Pseudonymous software engineer strange.rand is working on a compact wireless sensor node designed to run from solar power, collecting energy in a supercapacitor and discharging it to transmit to a remote gateway.
"I live in an apartment that faces south and west so I can get reliable temperature measurements from a standard thermometer (you know, such thing on your window with a little bit of colored ethanol inside) only till 1-2 PM," Rand explains. That's why I decided to start this project."
"The main idea is to create a relatively small device that measures temperature and humidity (or anything else using a low-power I2C sensor), sends data to a gateway, and never requires a battery change. In the current iteration, I [am] going to use an ATmega328P microcontroller, RFM69 transceiver, 1-1.5F supercapacitor, and some additional components to provide over/under voltage protection."
To make sure the project was possible, Rand started by totting up the power draw of the ATmega328P microcontroller and the voltage inputs compatible with all the components in use — plus the output of his compact panel, a 5.5V unit. "The easiest way was to use a low-drop voltage regulator and get a stable supply voltage (3.3V) for all components," Rand notes.
"But after a bit of testing I found out, that when the voltage drops below ~3.3V LDO becomes inefficient and consumes an additional 20uA (or even more) that's drain supercap too quickly. An alternative solution will be charging supercapacitor to 3.6V max and do not use any voltage regulators. It's much more efficient and in such a configuration I achieved 42 hours of work from a 1F supercap."
Rand has begun the prototyping process, but is still working on experimentally proving his project — which can be followed on Hackaday.io.