LED

18 LEDs using a 3.7v Li-Ion CELL
This circuit drives 18 white LEDs from a 3.7v Li-Ion cell. It has been designed by Samuel
Budiyanto budiyantosamuel90@gmail.com using components from an old Compact
Fluorescent Lamp. No data is available on the 1mH inductor and the circuit has been
provided for experimentation purposes ONLY.
It is an interesting circuit because the two transistors provide a constant brightness and the
BC547 provides feedback to keep the circuit oscillating.
The 10k base resistor seems very high but the circuit has been tested for 12 hours on a
1200mA-Hr cell and the brightness remained constant.
The brightness is determined by how hard you drive the 2SD882 transistor.
It is turned on by the 10k resistor and this will deliver very little current, but since the
transistor has a gain of 100 to 300, the collector current will be up to about 100mA.
Basically the circuit will over-drive the LEDs and the BC547 will limit the current to the
required brightness level.
The BC547 transistor has the effect of turning OFF the 2SD at a particular instant in each
cycle to reduce the time when it is turned ON. The BC547 gets its "timing" from the 10k and
1k resistors, by the fact that these resistors form a voltage divider to produce a voltage on
the base. When the 2SD turns ON, a voltage is developed across the 10R that adds to this
voltage but it is delayed slightly by the 1n capacitor. The 1n determines the frequency at
which the circuit will oscillate. By experimenting with these 4 components you get the
required brightness and this remains constant for the life of the cell.
All the LEDs are in series on each string and the brightness will depend on matching each
string. By swapping some of the LEDs you will be able to adjust the brightness to make
them all emit equally.
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