Automatic solar led

[digik]

Hi guys,

can someone tell me why this sucker



doesn't work at all?

My solar panel is a 5.5V one to charge a 3.6V Ni-Mh battery and I have a 3.2V led attached. Every time i cover the solar panel the led is very dim, it goes bright only if I short the panel (of course). It seems that I don't have enough current flow, Why? I have already made other circuits exactly like this with other led and it always worked, why this time didn't? Are there other ways to do with less parts possible? The transistor is a BC 557

[Efe_114]

i think its because of the high resistance of the solar panel when it is on shade. when solar panel is in shade its resistance increases and enough base-emitter current cannot flow. you can try replacing the single transistor with a PNP darlington pair so it can function with a lower base-emitter current(higher gain allows that).maybe you can try removing/decreasing the 10k resistor connected to transistor base to increase the base-emitter current, thus the collector-emitter current increases

[digik]

Thank you very much Efe_114, that was what I thought. I've already tried to remove the base resistor and the leds became slightly more bright but not enough. Will the darlington pair have some disadvantages (current consumption and/or voltage drops)? Can I connect them straight together without any additional resistor?

I've also tried the schematic that bigclive did in this video and it did't work either. Also it seems that the diode is connected in the wrong direction, but in his video everything worked perfectly.

[Zero999]

A MOSFET would be better.

I suspect the solar panel has a built-in diode. Move the 10k resistor to across the panel.

[Efe_114]

I agree with Zero999 about the MOSFET idea since MOSFETs have so big gain that might work well. You can (probably) keep the resistor if you use a darlington pair. Have a nice day and i hope it works soon!

[Zero999]

If the solar cell has a diode, there will be no leakage current. Try this.