Introduction
I created this circuit in an attempt to make the simplest possible
solar powered project. It would make for an excellent science fair
project, and would also serve as a good introduction to solar powered
circuitry. It may also have some practical uses, such as shedding some
light into a dark part of your house.
The idea is simple, the solar panel converts sunlight into a
trickle of electricity. The electricity is used to run a white LED.
Specifications
Operating Voltage: 3.7V DC
Solar Current: 25ma max.
LED Lamp Operating Current: 10-25ma.
Theory
The remote solar powered LED light takes advantage of the current
limited nature of solar photovoltaic cells. If light shines on the
solar array, current will flow through the circuit. For a typical size of
solar cell, there is a maximum current that can be produced. The maximum
solar cell current is simply matched to a value of current that the LED can
handle. If there is enough light to raise the solar panel's voltage above
around 3.7V, the white LED will light up. The LED regulates the maximum
voltage across the circuit to around 3.7V.
If the solar panel that you use produces more than 20ma, it may be
necessary to insert a series resistor between the LED and the
solar panel to prevent the LED from burning out. A 50 ohm 1/4 watt
resistor is probably about right for the job, the exact value may need
to be optimized according to the solar panel that you use.
This concept could easily
be expanded to systems with larger arrays of solar cells and more LEDs.
The capacitor is not required, but it will keep the LED from flickering
if the panel is briefly blocked, such as when a bird flies by.
With 7 solar cells, the LED will only light in fairly bright light,
if you use up to 10 solar cells, the circuit will work nicely in overcast
skies.
For an interesting modification to this circuit, replace the 1000uF
capacitor with a 1 Farad/5.5V "Memory Backup Capacitor". An Elna
DB-545D105 device was tested on the circuit, after charging up in the
sun for a few minutes, the capacitor was able to light the LED for
several minutes.
Construction
Most of the work goes into making the solar panel. Lay out the cells in
any pattern. Cut the two pieces of plexiglass and one piece of perforated
circuit board so that they are wider than the solar array.
Stack the three board layers together and drill holes for the mounting
screws. When the project is finished, the center circuit board will be
spaced away from the front and back plastic panels with extra nuts acting
as spacers on the mounting screws. The idea is to get an air gap above
and below the circuit board so that there is room for the solar cells and
wiring.
Mount the solar cells on the perf board and solder them into a series string.
An easy way to do this is to connect short segments of bare wire-wrap wire
to each cell, route the wires through the perf board and solder the ends
on the bottom. Connect two wires to the ends of the series string of cells
and secure the wires to the circuit board. For outdoor applications, seal
the edge of the panel with silicone caulk or other water proof material.
Also, seal the mounting screws where they pass through the plexiglass.
Connect the LED and capacitor in parallel, wire them across the two power
leads. Be sure to get the polarity correct, otherwise the LED won't light up.
Solder the parts together. Be sure to heat-sink the LED leads while soldering,
LEDs can be easily destroyed with too much heat.
Use
Place the solar panel in the sun, the LED will light.
The photo at the top of this page shows the circuit operating indoors
on a cloudy day.
If you put the LED on a long wire, it can be placed in a dark location,
such as a corner of a basement. As long as there is a fair amount of
light in the sky, the LED will light up. To get the best orientation for
the panel, aim it directly at the sun at noon during March or September
in a location that is free from shade.
Parts
7-10x photovoltaic cells, rated at 15-25ma each.
1x white LED, high efficiency types work best.
1x 1000uF 15V (or greater) electrolytic capacitor.
1 piece of perforated or printed circuit board.
2 pieces of clear plexiglass.
28 gauge bare wire-wrap wire.
24 gauge speaker wire.
miscellaneous screws, nuts, and washers.
silicone caulk.
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