Nowadays you can buy white LEDs, which emit quite a bit of
light. They are so bright that you shouldn’t look directly at
them. They are still expensive, but that is bound to change.
You can make a very good solid-state pocket torch using a few
of these white LEDs. The simplest approach is naturally to use
a separate series resistor for each LED, which has an operating
voltage of around 3.5 V at 20 mA. Depending on the value of the
supply voltage, quite a bit of power will be lost in the resistors.
The converter shown here generates a voltage that is high
enough to allow ten LEDs to be connected in series. In addition,
this converter supplies a constant current instead of a constant
voltage. A resistor in series with the LEDs produces a voltage
drop that depends on the current through the LEDs. This volt-
age is compared inside the IC to a 1.25-V reference value, and
the current is held constant at 18.4 mA (1.25 V ÷ 68 Ω).
The IC used here is one of a series of National Semiconductor
‘simple switchers’. The value of the inductor is not critical; it
can vary by plus or minus 50 percent. The black Newport coil,
220 µH at 3.5 A (1422435), is a good choice. Almost any type
of Schottky diode can also be used, as long as it can handle at
least 1 A at 50 V. The zener diodes are not actually necessary,
but they are added to protect the IC. If the LED chain is
opened during experiments, the voltage can rise to a value
that the IC will not appreciate.
The PCB shown here is unfortunately not available ready-
made through the Publishers’ Readers Services.