Now that ultra low-power options are coming into reach for JeeNodes, lots of new scenarios can be explored.
The most obvious one is probably a solar-powered JeeNode … so meet the latest new node #5 at Jee Labs:
It also uses a 0.47 F supercap, same as yesterday, but now hooked up to a small 4.5 V solar cell (which can only deliver a few mA in bright sunlight), and a Schottky diode between the solar cell and the capacitor.
Here’s the “power supply” in more detail:
As you can see, the solar cell is tiny. A few square cm’s only. In fact, it takes quite some time for it to charge the supercap to acceptable levels. I had to place the cell in moderately bright sunlight for about half an hour to get to a 4 Volt charge. It was inching along, taking several seconds per 0.01 V increase.
To avoid losing all that charge right away in the power-up cycle, I modified the ATmega’s fuses to start in 258 clock cycles after power down, and to start up within 4.1 msec after reset. That way it will start up as quickly as possible at all times. The 258 CK setting is particularly nice, because it means the ATmega can get out of total power down within about 16 µs, fast enough to respond to a byte RX/TX interrupt from the RFM12B!
Does it work? Check it out: after connecting the JeeNode with the “radioBlip.pde” sketch pre-loaded… away it went – sending one packet every 60 seconds as node 5:
OK 5 1 0 OK 5 2 0 OK 5 3 0 OK 5 4 0
While exposed to the current partly-sunny / partly-cloudy light levels, the voltage on the supercap is still increasing. This is good – it means there’s a surplus of solar energy, even with these transmissions going on. That extra energy will be crucial if this thing is to last through the night…
If everything works out, this little Arduino-compatible bugger could well be the first JeeNode to become completely autonomous and transmit wirelessly… forever!
Time will tell :)