Experiment 6: Very Simple Switching
50 Chapter 2
Introducing Schematics
In Figure 2-35, I’ve redrawn the circuit from Experiment 6 in a simplified style
known as a “schematic.” From this point onward, I will be illustrating circuits
with schematics, because they make circuits easier to understand. You just
need to know a few symbols to interpret them.
Compare the schematic here with the drawing of the circuit in Figure 2-17.
They both show exactly the same thing: Components, and connections be-
tween them. The gray rectangles are the switches, the zigzag thing is the resis-
tor, and the symbol with two diagonal arrows is the LED.
The schematic LED symbol includes two arrows indicating that it emits light,
because there are some kinds of diodes, which we’ll get to later, that don’t.
The triangle inside the diode symbol always points from positive to negative.
Trace the path that electricity can take through the circuit and imagine the
switches turning one way or the other. You should see clearly now why either
switch will reverse the state of the LED from on to off or off to on.
This same circuit is used in houses where you have a switch at the bottom of
a flight of stairs, and another one at the top, both controlling the same light-
bulb. The wires in a house are much longer, and they snake around behind the
walls, but because their connections are still the same, they could be repre-
sented with the same basic schematic. See Figure 2-36.
A schematic doesn’t tell you exactly where to put the components. It just tells
you how to join them together. One problem: Different people use slightly
different schematic symbols to mean the same thing. Check the upcoming
section, “Fundamentals: Basic schematic symbols,” for the details.220
Figure 2-35. This schematic shows the
same circuit as in Figure 2-17 and makes it
easier to see how the switches function.Figure 2-36. The two-switch circuit shown
in Figures 2-17 and 2-35 is often found in
house wiring, especially where switches
are located at the top and bottom of a
flight of stairs. This sketch shows what
you might find inside the walls. Wires are
joined with “wire nuts” inside boxes that
are hidden from everyday view.Larger versions of all schematics and
breadboard photos are available
online at this book's website: http://
oreilly.com/catalog/9780596153748.