Experiment 15: Intrusion Alarm Revisited
134 Chapter 3
essentIAls
All about diodes
A diode is a very early type of semiconductor. It allows electricity to flow in one
direction, but blocks it in the opposite direction. (A light-emitting diode is a
much more recent invention.) Like an LED, a diode can be damaged by revers-
ing the voltage and applying excessive power, but most diodes generally have
a much greater tolerance for this than LEDs. The end of the diode that blocks
positive voltage is always marked, usually with a circular band, while the other
end remains unmarked. Diodes are especially useful in logic circuits, and can
also convert alternating current (AC) into direct current (DC).
A Zener diode is a special type that we won’t be using in this book. It blocks cur-
rent completely in one direction, and also blocks it in the other direction until a
threshold voltage is reached—much like a PUT.
Signal diodes are available for various different voltages and wattages. The
1N4001 diode that I recommend for the alarm activation circuit is capable of
handling a much greater load at a much higher voltage, but I used it because it
has a low internal resistance. I wanted the diode to impose a minimal voltage
drop, so that the relay would receive as much voltage as possible.
It’s good practice to use diodes at less than their rated capacity. Like any semi-
conductor, they can overheat and burn out if they are subjected to mistreatment.
The schematic symbol for a diode has only one significant variant: sometimes
the triangle is outlined instead of filled solid black (see Figure 3-94).Figure 3-94. Either of these schematic symbols may be used to represent a diode,
but the one on the right is more common than the one on the left.