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Project Notes

#439 QX5252 Solar Night Light

Exploring the circuit inside most garden solar lights, which uses a QX5252/YX805 boost and charge controller.

Build

Here’s a quick demo..

clip

Notes

Inside most solar-powered garden lights is an ingenious little circuit comprising a rechargeable battery, solar cell, inductor, LED and a little IC that does all the magic:

  • when voltage is detected from the solar cell, the LED is off and the battery will charge
  • as the solar cell voltage drops off (gets dark), the chip runs a switching converter (using the inductor) to boost the voltage from the 1.x V battery sufficient to drive the LED
  • when the battery voltage falls too low, it tuens off the boost converter to prevent over-discharge

The chips that do the work are typically 4-pin TO-94 packages and come from various sources. Some of the most common are the QX5252 and YX805. I’m using a QX5252 here as it is the one I found a reasonable datasheet for.

There are DIP versions of these chips that have an additional light control pin that can either be used as a simple on-off, or to adjust the PVC voltage at which the LED cuts-off. I’m not using this as I have the TO-94 package version.

This project is a simple demonstration build of the standard solar-powered LED circuit.

Rechargeable Battery

I’m using an Energizer HR03 with a nominal voltage of 1.2V. I bought a pack of 2 with a charger (which I hopefully don’t need!):

energizer_hr03

Inductor Selection

The inductance will determine the current available to drive the LED load.

According to the QX5252 datasheet, indicative values are as follows for a nominal voltage of 1.3V:

Inductance Current
330µH 11mA
270µH 14.5mA
220µH 15.5mA
150µH 25mA
100µH 34.5mA
82µH 38mA
56µH 50mA
47µH 75mA
33µH 110mA

As I’m only going to use a few 3mm white LEDs in parallel (Vf = 2.7V), I’ve selected a low power 330µH choke as the inductor.

Solar Panel

For the final build I’m using a monocrystalline 5.5V 70mA 0.4W 9*4.3cm cell.

Construction

I’ve followed the typical application schematic from the datasheet: the circuit simply connects the solar cell and battery to their respective pins, and the the LED load is fed with an inductor so that the flyback voltage can power the LED when the chip is in switching mode.

Breadboard

Schematic

Proving things out on a breadboard…

QX5252SolarNightLight_bb_build

Demonstration Build

Turning once again to my store of Ikea cardboard packaging for an “all on display” demo build:

layout

Performance

When the voltage generated by the solar cell falls below about 0.2V, the boost converter switches on. Here’s the waveform of the switched output driving the LED:

boost_trace

We’re currently having a wet season “cool spell” with overcast days and afternoon showers, and I have the unit indoors behind mildly tinted glass and a chance of direct sublight for only a few hours a day.

But even so I’m seeing the battery charge to ~1.3V during the day and drop to ~1.2V overnight. To get an even better picture of what’s going on, I used another Arduino to monitor the voltage and log the data to Adafruit IO (see LEAP#440 for more on the data logging).

I’ll keep monitoring this for a while yet - basically to find out if net/net the solar panel is able to keep the battery charged while in continuous use under variable weather patterns - but over the first full 24 hour period I’m seeing roughly what I’d expect:

battery_voltage_chart

Is This How They Really Make Them?

I was recently in a Bunnings store in Australia and was absolutely blown away by the range of LED solar lights on offer - from $1 to over $30 AUD.

I picked up a “s*lar lytworx” solar LED spotlight for $3.95AUD to find out if this is what’s really inside them.

example_unit

And yes, the circuit I’ve demonstrated here is exactly what they contain:

  • 3 LEDs
  • a switch on the LED anode
  • the YX805 version of the LED control chip
  • an inductor (green - blue - black - silver, so I think 56µH)
  • a solar cell
  • and a 300mAh NiCad battery

The solar cell and battery are pretty small - but this was being sold on the Queenslad coast, where I’m sure there’s ample sunlight to keep these charged year-round.

example_unit_circuit2

Credits and References

About LEAP#439 PowerLEDSolar
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.