for Physics was awarded to a team that produced two-dimensional graphene; a substance one just atom thick. One method of doing so used Scotch tape and is mentioned in the video after the break as a technique that works but is not feasible for large-scale production.
The process seen here starts with graphite oxide because it can be suspended in water. This allows a lab technician to evenly distribute the substance on a plastic surface. Note the use of optical discs. The second part of the process involves hitting the dried layer of graphite oxide with a laser. It just so happens that this can be done with a consumer DVD drive. The result is graphene that can be used in circuits and may have potential as a fantastic super-capacitor.
Continue reading “Making Graphene With A DVD Burner”
[Gil] recently wrote in to tell us about some awesome research going on at UCLA. Apparently by layering some oxidized graphite onto a DVD and tossing it into a lightscribe burner, it’s possible to print your own super capacitors; some pretty high capacity ones at that.
For those that are unaware, supercapcaitors are typically made using two electrolyte soaked, activated carbon plates separated by an ion permeable film. Since activated carbon has an incredible surface area huge energy densities can be reached, in some cases 1kJ/lb.
Laser-formed graphite sponge replaces the activated carbon in the researchers’ printed capacitors. A video after the break discusses the whole process in moderate detail, meanwhile greater detail can be found in their two papers on the subject.
First one to print a transistor gets a bag of mosfets!
Continue reading “Print Your Own Supercaps”
You won’t find [Antoine] stumbling around in the dark. He just finished working on this LED flashlight which draws power from a super-capacitor (translated). He realized that lighting a high-efficiency LED takes so little power that there are many benefits in play when deciding to move away from batteries. When compared to a super capacitor, batteries have a shorter life span, are heavier, and take up more space.
The biggest drawback of a super capacitor in this situation is the low voltage operation. The output will start at 2.7V and drop as the current is discharged. [Antoine] used one of our favorite simple circuits to overcome this issue, the Joule Thief. That circuit is commonly seen paired with an LED in order to boost input voltage to a usable level. That’s precisely what’s going on here.
The final hack in his circuit is the addition of that red LED which you can see in the middle of the board. This takes the place of a Zener diode and drops the charging voltage to a safe level. That indicator light will not come on until the cap is fully topped off. This way it tells you when the device is done charging.