Page 238 - 15 Dangerously Mad Projects for the Evil Genius

Appendix ■ Electronics Construction Primer 213

Multimeters voltage you are about to measure).

■ Connect the black lead to GND. A crocodile
A big problem with electrons is that you cannot
clip on the negative lead makes this easier.
see them. A multimeter lets you view what they
are up to. It allows you to measure voltage, ■ Touch the red lead to the point whose voltage
current, resistance, and often other features, too, you want to measure. For instance, to see if an
like capacitance, frequency, and more. A cheap Arduino digital output is on or off, you can
$10 multimeter is perfectly adequate for most touch the red lead to the pin and read the
purposes. voltage, which should be either 5V or 0V.
Multimeters (Figure A-7) can be either analog Measuring current is different than measuring
or digital. You can tell more from an analog meter voltage because you want to measure the current
than you can from a digital, because you can see flowing through something, not the voltage at a
how fast a needle swings over, and how it jitters— particular point. So you put the multimeter in the
something that isn’t possible with a digital meter, path of the current you are measuring. This means
where the numbers just change. However, for a that when the multimeter is set to a current setting,
steady voltage, it’s much easier to read a digital there will be a very low resistance between the two
meter since an analog meter will have a number of leads, so be careful not to short anything out with
scales, and you have to work out which scale you the leads.
should be looking at before taking the reading. Figure A-8 shows how you could measure the
You can also get autoranging meters which, current flowing through an LED.
once you have selected whether you are measuring To measure current:
current or voltage, will automatically change
■ Set the multimeter range to a current range
ranges for you as the voltage or current increases.
higher than the expected current. Note that
This is useful, but some would argue that thinking
multimeters often have a separate high-current
about the range of voltage before you measure it is
connector for currents as high as 10A.
actually a good thing.
■ Connect the positive lead of the meter to the
To measure voltage using a multimeter:
more positive side from which the current will
■ Set the multimeter range to voltage (start at a flow.
range that you know will be higher than the

Figure A-7 A multimeter Figure A-8 Measuring current