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This white LED driver circuit is ideal for use in garden lighting. It automatically turns on the LEDs at night and operates from a single 1.2V nicad battery recharged by a solar cell during the day. Diode D1 allows the solar cell to charge during the day and prevents the battery from discharging into the solar cell at night. Transistor Q1 controls the LED driver circuit. This transistor is normally on during the day (i.e. when there is output from the solar cell) and so Q2 and the LED are off.
At night, Q1 is off allowing a simple blocking oscillator circuit based on T1, R2 and Q2 to operate. This circuit in turn drives LED1 through a 1W resistor to limit the peak current in the LED. T1 is a double-wound type, with both windings configured to produce a midpointed coil. The AB coil is the primary primary coil and the BC coil is the feedback coil. The number of revolutions and cores used doesn't matter. The prototype runs with a core taken from an old computer power supply, along with a small ferrite inhibitor and an Altronics L5110 core.
The toruses are wound using 10 turns of 0.25mm wire, while the ferrite bead works with only 5 turns of 0.25mm wire through the hole (that's all that fits). The oscillator works as follows: when Q1 is off, current flows through R2 and turns on Q2. This causes current to flow through coil AB and the core produces a flux. And this again causes the C-terminal of the transformer to exceed the voltage of the battery and turn on Q2. When the core saturates, the voltage at C drops to the battery voltage, thereby reducing the current in coil AB.
When this happens, the flux in the core starts to decrease causing the voltage at C to drop below 0.6V. As a result, Q2 turns off, and since there is no current in AB, the flux in the core begins to break down. What happens now is that the voltage at the A end of the coil exceeds the voltage of the battery. When 3.2-3.6V to ground is reached, LED1 "turns on" and current flows from the battery through BA, through the LED, and back to the battery. When the current runs out, LED1 turns off and the C-terminus returns to battery voltage. Current now flows through R2 and into the base of Q2 and the whole cycle starts again. Finally, when the Sun rises the next morning, Q1 lights up, robbing Q2's base drive, the oscillation stops, and LED1 goes out.