Page 162 - 15 Dangerously Mad Projects for the Evil Genius

140 15 Dangerously Mad Projects for the Evil Genius

actually do this no matter how much current flows resistor R1. It is the voltage dropped across R1 that
through the collector emitter of the transistor, will be amplified by the IC. So, using a higher
providing current amplification rather than voltage value of resistor here would increase the sensitivity
amplification. of the receiver; a lower value would decrease the
R7 is there to limit the current to the laser sensitivity. The value of 10k was found to be
diode. about right.

How the Receiver Works
Summary
Most of the work in the receiver is carried out
This has been an interesting project, in which
using a single audio amplifier IC. This device, the
we have learned something about amplification
TDA 7205, will amplify the signal from the
and modulation—the foundations of radio
phototransistor. The capacitor C1 provides AC
communication. While the end result is not as
coupling—that is, it only allows the changes to the
useful as a walkie-talkie or a cell phone, it does
microphone to pass through to the amplifier, not
work, at least over a fairly short range.
any DC bias.
In the next chapter, we can look forward to a
The phototransistor allows more or less current
project that the Evil Genius likes to use to torment
to flow depending on the light falling on it. The
his minions: flashing a camera flash in their faces.
more light, the more it conducts. This current will
flow through both the phototransistor and the