Page 87 - 15 Dangerously Mad Projects for the Evil Genius

66 15 Dangerously Mad Projects for the Evil Genius

heat when used as a switch that is either fully on a current of 10mA flowing through the base could
or fully off. cause up to 1A to flow through the collector and

They differ from more normal “bipolar” emitter. The actual current flowing will depend on
transistors in that they are controlled by voltage the value of the load resistor. In the case of the
rather than current. That is, it is the voltage at the balloon popper, this load resistor comprises a
“gate” connection of a MOSFET that determines variable resistor of 100k and a fixed resistor of
whether it turns on or not. Only a tiny current 1k . The fixed resistor limits the current when the
flows into the gate, making them ideal for variable resistor is set to zero.
switching from low-current sources like our A phototransistor is a variation on this theme. A
phototransistor. phototransistor will often have no base connection
and will be contained in a clear plastic case.

Phototransistors Instead of a small current being supplied to the
base, the current flowing through the collector and
To understand what a phototransistor is, we need emitter is controlled by the intensity of the light
to know a little about how a regular bipolar falling on the phototransistor.
transistor works.

Figure 5-31 illustrates how a regular transistor
works. Summary
This transistor has three leads: the emitter, the It is this kind of trickery that helps the Evil Genius
collector, and the base. The basic principal is that a maintain his mastery of the world. Nothing
small current flowing through the base will allow a impresses the minions like the threat of instant
much bigger current to flow between the collector death by ray gun.
and the emitter.
In constructing this project, we have learned a
Just how much bigger the current is depends on bit about sensing light and controlling power,
the transistor, but it is typically a factor of 100. So whether it is to the resistor used as a heating
element, or to the electric motor of the can jumper.
Collector Collector In creating this project, an alternative design
5V was considered for the can jumper: using a
mechanical mousetrap. These hold quite a lot of
Collector Current (lc)
energy in them, and they can be triggered by a
Base solenoid. However, in the end, the advantages of a
Base
system using a motor that would reset itself was
Base Current (lb)
considered the better option. It also seemed more
likely that the Evil Genius would retain more
fingers by avoiding said mousetraps.
0V
Emitter In the next chapter, we will look at adding a
Emitter touch-controlled laser sight to a BB gun.
Figure 5-31 The operation of a transistor