Continued from GeekPhysical - Sustaining Micropower Devices
Many (interactive) electronic devices spend most of their time doing nothing or very little. The ratio between usage and inactivity often results in an extremely low average power consumption. This opens the opportunity for powering such devices from very weak but ever-present power sources.
To explore the possibilities for having an active electronic microcontroller-based device without the need for any expendable power source we have developed an LCD door sign that will run day and night powered by a tiny pocket calculator solar cell.
Miniature solar cells like those found in pocket calculators will, in favorable conditions deliver a mere 100µA @ 3-4V and since most microcontrollers require at least a couple of milliamps to run some sort of power management is required.
The microcontroller used in the project(1) has the ability to shut itself down reducing power consumption dramatically. An internal timer can be programmed to 'wake up' the controller at regular intervals to do useful work and then go back to sleep. Using a capacitor or rechargeable battery to store the energy collected between small burst of microcontroller activity allows the average current draw to be reduced to a few µA. In effect the solar panel produces an excess amount of energy over time.
The controller used(1) also has the ability to measure the voltage across the battery/capacitor thus giving a measure of the available amount of energy at any given time. The microcontroller program has the ability to change its level of activity depending on available resources. Our project contains an AVR microcontroller, a small alphanummeric LCD, two pocket calculator solar cells and a tiny 80mAh LiPo rechargeable battery.
The controller scrolls the names of the inhabitants on the LCD thus demonstrating a simple and useful device.
Apart from the display function the microcontroller performs the power management functions required to maintain active over a 24 hour power harvesting period. Further, apart from changing the interval of the activity depending on power reserve the controller has the ability to completely shut down the LCD during times of power starvation.
This project does not imply interactivity but functions could be implemented to keep the device powered during interaction. Other weak energy sources such as electromechanics-, electromagnetic-, static electric or chemical could also be applied.
(1) Most modern microcontrollers have this functionality.