Ikea Solviden solar powered light teardown

[josem]

OK, so following Dave's sage advice as soon as I got home from Ikea today with a new Solviden solar light (on sale for £3!) I immediately proceeded to tear it down without turning it on 

I wanted to see how it charged the included AA 1.2v Ni-MH battery and turned on the 3 LEDs only when it was dark. What I found left me intrigued, which I guess comes down to my inexperience.

There's only one tiny PCB with the control circuit, two solar panels connected in series, one switch,  the AA Ni-MH 1.2v battery (1000mAh) and a separate board containing a cluster of 3 LEDs connected in parallel.

I measured the voltage going to LED cluster with the mutimeter and showed something around 1.2v. Oddly enough it didn't matter if the light was shining or not, the voltage to the LEDs was always around 1.2v.. however the LEDs seemed to "know"  . A current measurement confirmed this, when there was light shining on the solar panels the current to the LEDs would drop to 0, otherwise there was about 5mA.

How can we have a stable voltage but 0 current? Seemed sorcery to me.

So I connected my oscilloscope to the LEDs board and something very odd showed up. When it's dark and the LED goes on, I see a stable 1.08-1.4v line, OK makes sense... but when there's light, the solar panels are charging the battery and the LEDs turn off their input becomes these very odd waveforms at about 110kHz.

So I guess this why my multimeter is still showing voltage, but the LEDs don't light up?

I've traced the schematic of the PCB, hopefully it's correct to the best of my knowledge. There's one IC on the board which I don't know what it is, it has 4 legs and no markings other than "0116".

Can anyone explain what's going on and why they decided to do it this way? It's way too smart for me but I'm keen to understand :-)

Thanks!

[SeanB]

It is a simple switch mode boost converter. When there is light the voltage on pin 1 keeps the oscillator off. When the light drops to a low enough level the IC starts to drive the output by pulling pin 3 low via R3 ( which is actually 10R) and then turning off, with the energy stored in L1 then forward biasing the LED to emit light. This then repeats until the battery is flat or there is daylight again. Those are nice little low power switchers, and work well as a LED driver off a single AA cell in a torch conversion, providing a low power light that lasts a long time. At least that is what I did with one where the solar panel failed after a few months from water ingress, using it to replace an incandescent lensed lamp in a magnifier.

[josem]

Great explanation! Thanks Sean.

(and yes 100 = 10x10^0 nice catch)