This is simplest automatic solar led light circuit that my son try to make it for basic small solar charger.
This circuit is a simplest circuit. He trial it to learn about solar LED light, and makes it all by himself.
He needs one LED light that brightest at night. But in daytimes it will go out and charged current to battery.
He choose to use 2 x AA nickel-metal hydride battery total voltage is 2.4V at 500mAh. It need to used current about 50mA – 80mA. Thus he cannot use a white LED because it use 3.3-3.6V.
As this circuit. We use the relay to control current because it’s easy. But Relay works by Coil which use high current about 80mA. ??? the solar cell have 100mA only so cannot use both the relay and to charge the battery.
Thus, I choose to use transistors instead relay, until is the circuit as Figure 1. The small base-emitter current controls the larger collector-emitter current.
During the day, the solar cell has voltage to D1-diode to bias transistor Q1 it conducting collector and Emitter closed, so it not has the current bias to base of Q2. It is OFF so LED will go out.
At the same time, the solar current will charge to the batteries B1.
When no sunshine so no the solar current to base Q1 it not works. Then, the battery’s current will flow to R2 to base Q2 makes it turn ON, the LED glow more brightly.
Parts you will need
D1 = 1N4007____1000V 1A Diodes
D2 = 1N4148____75V 150mA Diodes
Q1,Q2_____2N2222____75V 0.6A TO-18 NPN transistor
R1,R2___1K 0.25W Resistors
Solar cell 3.3V-3.6V 100mA
1.2V x 2 AA batteries 500mAh
How to builds
He assembles the circuit on breadboard as Figure 2 and test on sunshine.
I test Simplest automatic solar led light circuit as video below.