Zak Kemble's ATtiny10-Based Smart Power Bank Keep-Alive Boards Do Exactly What They Promise
Designed to let low-power projects run from USB power banks that would otherwise go to sleep, Kemble's boards are highly configurable.
Zak Kemble has come up with a novel solution for running low-power projects from USB power banks that would otherwise go back to sleep: a "handy and slightly over-engineered USB module" design that acts as an adjustable pulsed constant current sink.
USB power banks are a fantastic way to run portable electronics projects, offering high capacity batteries in a rugged housing and a regulated DC output on the ubiquitous USB connector. Unfortunately, all too many of them are "smart" — switching themselves off automatically if they don't detect a certain minimum power draw, often set above the level low-power projects would require.
"A quick hack to keep power banks alive is to use something like a 150R resistor across the power output to draw an extra 33mA, but some power banks might need as much as 100mA to stay on, requiring a 50R 1W power resistor," Kemble explains. "Usually, power banks don’t need to have current continuously flowing to stay on, where a 2 second pulse every 15 seconds might be enough to keep it alive. This pulsing technique drastically improves the battery life of the power bank, perfect for powering a small project for a few weeks."
That's what Kemble's Smart Power Bank Keep-Alive board is designed to do. "This handy USB module uses a pulsed constant current sink to keep the power bank alive, and supports USB 3 pass-through," he notes. "The smarts of this device is an ATtiny10 which controls the on and off cycling of the constant current sink. The current is adjustable via a small trim potentiometer from 0mA to 140mA and the supply voltage can be anywhere from 1.8V to 5.5V."
By default, the pass-through board pulses for three seconds every six seconds, but by flashing a modified firmware to the ATtiny10 module it's possible to configure this alongside the current and voltage — handy given that of the three unbranded power banks Kemble tested during development, all three had different requirements.
More information is available on Kemble's blog, while the boards are available to buy Tindie for $14. The design files and source code, meanwhile, can be found on Kemble's GitHub repository.