Electronic Building Blocks
By Julian Edgar
Quick and easy construction
Great results on a low budget
Modifying solar garden lights
Fig.1. Good quality solar lights can be
adapted to a range of uses. Try to buy those
that have a decent sized solar panel and
replaceable Ni-MH cells. If you wait for ‘on
special’ prices, you can get lights with these
attributes at very low prices – in fact, cheaper
than the cost of the solar panel alone. This
one cost £5.
D
espite the price of solar
photovoltaic panels falling dramatically over the last 10 years
– in terms of cost per watt – small panels
(eg, 2-5W) have remained relatively
expensive. But there is a way of gaining
a good quality solar panel at low cost
– and as a bonus, you also get battery
storage, a charge controller and an LED
light! How? By buying a solar garden
light. But note, not just any garden light.
Selecting
I first started looking at cheap garden lights as a source of solar power
a few years ago. Since then, I have
bought lots of them – but I’ve often
been disappointed. Why? Well, first,
many garden lights have ineffective
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battery storage – some even still use
nickel-cadmium (Ni-Cd) batteries!
(These batteries have bad memory effects, and their capacity is also much
less than more modern nickel-metal-hydride (Ni-MH) batteries.) Other
lights use non-replaceable, tiny batteries – flat packs about as big as your
little fingernail.
Second, the cheapest solar lights often have a very low operating voltage
– that is, the solar panel develops an
output of only 1 or 2V. To run a white
LED, they use an oscillating circuit
that develops an AC output voltage,
with the LED lighting-up only on the
positive-going part of the waveform.
To make use of the power for other
purposes, you’re starting off with a
very low voltage, which in turn makes
everything that follows more difficult.
Finally, many solar lights hide their
low power with huge reflectors. When
you examine the actual source of illumination, it’s often only milliwatts.
To avoid these issues, look for a
powerful light with a large solar panel
that uses a replaceable Ni-MH battery
– preferably comprising one or two
18650 cells.
Lights of this sort often come up as
specials. (I think perhaps it’s because
they’re normally much more expensive than run-of-the-mill solar lights,
so people don’t buy them and hence
stores discount them.)
In front of me on my desk is a good
example. It uses a 2.3W solar panel, a
Ni-MH battery pack comprising two
18650 cells wired in parallel, full electronic control, and a light comprising
30 surface-mount LEDs providing 550
lumens at full brightness. Furthermore,
it has an inbuilt motion sensor. It’s also
well made, with stainless steel hardware
and good internal waterproofing… and
it cost me (on special) just £5! After I’d
taken one home and pulled it apart for
inspection, I went back to the shop and
bought a whole lot more.
Now, by the time you read this, the
chance of your finding that specific
light, at that price, is slim. But lights
like these do pop up – and so this article is more about what you can do with
such lights as and when you find them.
Adding a 5V USB output
With 5V USB power being used to
do everything from charging phones
and Bluetooth speakers, to running
microcontroller boards and electronic instrumentation, adding a 5V USB
output to the solar light immediately
makes it far more useful.
In most cases, the operating voltage
of the solar light is less than 5V, so you
will need a boost converter. Such converters are readily available on eBay,
Fig.2. This light uses two 18650 Ni-MH
cells. Despite appearances, they are wired
in parallel, so the system operates at a
nominal 3.7V. Note the seal around the
battery compartment lid.
Practical Electronics | July | 2020
Fig.3. The electronics are mounted under the LED light panel. The
PCB is well-labelled, making connecting to the panel, the LED light
or the battery quite easy. When the lid to this compartment is in
place, the electronics are fully sealed.
with some even incorporating the USB socket. Search under
‘DC-DC 0.9-6V to 3.3-9V Solar Boost USB Adjustable Stepup Power Supply Module’ or similar, and expect to pay only
a few pounds. I’ve found that those with an adjustment pot
are preferable over those with a pre-set output, because you
can then set the output voltage under load. You also want
the module to be as small as possible – and that’s OK, because they are usually tiny.
With the solar light I purchased, there was room inside to
locate the small 5V power supply board, and filing a slot in
the case allowed external access to the USB socket. Of course,
with the USB socket installed, the solar light is no longer
waterproof. If you want the assembly to remain waterproof,
then mount the board entirely inside the housing and run
a charging cable out through a grommet further sealed with
silicone. Depending on the use you are putting it to, you
may also want to add an on/off switch for the USB supply.
Remote-mounting the LED light
Solar garden lights invariably have the panel, light, battery
and electronics in one assembly. That lowers manufacturing
costs and also means that you just need to ‘plant’ the light
in the garden, aim the panel the right way and then leave it
– no further work required. But such lights are far more versatile if the light can be mounted remotely. (Solar-powered
lights like this are available but they tend to be much more
expensive than garden lights.) So how easy is it to remote
mount the LEDs? In many cases, very easy!
To remote mount the LED light, you will need to extend
its connections to the rest of the assembly. Depending on
Fig.4. This boost USB converter can be easily
added to make the solar light into a powerful,
cheap and effective solar-charged power
bank. It is also small enough to fit within the
body of most lights.
Practical Electronics | July | 2020
Fig.5. The solar light, fitted with a USB boost power module, charging
a Bluetooth speaker.
the construction of the light that might require the use of
just two wires – power and ground for the LEDs. But in
other cases, additional connections may be needed. In the
lights on which I was working, the solar panel assembly
also contains the battery, so in this case, four wires were
needed – two for the solar panel and two for the battery. If
the light has movement sensing, ensure that the movement
sensor stays with the LED light (usually the case, without
any wiring changes).
When making these wiring changes, don’t forget to remove the battery first. There’s often no protection against a
battery short-circuit – which is quite easy to achieve when
extending the connections! If you are going to be making a
long extension, go up in cable gauge over the original wiring to lessen the voltage drop.
Note that with the LED light remote-mounted, it will still
function as it did previously. For example, the light will
not illuminate until the solar panel is in the dark. So if the
light is mounted inside a dark cellar, it will not operate until night has fallen outside.
Other uses
Good quality solar lights like the one shown here can also
be used in a range of other uses; for example:
Powering a remote transmitter – eg, for a weather station
Add a voltage switch and buzzer – acts as a motion-sensing burglar alarm
Emergency household or car light.
So, keep an eye out for these lights when they are being
sold on discount.
Fig.6. In some applications (for example, lighting the interior of a shed or outhouse) separating
the solar panel from the light gives more versatility. In this light, where the battery is mounted
on the back of the panel, connections for both the light and the battery needed to be
extended. This is the rear view of the LED panel before the panel was replaced – the two
new cables exit the case at the bottom of the photo.
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