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More  Solar Cell Circuits

Solar Powered Quartz Clock - September 25, 2017
designed by David A. Johnson, P.E.

One of my homes in Colorado had a large battery powered quartz clock above the fireplace.  It was powered by an AA size 1.5v alkaline cell.  The battery would run the clock for about a year or so, then I would have to break out a tall step ladder and replace the battery for another year or so of operation.  One day I decided I was going to fix up the clock so it would run for many years without having to replace the battery.

The living room where the clock was situated had three skylights.  During the day, the room was filled with natural bright light.  So, I thought I could harvest some of that light with a solar panel mounted on top of the clock and use that energy to run the clock.  The circuit I used is shown below. 

I used a 4 cell 2v solar panel which under a full sun condition would crank out about 500ma of current.  During almost any day, the panel could produce about 5ma of current from the diffuse ambient light in the room on top of the clock.  That averaged out to about 25ma-hours per day.  If the clock averaged about 100uA of current or about 2.5ma-hours each day, then the solar panel should be able to keep a battery fully charged, even if there were many days of cloudy weather.

I connected the solar panel to a quality AA size 1.2v NiMH battery cell.  I tested the clock motor and found that the clock would still function down to a voltage of 1.0v.  It worked fine with the 1.2v from the single NiMH cell, even if the cell were nearly fully discharged.  That gave me a great 10:1 energy margin.  As long as the solar panel could pump more energy into the battery than needed by the clock each day, then the circuit should last a long time.

Click on Drawing Below to view PDF version of Schematic

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