The circuit design is extremely straightforward and may be understood with the following points:
As can be seen in the given circuit diagram, the design basically consists of a solar panel, a PNP transistor, few LEDs, a battery and a few resistors.
The transistor is the only active component which is positioned as a switch for preventing the battery voltage from reaching the connected LEDs during day time.
During broad day light, the solar panel produces the required amount of voltage which is applied across the rechargeable battery via the 1N4007 diode and the resistor R*. This voltage charges the battery gradually from dawn to dusk.
The resistor R* value should be adjusted as per the specs of the battery for limiting excessive current to it.
The resistor also serves as the current limiting resistor for the connected LEDs when the transistor is switched ON.
Here it has been calculated as 10 Ohms.
As long as the solar panel generates the optimal amount of power, the positive potential at the base of the transistor keeps it switched OFF.
However when dusk sets in the solar voltage begins to drop, and when it drops below the zener diode rating, the transistor slowly starts conducting, illuminating the LEds gradually.
With complete absence of sun light or when its completely dark, the transistor conducts fully with the help of the 1K resistor, and produces full brightness over the LEDs.
The next morning, the cycle repeats all over again.
The circuit can be modified in many different ways.
The above diagram may also be built in the following manner. It looks more sensible now as the resistor is removed from the emitter for facilitating efficient triggering of the transistor.
The diagram shows an incorrect transistor number (8050), use 8550 instead.
Recommended Solar Panel Specs
6 to 8V/2 watt
Voltage - 6V
Current - 330 mA
Solar Garden Light Circuit with Constant Voltage
If a Li-Ion battery is intended to be used for the above explained circuit, a constant voltage feature becomes crucial for safeguarding battery life and prolonging it.
The following circuit show how this may be done by adding a simple voltage follower regulator circuit:
If a 3.7V Li-Ion battery is used, make sure to adjust the 10K preset to achieve precisely 4V across the output points where the battery is supposed to be connected, do this adjustment without connecting the battery. The 4V level ensures that the battery is never overcharged (at 4.2V) and this also allows the circuit charge the battery without a constant current supply.