Page 93 - 15 Dangerously Mad Projects for the Evil Genius

72 15 Dangerously Mad Projects for the Evil Genius

A laser diode on its own is not enough to presence of potential at the gate. To turn on the
produce the narrow beam of light we want. We current and have the laser light up, simply increase
need a lens to do that. Since it is quite difficult to the voltage at the gate to an amount above its
get the lens lined up in exactly the right place, the threshold voltage.
solution is to buy a laser module. A laser module For the 2N7000, this threshold voltage is around
consists of both a laser diode and lens in one 2V. When the transistor is turned on, it has a very
sealed unit with two leads coming out the back to low resistance (typically 1.2 ). On the other hand,
supply it with power. Like laser modules that shine when it is switched off there is a tiny “leakage”
a dot, certain varieties shine a cross or a line. You current of around 1μA, or a millionth of an amp,
can also now buy green laser modules. Blue laser which is the reason we don’t need an on/off
modules are also becoming available as a result of
switch.
Blu-ray disk players, which use high-powered blue
In Chapter 5, we encountered a type of FET
lasers.
called the MOSFET, which was employed to
The laser module we are using is very low
control the much higher power of a motor and
power—only 3 mW. However, it is possible to buy
resistor used as a heating element in the balloon
laser diodes of 300 mW or more, which when
popper.
focused are capable of popping a balloon and even
burning and cutting. Clearly, such devices must be
handled with care and respect. Summary

In this simple project, we explored how to use a
FETs
laser diode and FET. You can adapt the basic
You may have noticed that our design does not circuit to make other things “touch-controlled” as
have an on/off switch. This is unnecessary because well. The FET can control a current of up to
the transistor we use is a type called a FET (field 400mA. This is more than enough to power a high-
effect transistor). These transistors differ from the brightness LED (HB LED). So you may feel the
more common bipolar transistors because, rather urge to experiment and see what else you can do
than having a “base” connection, they have a so- with this circuit.
called “gate.” The gate is insulated from the rest of In the next chapter, we will move away from
the transistor through which the main current weapons, toward security, starting with another
flows. It exerts its influence over the current laser project. The laser beam alarm will use
through the transistor by way of the charge on it. mirrors and another laser module to protect the
In other words, unlike an ordinary bipolar Evil Genius’ Lair.
transistor, which relies on a current to switch the
transistor, the FET can be switched just by the