Page 180 - 15 Dangerously Mad Projects for the Evil Genius

Chapter 12 ■ High-Brightness LED Strobe 157

Theory The LEDs used in this project have a
recommended I of 30mA, at which there will be a
f
LEDs are the component of choice for generating V of 3.3V. So to get them as bright as possible
f
light. In this section we take a closer look at LEDs without damaging them, we need to set the current
and how to use them. through them to be about 30mA. The problem is

that batteries and power supplies are voltage
Using LEDs sources rather than current sources. That is, you
choose a battery that will maintain a more or less
LEDs have been used as indicator lights for many
constant voltage of, say, 9V, no matter how much
years. They have a great advantage over
current you draw from it. But what we really want
incandescent bulbs in that they last almost
for LEDs is something that will maintain a current
indefinitely (if treated well) and generate light
of 30mA even if the voltage supply goes up or
efficiently without large amounts of waste heat.
down.
In recent years, LED technology has improved
Some circuits can provide a current source in
so that all colors of light can be generated, and at
this way—many based on the L200 IC. The
intensities that make LEDs suitable for
cheaper alternative is to use a current limiting
illumination rather than just indication. Indeed,
resistor, as shown in Figure 12-11.
where the author lives, LEDs are now being used
Figure 12-11 shows how if we know V (the
in street lighting.
supply voltage), V (the forward voltage of the
Although LEDs are pretty wonderful, they are f
more difficult to use than conventional
incandescent bulbs. For a start, they have a
polarity, so you have to drive them from DC and
you have to connect them the right way around.
Second, they cannot normally be connected
directly to a power source like a battery. They
require a resistor to limit the current; otherwise,
the LED will draw too much current and burn out.
This design is typical of LED applications in
R V R
that it requires a number of LEDs to achieve the
desired brightness. In this case, quite a big number.
V
So we have to work out how to wire up our LEDs
so they shine brightly but without burning out. The
key to this is to control the current that flows I f
through the LED. This figure will typically be
between 10mA to 30mA for a small regular LED
and considerably higher for special-purpose LEDs LED
such as the Luxeon LEDs, which can be up to a V f
few watts in power. This figure goes under the
name of I , or forward current. The other value we
f
need to know regarding the LED is called V , or
f
forward voltage. This is the voltage across the
Figure 12-11 LED and current limiting resistor
LED when it has I flowing through it.
f