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CIRCUIT NOTEBOOK
Interesting circuit ideas which we have checked but not built and tested. Contributions will be
paid for at standard rates. All submissions should include full name, address & phone number.
A reliable solar lighting system
The steps to my shed do not all have
the same tread depth and riser height. I
am used to them, but I still have some
difficulty on moonless nights. A few
years ago, when solar-powered garden lights became available cheaply, I
bought a few of them and poked them
into the ground alongside the steps, and
I could finally see the steps at night.
The problem is, these things are only
made to have a life of about 12 months
if you’re lucky, and soon they needed
repair or replacement.
After going through a couple of repair/replacement iterations, I decided
that I needed a more permanent solution. I had collected a few 18650-size
Li-ion cells which still seemed to have
some life left, so I decided to use these
in a lighting system for the steps.
I needed to mount the actual lights
on something solid so they wouldn’t be
destroyed by a brush-cutter. I mounted the LEDs in a length of rectangular
steel tube from a discarded swing set.
I mounted this so that it also formed
a handrail for the steps using tubular steel supports concreted into the
ground with welded brackets.
The four LEDs came from a defunct
garden light. Once they were mounted
siliconchip.com.au
to the underside of the handrail, I ran
a cable into the shed. The accompanying circuit diagram shows the solar panel, battery charger and battery
manager for the lights.
The circuit switches on the lights at
nighttime but switches them off once
the battery is flat.
The solar panel is a 10W, 12V type
left over from a previous project, already mounted on the shed roof. The
XL6009 buck-boost regulator module reduces the voltage from the solar
panel to the 5V required to run the
TP4056-based Li-ion battery charger.
The low voltage cut-off circuit below prevents damage to the Li-ion cells
from over-discharge. Op amp IC1a acts
as a voltage comparator with the reference coming from VREF1, an LM385BZ. This device only requires tens
of microamps to operate and produces
an accurate 1.2V which is fed to pin
2 of IC1a. Its other input pin samples
the battery voltage via a 120kW/100kW
divider.
When the battery voltage falls to
2.7V, the output of IC1a goes low and
so NPN transistor Q2 switches off. The
gate of Mosfet Q1 is then pulled up to
its source voltage by the 100kW resis-
Australia’s electronics magazine
tor, so Q1 turns off, preventing current from flowing from the battery to
the LEDs. About 200mV of hysteresis
is built into the switching point by
the 560kW positive feedback resistor.
The LEDs are also switched off during the day due to the action of NPN
transistor Q3. When the solar panel
voltage rises above about 1.2V, Q3’s
base-emitter junction is forwardbiased, and so it switches on, pulling
the battery-related voltage at pin 3 of
IC1a low, close to 0V. This is sensed
by IC1a as if the battery is flat, so again
it switches the LEDs off.
The rest of the time (ie, when the battery is above 2.7V and the solar panel
is in darkness), the LEDs are connected across the battery and so they light
up. The garden light LED modules incorporate current-limiting resistors to
prevent them burning out with a fully
charged (4.2V) battery, not shown on
the circuit diagram.
Note that Q1 was chosen so that it
would present a low channel resistance
with its gate at -2.7V compared to its
source, ie, just before the low-battery
cutout activates. It must also have a
sufficiently low on-resistance to avoid
getting too hot (~20mW in this case).
K. G.,
One Tree Hill, SA. ($80)
January 2021 75
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