In the circuit diagram shown above, the solar panel charges a 2-cell NiMH battery (2.4 V). Between the two is a “blocking” diode. This one-way device allows current to flow from the solar panel to the battery, but does not allow current to flow backwards out of the battery through the solar panel - because the small solar panel can sink up to 50 mA in the reverse direction and drain the battery when there is no light. We are using a 1N914 diode for blocking reverse current through the solar panel. We are “trickle charging” the battery when sunlight is present. For NiMH batteries it is generally safe to “trickle” charge them by circulating through them current following rule of thumb, at a rate below “C/10”. For 1300 mAh battery cells, C/10 is 130 mA, so we keep our charging current below 130 mA – this is perfect since our solar panel only sources up to 80 mA. The other thing to notice about this circuit is that, the LED is on all the time. The circuit in bright sunlight will waste energy by keeping the LED on: Most of the solar panel current goes to driving the LED, not to charge the battery. We don’t need the LED to glow when there is light - meaning, it should glow only when it is dark.
Detection of Darkness:
We need to add a darkness detecting capability to our solar circuit. Because a solar panel produces a voltage during daylight, it can directly serve as a sensor to tell when it’s dark and we can build a darkness detecting circuit based on it.