Building Robot Electronics—the Basics--Part I

In this chapter you’ll discover the electronics that endow bots with the appearance of life. In this and future chapters you’ll find out ways to use modern (but still inexpensive) advances in electronics to create fully programmable robots able to truly think on their own. It’s an exciting endeavor, so let’s get started.

Tools for Electronics You Should Have

Compared to mechanical construction, you need relatively few tools to build the electronic centerpieces of your robots. You can always spend lots and lots of cash for all kinds of testing equipment and specialized electronic gear, but what follows are the basics that will get you started.

MULTIMETER

A multimeter, also called a volt-ohm meter, VOM, or multitester, is used to test voltage levels and the resistance of circuits—among other things. This moderately priced tool (See Figure 30-1) is the basic prerequisite for working with electronic circuits of any kind. If you don’t already own a multimeter, you should seriously consider buying one. The cost is minimal, considering its usefulness.

Figure 30-1. A digital multimeter checks resistance, voltage, and current. This model also performs simple checks of a number of common electronic components, including capacitors, diodes, and transistors.

There are many multimeters on the market today. A meter of intermediate features and quality is more than adequate. Meters are available at RadioShack and most online electronics outlets. Shop around and compare features and prices.

Digital or Analog

There are two general types of multimeters available today: digital and analog. The difference is not in the kinds of circuits they test, but in how they display the results. Digital multimeters, which are now the most common, use a numeric display not unlike a digital clock. Analog meters use the more old-fashioned—but still useful—mechanical movement with a needle that points to a set of graduated scales.

Automatic or Manual Ranging

Many multimeters require you to select the range before it can make an accurate measurement. For example, if you are measuring the voltage of a 9-volt battery, you set the range to the setting closest to, but above, 9 volts. With most meters it is the 20- or 50-volt range. Because you need to select the range, there are lots of options on the dial, but in reality these options are really just variations on a theme. The meter is easier to use than it looks.

Autoranging meters don’t require you to do this, so they are inherently quicker to use. When you want to measure voltage, for example, you set the meter to volts (either AC or DC) and take the reading. The meter displays the results in the read-out panel.

Note:  For the sake of completeness, the examples in this book that explain how to use a meter assume you have a manual (nonautomatic) ranging model. If yours has automatic ranging, then just skip the step that says to dial in the upper range of your expected measurement.

Accuracy

The accuracy of a meter is the minimum amount of error that can occur when taking a specific measurement. For example, the meter may be accurate to 2000 volts, plus or minus 1 percent. A 1 percent error at the kinds of voltages used in robots—typically, 5 to 12 volts DC—is only 0.1 volts. Not enough to quibble about.

Digital meters have another kind of accuracy: the number of digits in the display determines the maximum resolution of the measurements. Most digital meters have three and a half digits, so they can display a value as small as 0.001—the half digit is a “1” on the left side of the display.

Functions

Digital multimeters vary greatly in the number and type of functions they provide. These functions are selectable by rotating a dial on the front of the meter. At the very least, all standard multimeters let you measure AC and DC voltage, DC amperage, and resistance.

The maximum ratings of the meter when measuring volts, milliamps, and resistance also vary. For most applications, the following maximum ratings are more than adequate:

One very important exception to this is when you are testing the amount of current draw from motors. Many DC motors draw in excess of 200 milliamps.

Better multimeters have a separate DC amperage input that allows readings of up to 10 amps (sometimes as high as 20 amps). If you have the budget for it, I highly recommend that you get a meter with this feature. In most cases, it’s a separate input on the front of the meter and is clearly labeled, like that in Figure 30-2.

The high-amperage input may or may not be fuse-protected; if it is fuse-protected and you exceed the current rating for the input, a fuse will blow and you’ll have to get it replaced. On some inexpensive multimeters the inputs are not fused, and exceeding the maximum ratings could result in permanent damage to the device. So be careful!

Meter Supplies

Multimeters come with a pair of test leads, one black and one red. Each is equipped with a pointed metal probe. The quality of the test leads included with the multimeter is usually minimal, so you may want to purchase a separate set that’s better. The coiled kind is handy; the test leads stretch out to several feet, yet recoil to a manageable length when not in use.

Figure 30-2. For robotics work you’ll want a multimeter with a high amperage (10 amps or higher) input. You use this to easily test the current draw of motors, among other tasks

Standard point-tip leads are fine for most routine testing, but some measurements may require that you use a clip lead. These attach to the end of the regular test leads and have a spring-loaded clip on the end. You can clip the lead in place so your hands are free to do other things. The clips are insulated with plastic to prevent short circuits.

Using the Meter: The Basics

To use your multimeter, first set it next to whatever circuit you’re testing. Make sure it’s close enough so the test leads reach the circuit without any risk of pulling either the meter or the circuit into your lap. Then start with this:

1. Plug in the test leads; the black lead of your meter goes into the - or COM jack, and the red lead of your meter goes into the + or labeled function jack (the label may be something like VΩmA, which represents the kinds of tests you can do when the lead is inserted into that jack—in this case, voltage (V), resistance (Ω), and low-milliamp (mA) current testing.

2. Check for proper meter operation by doing a continuity test. This involves selecting one of the following operating modes. Depending on the features of your meter, choose Resistance (Ω), Diode check, or Continuity. If using Resistance and the meter is not autoranging, choose the lowest Ω setting.

Touch the metal tips of the test probe together. If the meter is functioning properly—the battery is good, the test leads are not broken—the results should be as shown here: