In precedent posts I've introduced some informations about power consumptions. As a rule of thumb, when configured as battery-operated devices, panStamps may drain a few microamps in power-down mode and a few miliamps when measuring and transmitting. Under these circumstances, powering a panStamp based board from a couple of AA batteries should give us an autonomy between one and three years, depending on the particularities of each project (consumption of the sensor, DAQ and transmission intervals, time remaining in Rx after each transmission, etc).
Now I want to go a step further and create real autonomous wireless motes; no need to replace batteries at all since solar energy is available for free almost everywhere. Before adding more complexity to the hardware, I thought about checking this concept with a common solar battery, one of those cheap USB chargers for cell phones.
Figure 1: panStamp base board being powered from a solar phone charger
The solar charger appearing in the above picture may seem somehow overkilled for our purpose; the most simple (and cheap) solar chargers may be used instead. The base board where the panStamp is plugged onto was originally designed to work as programmer and USB interface. It also includes a LDO, independent from the ft232 on-chip voltage converter, so that the FTDI USB chip can remain unpowered (through an external jumper) whilst the board can still be powered from an external 5VDC power supply.
However, the above solution is not what I had in mind when I planned to build cheap and simple solar-powered devices. Thus, I needed to go back to the basis, restart my solar design and do things step by step.
My first decision was to build a boost power supply where to base my solar design on. Powering a microcontroller from a solar cell is a bit more complicated than simply connecting the cell to the Vcc and GND pins. Voltages provided by solar cells are far from being constant and regulated. Moreover, I need to back up some of the power produced into a battery or capacitor. Finally, I want my solar kit to cost less than USD10. Otherwise I'd see no reason for abandoning the (above) USB solar battery approach.
Regarding the step-up voltage regulator, the NCP1400 chip, made by ON Semiconductor, appears as a good candidate. It costs less than USD1.0 and requires only a few passive components. This regulator is capable to start converting 0.8 V into other more usable voltages and keep conversions when the voltage source goes below 0.2 V. In order to test the samples received from the manufacturer, I built some small boards that will let me plug different power sources and storage devices.
Figure 2: NCP1400 test board schematics
This step-up regulator is not only suitable for solar designs. Since it accepts low voltage ranges, I'll be able to power my panStamps from single AA/AAA batteries, ensuring that their charge will be drained more efficiently than simply connecting AA/AAA pairs as described in the precedent post.
Figure 3: NCP1400 test board
Finally, I ordered some cheap solar cells from Internet and a supercapacitor where to store the excess of power and keep the panStamp running through the night. The supercapacitor is a 6.6F 2.7V ELNA DZ one, purchased from eBay, that should show a low ESR (Equivalent Series Resistance), 0.2 ohms or so. Choosing a good supercapacitor is very important, not only for the amount of charge that it will be able to store but also for its rate of discharge. A low ESR ensures low current leakages.
Figure 4: Cheap solar cells. 29 x 59 mm. 2V 45 mA under direct sunlight.
Once I got all the necessary pieces I assembled them all together and run some tests. I initially expected the ensemble to work under low daylight conditions and still be able to back up the necessary energy to work through a whole night...
Figure 5: My first solar engine
… Unfortunately, none of my initial wishes fulfilled. Firstly, the solar cells resulted to work only under direct sunlight. Under the shade, even with good indirect daylight, these cells do not provide the necessary voltage to make the regulator work. My second frustration arrived when I checked that the supercapacitor was not able to keep its charge more than 7 minutes, even when my panStamp was not consuming more than 0.2 mA. All these bad news have led me to review my design and get the following conclusions:
1. I need to look for more efficient solar cells, capable to produce a minimum of voltage and current even under low daylight conditions. In household applications, these solar-powered sensors won't rarely be placed under direct sunlight so my current solar cells are useless.
2. The NCP1400 board requires low ESR capacitors. Placing a good supercapacitor besides not so good filtering capacitors makes the whole design discharge quicker. As filtering capacitors I initially used some tantalum ones that I had in stock but later I realized that their ESR's were not acceptable for my design. Instead, I'll have to search better ones, with ESR's below 0.3 ohms.
3. I need to try other supercapacitors. The one purchased from eBay theoretically has an ESR of 0.2 Ohms but I've not been able to find the original data sheet. Indeed, I'd say that ELNA no longer makes this reference and, instead, Mouser and other distributors provide the 4.7 F value and lower ones. I'm beginning to question the quality of this 6.6 F supercapacitor.
In summary, I guess it's an issue of replacing pieces in my current design. I'll try to document in future posts my progresses and other experiences about this exciting topic.