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Tiny Solar Panels Using Photodiodes
By: Dave Johnson

There are times when you would like to harvest just a little bit of energy from the sun.  Unfortunately, there are not many miniature solar panels available.  Most panels are 10 to 50 times larger than needed.


One way to make your own miniature solar panel is to wire multiple small photodiodes in series. The result might look something like the drawing below, which is a made from 12 small BPW34 photodiodes.  This panel is about of an inch in each side and can produce about 6 volts at a milliamp.  This may not sound like much but for many applications it is plenty.  Even smaller arrays are possible if less power is needed.
12 Photodiode Array     BPW34 Photodiode
Below are examples of some photodiodes which measure about 2mm to 3mm on a side.  Make sure you get diodes with a clear epoxy flat profile.  These will maximize the solar energy collection during the day.  As in solar cells, each device will have an open circuit voltage of about 0.5v.  The current will depend on the diode area and sunlight intensity.  Figure about 1ma of current for each 10 square millimeters.  I have seen some of these devices selling for as little as $0.25 each.
Just as in the larger panels, be sure to insert a small signal diode between the photodiode array and any battery or super capacitor being charged.  The diode will prevent current from leaking back into the array at night.
What can you do with one of these devices?  Many could be used to power a remote sensor of some kind.  Some Zigbee RF systems are so efficient that very little average power is needed.  Remote temperature, pressure, flow or current sensors could all be powered by a small solar panel.  Another application is to extend the life of a battery powered device.  Rather than use a battery large enough to last 5 years or more, a small solar panel could be used in conjunction with a small rechargeable battery, super capacitor or even a non-rechargeable alkaline or lithium battery. As an example, an outdoor clock might be powered by a single 1.5v AA alkaline cell.  Such a battery might run the clock for a year or so.  That works out to an average current of 0.2 milliamps or 5.5 milliamp-hours per day.  A photodiode array made of 5 photodiodes in series and a 1N4148 series signal diode connected to the battery, would pump about 1ma of current into the battery during the day and might allow the battery to operate for 5 years.
Outdoor Clock ZigBee RF System
Solar Powered 1.5v Battery Charger  Remote Temperature Sensor

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November 2011

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