Hi Guys
This topic is of the back of another recent thread that can be found here.
I did not want to hijack that thread with my question so I’m starting a new thread.
Iv been doing test on the sleep library and example code of Nick Gammon(Arduino Master in my eyes)
But I need help with alternative ways to power the device while is sleep mode.
a couple of years ago.
I bough a few of these https://www.sparkfun.com/products/9962 solar chips with the original idea that that would be able to charge a small coin battery but they did not make the cut. now after playing and getting my Arduino pro mini 3v3 to run at 4.8 uA in sleep mode these devices has a new meaning.
these solar chips has a max current delivery of 50 uA in direct sunlight at 4V, i have 4 of these chips connected in parallel and in ambient light setting i get around 11-16 uA out. 
this is enough (tested) to run the modified pro mini in sleep mode with no battery connected.
Interested yet ? I AM !!!.
My problem is as follows and this is where i need input and advice.
When the solar chip(s) is connected and there is no light on them they actually draw 1 uA more from the battery then just a pro mini alone in sleep mode. So how can i detect that there is enough voltage on the solar chips to disconnect the battery (save power) without using more CPU power to monitor this. ?
Can I use a mosfet or transistor as a stand alone hardware solution or what can you guys suggest ?
PS here is the code with a few modes
I managed to create a crude timer form the internal watchdog to wake the chip up at any desired time.
#include <avr/sleep.h>
#include <avr/wdt.h>
const byte LED = 13;
int ATimer;
void AwakeTimer()
{
ATimer++;
}
void flash ()
{
pinMode (LED, OUTPUT);
for (byte i = 0; i < 2; i++)
{
digitalWrite (LED, HIGH);
delay (1000);
digitalWrite (LED, LOW);
delay (50);
ATimer = 0;
}
pinMode (LED, INPUT);
} // end of flash
// watchdog interrupt
ISR (WDT_vect)
{
wdt_disable(); // disable watchdog
} // end of WDT_vect
void setup () { }
void loop ()
{
AwakeTimer();
if ( ATimer == 7) // 45 = 6 min// 75 = 8 min // 450 = 1 Hour
{ flash (); }
// disable ADC
ADCSRA = 0;
// clear various "reset" flags
MCUSR = 0;
// allow changes, disable reset
WDTCSR = bit (WDCE) | bit (WDE);
// set interrupt mode and an interval
WDTCSR = bit (WDIE) | bit (WDP3) | bit (WDP0); // set WDIE, and 8 seconds delay
wdt_reset(); // pat the dog
set_sleep_mode (SLEEP_MODE_PWR_DOWN);
sleep_enable();
// turn off brown-out enable in software
MCUCR = bit (BODS) | bit (BODSE);
MCUCR = bit (BODS);
sleep_cpu ();
// cancel sleep as a precaution
sleep_disable();
} // end of loop