>
> On Dec 4, 2009, at 9:57 AM, Michael Rigby-Jones wrote:
>
> This is a fairly simple (and quite cunning IMO) circuit that does what
> you want:
>
> http://www.solarbotics.net/library/circuits/misc_switching.html
>
> Note that there will be a small quiescent current drawn from the  
> battery
> (mainly through R3), the value of which will depend on your battery
> voltage which you don't seem to have stated.  At 4.2v (fully charge
> lithium polymer voltage) LTSpice says it will be of the order of 45uA
> with the component values shown.  You can probably reduce this  
> somewhat
> by increasing the value R3 since the circuit is designed for lower
> voltages.
>
> Regards
>
> Mike

I looked at the circuit and I don't understand how the 2N3906 can turn  
on when the negative terminal of the solar cell goes below zero.  It  
looks to me as though the negative terminal has to go below negative  
0.7 volts.  Where did I go wrong ?
Gus

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