This is a different twist to a super-simple solar charge lighting circuit. I’ve never used reverse leakage current through a solar panel in this way before, but it seems to work.
The project uses a cheap eBay solar panel (5 or 6V) and a standard phone lithium battery with built in protection. The charge current is limited purely by what the solar panel can deliver, in this case about 100 to 200mA. The intensity of the LEDs will depend on the value of the series resistor and the transistor’s base resistor. The unit is more intended for decorative use than area lighting.
I built a similar circuit years ago and use it to drive a string of meteor lights. They’re still going strong years later and even work well into the night in winter.
I’m wondering how consistent the reverse leakage current from the solar panel will be between panels. It seems common enough to require the inclusion of a reverse discharge diode in most solar chargers.
The component list is:
Nokia or other protected phone battery.
Solar panel from eBay 5 or 6V 500mA output max (100ma is fine).
1N4001 1A diode. (or any from the 1N400X range)
10 ohm resistor to limit LED current.
10K resistor for transistor base, adjust if needed.
BC547 or any other similar NPN transistor with high gain.
Double sided foam tape and some insulated wire.
Some LEDs, Parallel LED string, meteor lights or whatever you want to run.
Keep in mind that the cell may have a charge while building the circuit, so be careful not to short it out, although it should have overcurrent protection if you do.
This circuit is intended for low current LED loads only.
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via YouTube Simplest solar light possible, using old phone battery.