How many LEDs in series can a solar circuit drive?

An experiment to see how many LEDs in series a standard solar garden light can drive with its boost circuit. In the past I've hacked existing lights with two LEDs to give more colour options, but I've never tested the maximum number of LEDs that will work.

There's an advantage to using at least two in series, in that the classic gallium nitride LEDs often fail with a shunt resistance. So using two in series gives redundancy to still get light and allow the NiMH cell to charge up.

I'm not sure if the chip has a deliberate shunt circuit to protect it from high open circuit voltage, or if the transistor itself starts shunting at higher voltages. The cap voltage is high enough that the circuit could be used to drive 5V microcontroller circuitry. That would require the addition of a 1N4147 diode and a small capacitor (100nF) to provide smooth DC though.

I also added a socket to allow the LEDs to be swapped, but past experience has shown that the contacts need well greased to avoid moisture causing surface corrosion. The grease (or lacquer) can also be applied to the PCB to protect it from moisture, and as always the switch should be bridged with a blob of solder one the light is in use, as they often fail open circuit.



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