Components 119a simple voltage source (e.g., a 9-volt battery). The resistor midpoint is half the source
voltage, that is, 4.5 volts. This can be checked with a multimeter set to measure DC volts.
Ohm’s law tells us that we can fi nd the current fl ow in a resistor by dividing the applied
voltage by the resistance. The voltage source for the projects in this book is always a
9-volt battery. For the ease of the arithmetic I just round this up to 10 volts. So if we’ve
got a 10-ohm resistor, the current is just under 1 mA, actually, it is 0.9 mA, as the current
is the ratio of the voltage to the resistance. That quick calculation gives us an idea of what
to expect for our meter reading.
The same multimeter set to the ohms or resistance range can be used to check out resistor
values. There are two precautions if you’re going to do this now. The fi rst is to: keep your
fi ngers away from the resistor terminals because your body has a fi nite resistance, more
if your hands are sweaty and less if they’re dry. What you’re doing when you touch the
resistor terminals is adding your body resistance to that of the resistor you’re trying to
measure. The other precaution is to zero the resistance meter fi rst. Do this by shorting the
meter terminals and adjusting the “ zero knob ” until the meter reads zero. You need only
do this with the analog type of multimeter.
The value of a particular resistance is marked on the component body, typically with
a three-color band code. A fourth band represents the tolerance, but for the sake of
simplicity you may ignore this if you just want to read off the resistor value (which is
generally the case). As you almost certainly will want to be able to read resistor color
codes, here they are:
Color band Equivalent number code
Black 0
Brown 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9