Another Junk Box Solar LED Candle

Solar powered novelty items such as an electronic candle have become very inexpensive and common place. Looking from the outside, it consists of a small solar panel to harvest power from daylight and some sort of rechargeable battery as a power reservoir that energizes a flickering lamp at night. A typical example, as shown below, costs about $1.00 each at the local store.

Solar LED Candles

But rather than buying something, make it yourself. So, this post is another entry exploring the design idea of a pretty cute solar LED candle!

The complete circuit is shown in the below figure. There are a total of four key components. A small 6V/100mA solar panel, a 2.4V/600mAH Ni-MH battery pack, a S8550 PNP transistor, and a yellow 5mm candle flicker LED.

Solar LED Candle Schematic

Below is a snip from the datasheet of the candle flicker LED

UTC datasheet of the S8550 transistor

Parts list in detail

  • 6V/100mA Solar Panel x1
  • S8550 Transistor x1
  • 1N4007 Diode x1
  • Yellow 5mm clear Flicker LED x1
  • 1.2V/600mAH Ni-MH battery x2
  • 100KΩ ¼ w resistor x 1
  • 10KΩ ¼ w resistor x1

A blueprint for the final build, but I leave the rest to you…

Following my usual practice, the first test was done on a mini breadboard and it worked as expected. All the parts were taken from my junk box, so obviously no expense or effort was spared when building the breadboard version.

It is not a secret; a suitable candle flicker LED was found in a dismantled pool lamp.

First, my test setup did not include the 100KΩ load resistor, but even in dark the candle never lit as there was a feeble output on the solar panel. And the addition of the modest load resistor quickly cleared the bug.

Note at this point that the solar panel in this circuit is also the ambient light sensor. If it provides sufficient voltage, the driver transistor is switched off.

Later, the second version was built on a flake of prototyping circuit board as shown below. The test in the garden was exciting, it came on a little before the sun set.

Solar LED Candle Veroboard

Just a quick and dirty construction, but it works flawlessly!

Solar LED Candle Vero Closeup

I have not built the real-world model yet as I am still looking for a compact enclosure. Maybe I will reuse the “flame head” of the aforementioned pool lamp (see below) in my final version.

Getting back to the design revisions, this little circuit also runs well on a 1S lithium-ion battery (3.7V) if there is enough light to harvest during the day.

So that is all there is to this Solar LED Candle. There are countless ways to design and build a solar LED candle. But this stark concept requires only a few cheap, common parts you might have in your junk box. So, it would be good to give it a try.

I am sure there are other potential applications for this design idea, so as always, I welcome comments and suggestions from the readers community out there!

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