We’ve been tuned into coin cell designs lately given the coin cell challenge, so we were interested in [CNLohr]’s latest video about pushing the ESP8266 into the lowest-possible battery drain with coin cells. The result is a series of hacks, based on a reverse-engineered library and depends on a modified router, but that gets the power consumption down by more than a factor of ten!
Although the ESP8266 has a deep sleep mode that draws only 20 microamps or so, that isn’t as rosy as it seems. If you could go to sleep for a while, wake up for just a moment, send your data, and then go back to sleep, that might be one thing. But when you use conventional techniques, the device wakes up and has to do about ten seconds of work (at high power) to connect to a nearby access point. Then it can do what you want and go back to sleep. That ten-second hit is a killer on small batteries.
Since that’s all you can do with the standard libraries, the next step was to find [pvvx] who has reverse engineered a great deal of the libraries and provides a library with no WiFi capability. That’s a two-edged sword. The pro is you get a 30 ms startup from a deep sleep. The downside is — well — you don’t have WiFi.
At its heart is a modified Arduino Nano clone that draws a measured 608 nA from a CR2450N. From the specification of the cell he has calculated the 50 year maximum figure, as well as a possible 29 years for a CR2032 and 64 years for a CR2477. He does however note that this does not take self-discharge into account, but you can probably afford a new battery in a decade or so.
The Arduino clone carefully selected for its “P” version low-power processor has had its serial bridge IC removed to achieve this power consumption, as well as a voltage regulator and some discrete components. Interestingly he notes that the ATMega168P is even more frugal than its 328 cousin, so he’s used the former chip. A selection of internal flags are set for minimal power consumption, and the internal oscillator is selected to use as low a clock speed as possible. There is an Intersil ISL1208 low power RTC chip mounted on a piece of stripboard to provide the timing, and of course an LED to provide the essential birthday alert.
Here’s a fun entry into our coin cell challenge. The power source is the actuating force in [Frank]’s blinky LED Christmas tree, which takes advantage of the physical structure of coin cells and our old pal gravity to roll out some holiday cheer. Talk about forward voltage!
We love the concept, and the circuit couldn’t be more simple. A coin cell is released at the top of the tree and rolls down a series of angled foam board railings covered with 1/4″ copper tape. As the coin cell travels, the negative terminal shimmies along the face of the tree, which has corresponding ground rail tapes. There’s no microcontroller here—all that’s needed for blinks are breaks in the negative rail tape.
The challenging part of a project like this is the execution. Getting a coin cell to ride the rails without falling off required angle experimentation prior to and during the build. Now that it’s done, keeping the tree tilted back against the wall is key. [Frank] explored several options for returning the coin cell to the top using a camera motor and the gear assembly from an old inkjet, but for now, his six-year-old does the job without complaint. Check out his work ethic after the break.
This is it. This is the last weekend you’ll have to work on the most explosive battery-powered contest in recent memory. This is the Coin Cell Challenge, and it’s all ending this Monday. You have less than 48 hours to create the most amazing thing powered by a coin cell battery.
Right now, we’re looking at the entries to the Coin Cell Challenge, and there are some real gems here. Did you know the Rickroll Throwie maddeningly distributed around the dorms at Cornell is an entry? Yes, with just a coin cell, an ATtiny85, and a piezo, you can rickroll people for an entire year.
The top twenty projects for this contest will each receive $100 in Tindie credit to pick up some fancy kits and cool gear. The three top winners will each receive a $500 cash prize. We’re looking for three things specifically — a Lifetime Award that keeps a project going longest, a Supernova Award that drains a coin cell in the blink of an eye, and a Heavy Lifting Award that demonstrates what shouldn’t be possible with a simple coin cell.
This is your last weekend to submit a project, and the contest ends Monday afternoon, Pacific time. Enter now!
The year is drawing to a close, and we have a weekend project for you to while away the remaining hours. Take the Coin Cell Challenge!
The point of the challenge is to do something interesting with a coin cell. That’s it! It’s a challenge that can be as simple or as involved as you want. Low power is where it’s at these days, so if you’ve never used the hardware sleep modes in your favorite microcontroller, that would make an excellent challenge entry. Show us what you’re able to do with short wake periods, and talk about when and why that wake happens. Or go a completely different route and build your own cell!
The top twenty entries will each receive a $100 Tindie credit so they can score some excellent gear. Three top winners in some special areas who will each be awarded a $500 cash prize. We’re looking for something interesting that demonstrates longest life (Lifetime Award), something that burns through that coin cell as if it’s going out of style (Supernova Award), and something that fills us with disbelief (Heavy Lifting Award) because it shouldn’t be possible with “just a coin cell”.
We didn’t include a “Most Ornate” category in this year’s Coin Cell Challenge, but if we had, the environmentally reactive jewelry created by [Maxim Krentovskiy] would certainly be the one to beat. Combining traditional jewelry materials with an Arduino-compatible microcontroller, RGB LEDs, and environmental sensors; the pieces are able to glow and change color based on environmental factors. Sort of like a “mood ring” for the microcontroller generation.
[Maxim] originally looked for a turn-key solution for his reactive jewelry project, but found that everything out there wasn’t quite what he was looking for. It was all either too big or too complicated. His list of requirements was relatively short and existing MCU boards were simply designed for more than what he needed.
On his 30 x 30 mm PCB [Maxim] has included the bare essentials to get an environmentally aware wearable up and running. Alongside the ATtiny85 MCU is a handful of RGB LEDs (with expansion capability to add more), as well as analog light and temperature sensors. With data from the sensors, the ATtiny85 can come up with different colors and blink frequencies for the LEDs, ranging from a randomized light show to a useful interpretation of the local environment.
It’s not much of a stretch to imagine practical applications for this technology. Consider a bracelet that starts flashing red when the wearer’s body temperature gets too high. Making assistive technology visually appealing is always a challenge, and there’s undoubtedly a market for pieces of jewelry that can communicate a person’s physical condition even when they themselves may be unable to.
Some of the entries for the 2017 Coin Cell Challenge have already redefined what most would have considered possible just a month ago. From starting cars to welding metal, coin cells are being pushed way outside of their comfort zone with some very clever engineering. But not every entry has to drag a coin cell kicking and screaming into a task it was never intended for; some are hoping to make their mark on the Challenge with elegance rather than brute strength.
A perfect example is the LiquidWatch by [CF]. There’s no fancy high voltage circuitry here, no wireless telemetry. For this entry, a coin cell is simply doing what it’s arguably best known for: powering a wrist watch. But it’s doing it with style.
The LiquidWatch is powered by an Arduino-compatible Atmega328 and uses two concentric rings of LEDs to display the time. Minutes and seconds are represented by the outer ring of 60 LEDs, and the 36 LEDs of the inner ring show hours. The hours ring might sound counter-intuitive with 36 positions, but the idea is to think of the ring as the hour hand of an analog watch rather than a direct representation of the hour. Having 36 LEDs for the hour allows for finer graduation than simply having one LED for each hour of the day. Plus it looks cool, so there’s that.
Square and round versions of the LiquidWatch’s are in development, with some nice production images of [CF] laser cutting the square version out of some apple wood. The wooden case and leather band give the LiquidWatch a very organic vibe which contrasts nicely with the high-tech look of the exposed PCB display. Even if you are one of the legion that are no longer inclined to wear a timepiece on their wrist, you’ve got to admit this one is pretty slick.