Page 181 - 15 Dangerously Mad Projects for the Evil Genius

158 15 Dangerously Mad Projects for the Evil Genius

LED), and I (the forward current of the LED), we when it is new. This would give 3.3V per LED,
f
can calculate a suitable value of resistor to use. which is still fine.

Since V V V , we know that V V – V We then repeat this pattern six times to light all
f R R f
and since the current through the resistor will be of our LEDs.
the same as the current through the LED (I ), then
f
using Ohm’s law we have R (V – V ) / I .
f f
Summary
For example, if we have a 9V battery and we
want to drive an LED with a forward voltage of 2V This is a great project if you like soldering. With
and a forward current of 20mA, we need a resistor 36 LEDs to solder, there are a lot of connections to
of value: be made.
(9 – 2) / 0.02 350 As a way of driving LEDs, this design is far

So, you may be wondering how we are from ideal, and if the reader is interested in such
managing to drive all 36 of our LEDs without a things, they are encouraged to look at more
single current-limiting resistor. The answer is that advanced techniques for driving LEDs, such as
we are relying on the fact that if we drive three constant current power supplies.
LEDs in series from 9V, each will drop 3V, which In the next chapter, we will turn our attention to
should still allow our LEDs to draw enough gravity, and more specifically, how to defy it—
current to shine brightly without burning out. with a levitation machine.
However, a 9V battery will often supply up to 10V