Energy harvester/perpetual sensor

Energy harvester/perpetual sensor, a set on Flickr.

Some time ago I was presented with the challenge to make a device that would collect information such as temperature, light, radiation and similar without ever being serviced or recharged. The device could possibly even be integrated into a wall or a rock for all eternity.
I started experimenting with running micro controllers in really low power configurations and developing algorithms for extreme power consumption.
The images above show a couple of these devices. They are all powered from a single 2.5V/100µA solar cell and use super-capacitors for power storage.
One have a simple LCD that displays the power stats for the device. This allowed me to get an idea of what kind of power an computation power was available.
The others uses a power booster circuit to blink a white LED that need higher voltage than 2.5V.
The LED blinks once per second. The duration of the blink is determined by the amount of stored energy allowing the blinking to go on during low energy times. Once per hour, if the stored power allows for it, the blink is modulated in a way that allows collected data to be transmitted to an external device. The average current drawn by these devices is around 6.2µA with a bus voltage of about 2.5V.
During energy starvation the bus voltage may drop as low as 1.3V. If this happen the firmware will put the device into 8 second inactivity cycles until better times.