This white-LED driver circuit is ideal for use in a garden light. It automatically turns the LED on at night and runs from a single 1.2V nicad cell which is recharged by a solar cell during the day.
Diode D1 allows the solar cell to charge the battery during the day and prevents it from discharging back into the solar cell at night. Transistor Q1 controls the LED driver circuit. This transistor is normally on during the day (ie, when there is output from the solar cell) and so Q2 and the LED are off.
At night time, Q1 is off and this allows a simple blocking oscillator circuit based on T1, R2 and Q2 to operate. This circuit in turn drives LED1 via a 1W resistor which limits the peak current into the LED.
T1 is wound bifilar, with the two windings configured to produce a centre-tapped winding. Winding AB is the main primary winding and winding BC is the feedback winding. The number of turns and the core used are not critical. The prototype worked with a toroid scrounged from an old computer power supply, as well as with a small ferrite suppression bead and an Altronics L5110 core. The toroids were wound using 10 turns of 0.25mm wire, while the ferrite bead worked with just five turns of 0.25 mm wire through the hole (that’s all that would fit)
Source: Silicon Chip