Garden Lighting Using Solar Cells


Completely
‘self-supporting’ garden lamps using solar cells as their energy source
are gradually becoming more and more common. How do they actually work?
We took one apart to find out. From environmental and technical
considerations, buying such a solar-cell garden lamp has a lot to
recommend it. It’s a great thing that the energy necessary for the lamp
to burn in the evening can be drawn from the sunlight that is available
for free during the day. In addition, such a lamp is enormously
practical, since you can place it in any desired location in the garden
without having to dig a trench through the lawn or flowerbeds.

You are also free to change your mind about the best location for
the lamp – something that would have unpleasant consequences with
ordinary garden lamps. What makes up a typical solar-cell garden lamp? A
certain number of elements are in any case necessary for it to
function. It’s clear that there must be a light bulb and some solar
cells. However, the bulb is naturally not powered directly from the
solar cells, so there must be a storage battery and a suitable charging
circuit to allow the battery to be charged by the solar cells. In
addition, the idea is that the lamp should only burn during the evening
and the night, and that needs a twilight switch with a light-sensitive
cell.

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It’s not necessary to do anything to switch off the lamp, since that
happens automatically as soon as the battery is fully discharged. Some
of the more luxurious models have a small fluorescent tube in place of a
normal light bulb, and in this case a small converter is also
necessary. However, the model that we examined contained a small
2.5V/75-mA halogen bulb, and thus did not need a converter. As far as
the electronics are concerned, the whole thing can thus remain very
simple.

Simplicity wins out:

Our garden lamp consists of a simple plastic structure. Eight solar
cells are mounted at the top, and inside there are a small halogen bulb,
two penlight NiCd cells and a small printed circuit board for the
electronics. As can be seen from Figure 1, there isn’t all that much
inside. This lamp costs around 15 pounds, and it can be found in several
different shops. The electronics also turn out to be extremely simple.
Figure 2 shows the complete schematic of the internal circuitry. The
twilight switch is on the left, and its output controls the lamp via
transistor T4. To the right are the on/off switch, a diode and the eight
solar cells.

Charging:

During the day, as long as there is sufficient light, the voltage
generated by the solar cells is 8 × 0.45 V under ideal conditions, with a
current that depends on the size of the cells — in this case,
approximately 140mA. With less light, less current is supplied. The
charging circuit consists simply of a single Schottky diode (D1). The
current generated by the solar cells passes through this diode, with its
typical low voltage drop of 0.3 to 0.4 V, and charges the NiCd cells.
There is no overcharge protection. It is not actually necessary, since
all NiCd cells can handle a continuous charging current equal to 1/10 of
their capacity (60mA in this case), while modern cells are so robust
that twice this amount of current does not cause any problems.

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The advantages of using a somewhat higher charging current are
naturally that the battery is already fully charged after several hours
of sunlight, and that a certain amount of charging takes place even on
rainy days and during the winter. Solar cells act as light-dependent
current sources, so the more light there is, the more current they
produce. The voltage is determined by the load, but it can never be
higher than the previously mentioned 0.45 V per cell. Approximately 2.8 V
is necessary to charge two NiCd cells. If we add the voltage drop
across D1, we arrive at a required voltage of 3.2 V. This is 0.4 V per
solar cell.

Charging takes place continuously, even when switch S1 is off. It is
important to make sure that both NiCd cells are fully charged the first
time. Otherwise, one cell may become fully discharged before the other
one when they are discharged. As a result, this cell may have a
reverse-polarity voltage applied to it, which will shorten its useful
life. Therefore, when first putting the lamp into service, you should
place it outside with S1 switched off for at least one day in full
sunlight, or two days if the weather is cloudy.

Burning:

When S1 is closed, voltage is applied to the part of the circuit containing the light bulb. An LDR is used to determine whether it is light or dark outside. During the day, the resistance of the LDR
is low, and the voltage on the base of T1 is also low, so that it is
cut off. T2, T3 and T4 are then also cut off, so that the bulb is not
illuminated. As soon as it becomes dark, the resistance of the LDR
increases, and the voltage on the base of T1 rises. T1 starts to
conduct when the voltage is around 0.65 V. This causes T2, T3 and T4 to
conduct as well, and the lamp starts to burn. T1 then receives a bit of
extra current via R4, so that positive switching takes place when the
circuit is sitting ‘on the edge’. This is called hysteresis. It means
that a threshold is set such that the light level has to drop a bit more
before the lamp will switch on again once it is off, and vice versa.

This means that the circuit does not react to every passing cloud or
insect that is flying around. As long as it remains dark, the lamp
continues to burn until the battery is fully discharged. A fully charged
battery has a capacity of 600 mAh, which is enough to supply the 75-mA
bulb for approximately eight hours. This is sufficient for the evening
and a large part of the night. In the winter, this is not possible,
since the battery will probably not be fully charged due to a lack of
sunlight. When the battery becomes fully discharged, its voltage drops.
If the voltage drops below 1.25 V, T2 and T3 are cut off, since their
base-emitter junctions are in series and thus need at least this amount
of voltage. The lamp is then switched off, and the battery is not
further discharged.

In the long term:

NiCd batteries usually have a lifetime of around 500 to 1000
charge/discharge cycles. After two to three years of continuous use,
therefore, the two penlight cells of the garden lamp will probably be
ready for replacement. However, these cells are presently so inexpensive
that this is not a serious disadvantage. Naturally, there is also a
limit on the life-time of the light bulb, but here again, making a
replacement is quick and inexpensive.


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