Solar! Solar! Solar!

I have always wanted to do this, hook up a solar cell to one of my wireless temperature sensors.  I was out on SparkFun the other day and found these step up converters.  As I looked at those step up converters I realized that they would take a single cell battery that output ~1.2 volts and convert it to 3.3 volts the exact voltage I needed for my wireless sensors.  That would be perfect.  So, I ordered some solar panels from eBay.

The panels came and I scrambled to order the step up converters.  The step up converters came in short order.  I hooked up a single panel as they output 2 volts at 22ma.  I thought one would do it but when I put the required diode in series with the panel the voltage drop was too much and I didn’t have enough voltage to charge the battery.  I put two panels in series to up the voltage to about 4 volts.  That was enough voltage with a .7 volt drop across the diode to charge the battery.

Now that I could charge the battery I hooked up the output of the solar panel and battery to the step up converter.  I looked at the output of the step up converter and it was at 3.295 volts.  Perfect for my wireless sensor.  I checked the input to the step up converter and had 1.32 volts going into it.  Looks like the battery is fully charged.  I believe I charged the battery before I put it in the battery holder to make sure it had a good start.  I should discharge a battery and put a dead battery into the unit to see if the solar panel can charge the battery and still keep the sensor running.

With a coin sell in my wireless sensor I have been getting a little over a year of service.  I calculated 5 years but you know how that goes.  I guess it takes more power than I realize when the processor comes out of sleep mode and actually reads the sensor and powers on the transmitter to send the information.  I thought with the solar panels I should get many years of service from a sensor.  I have had some yard lights that have been in service for over 5 years and still going.  They do discharge most of the battery at night but if there is good sun during the day they are back to full strength that night.  I believe that my sensors aren’t draining the battery much at all at night so, they should run for many days without sun shine.  I’m using AAA rechargeable batteries which have a capacity of about 700ma where as the coin cells I was using had about 230ma.  The AAA’s should last a lot longer than the coin cells with out the solar panels but, with the solar panels, if there is enough sun to charge the battery, the sensor should last a long time, years or the life time of the battery and solar panels.

You can see how I glued the solar panels to the top of the case.  I used hot glue to create a weather tight seal.  The problem is my case isn’t exactly weather proof but has survived a couple of rain and snow storms.

You can see that I drilled two holes in the top of the case to pass wires from the solar panels to the rest of the circuit.  Here you can see the AAA battery and sparkfun step up converter I used to complete the circuit.  The black hot shrink insulation has the diode I used in it.

This is the schematic.  You can see the two solar panels in series, the blocking diode, the battery, step up converter and the sensor circuit.  The battery keeps the voltage of the solar sell no higher than about 1.3 volts.  This is perfect for the step up converter.  It takes anything from 1 volt to 3 volts.  They have been working very well for me.