This Skill Builder is excerpted from the 2nd edition of Make: Electronics, available at Maker Shed and fine retailers everywhere.

This Skill Builder is excerpted from the 2nd edition of Make: Electronics, available at Maker Shed and fine retailers everywhere.

Of all electronics tools, I consider the multimeter to be the most essential. It will tell you how much voltage exists between any two points in a circuit, or how much current is passing through the circuit. It will help you to find a wiring error, and can also evaluate a component to determine its electrical resistance — or its capacitance, which is the ability to store an electrical charge.

If you’re starting with little or no knowledge, these terms may seem confusing, and you may feel that a multimeter looks complicated and difficult to use. This is not the case. It makes the learning process easier, because it reveals what you cannot see.

Figure A multimeter

Figure A

Before I discuss which meter to buy, I can tell you what not to buy. You don’t want an old-school meter with a needle that moves across a scale, as shown in Figure A. That is an analog meter.

You want a digital meter that displays values numerically — and to give you an idea of the equipment available, I have selected four examples.

Figure B

Figure B

Figure B shows the cheapest digital meter that I could find, costing less than a paperback novel or a six-pack of soda. It cannot measure very high resistances or very low voltages, its accuracy is poor, and it does not measure capacitance at all. However, if your budget is very tight, it will work for basic projects.

Figure C

Figure C

The meter in Figure C offers more accuracy and more features. This meter, or one similar to it, is a good basic choice while you are learning electronics.

Figure D

Figure D

The example in Figure D is slightly more expensive but higher quality. This particular model has been discontinued, but you can find many like it, probably costing two to three times as much as the NT brand in Figure C. Extech is a well-established company trying to maintain its standards in the face of cut-price competitors.

mele_0105

Figure E shows my personal preferred meter at the time of writing. It is physically rugged, has all the features I could want, and measures a wide range of values with extremely good accuracy. However, it costs more than 20 times as much as the lowest-priced, bargain-basement product. I regard it as a long-term investment.

How do you decide which meter to buy? Well, if you were learning to drive, you wouldn’t necessarily need a high-priced car. Similarly, you don’t need a high-priced meter while you are learning electronics. On the other hand, the absolute cheapest meter may have some drawbacks, such as an internal fuse that is not easily replaceable, or a rotary switch with contacts that wear out quickly. So here’s a rule of thumb if you want something that I would regard as inexpensive but acceptable: Search eBay for the absolute cheapest model you can find, then double the price, and use that as your guideline.

Regardless of how much you spend, the following attributes and capabilities are important.

Ranging

A meter can measure so many values, it has to have a way to narrow the range. Some meters have manual ranging, meaning that you turn a dial to choose a ballpark for the quantity that interests you. A range could be from 2 to 20 volts, for instance.

Other meters have autoranging, which is more convenient, because you just connect the meter and wait for it to figure everything out. The key word, however, is “wait.” Every time you make a measurement with an autoranging meter, you will wait a couple of seconds while it performs an internal evaluation. Personally I tend to be impatient, so I prefer manual meters.

Another problem with autoranging is that because you have not selected a range yourself, you must pay attention to little letters in the display where the meter is telling you which units it has decided to use. For example, the difference between a “K” and an “M” when measuring electrical resistance is a factor of 1,000. This leads me to my personal recommendation: I suggest using a manual-ranging meter for your initial adventures. You’ll have fewer chances to make errors, and it should cost slightly less.

A vendor’s description of a meter should say whether it uses manual ranging or autoranging, but if not, you can tell by looking at a photo of its selector dial. If you don’t see any numbers around the dial, it’s an autoranging meter. The meter in Figure D does autoranging. The others that I pictured do not.

Values

The dial will also reveal what types of measurements are possible. At the very least, you should expect:

Three samples of the Greek symbol omega, used to represent electrical resistance

Three samples of the Greek symbol omega, used to represent electrical resistance

Volts, amps, and ohms, often abbreviated with the letter V, the letter A, and the ohm symbol, which is the Greek letter omega, shown in Figure F. You may not know what these attributes mean right now, but they are fundamental.

Your meter should also be capable of measuring milliamps (abbreviated mA) and millivolts (abbreviated mV). This may not be immediately clear from the dial on the meter, but will be listed in its specification.

DC/AC stands for direct current and alternating current. You may select these options with a DC/AC pushbutton, or choose them on the main selector dial. A pushbutton is probably more convenient.

Continuity testing is a useful feature that enables you to check for bad connections or breaks in an electrical circuit. Ideally it should create an audible alert, in which case it will be represented symbolically with a little dot that has semicircular lines radiating from it, as shown in Figure G.

Figure G

Figure G. This symbol indicates the option to test a circuit for continuity, with audible feedback. It’s a very useful feature.

For a small additional sum, you should be able to buy a meter that makes the following measurements, in order of importance:

Capacitance. The majority of electronic circuits require small components called capacitors. Because small ones usually don’t have their values printed on them, the ability to measure their values can be important, especially if some of them get mixed up or (worse) fall on the floor. Very cheap meters usually cannot measure capacitance. When the feature exists, it is usually indicated with a letter F, meaning farads, which are the units of measurement. The abbreviation CAP may also be used.

Transistor testing is indicated by little holes labeled E, B, C, and E. You can plug the transistor into the holes to verify which way up the transistor should be placed in a circuit, or if you have burned it out.

Frequency is abbreviated as Hz.

• • •

Taste the Power

Can you taste electricity? It feels as if you can.

mele_0116

What you will need:

  • 9-volt battery
  • Multimeter

Caution: No More Than Nine Volts! This experiment should only use a 9-volt battery. Do not try it with a higher voltage, and do not use a bigger battery that can deliver more current. Also, if you have metal braces on your teeth, be careful not to touch them with the battery. Most importantly, never apply electric current from any size of battery through a break in your skin.

Procedure

Moisten your tongue and touch the tip of it to the metal terminals of a 9-volt battery.

Do you feel that tingle? Now set aside the battery, stick out your tongue, and dry the tip of it very thoroughly with a tissue. Touch the battery to your tongue again, and you should feel less of a tingle.

What’s happening here? You can use a meter to find out.

Setting Up Your Meter

Does your meter have a battery preinstalled? Select any function with the dial, and wait to see if the display shows a number. If nothing is visible, you may have to open the meter and put in a battery before you can use it — check the instructions that came with the meter.

Figure 1

Figure 1. Leads for a meter, terminating in metal probes.

Meters are supplied with a red lead and a black lead. Each lead has a plug on one end, and a steel probe on the other end. You insert the plugs into the meter, then touch the probes on locations where you want to know what’s going on. See Figure 1. The probes detect electricity; they don’t emit it in significant quantities. When you are dealing with small currents and voltages, the probes cannot hurt you (unless you poke yourself with their sharp ends).

Figure 2

Figure 2. Note the labeling of sockets on this meter.

Figure 3

Figure 3. Socket functions are split up differently on this meter.

Most meters have three sockets, but some have four (see Figures 2 and 3). Here are the general rules:

One socket should be labeled COM. This is common to all your measurements. Plug the black lead into this socket, and leave it there.

Another socket should be identified with the ohm (omega) symbol, and the letter V for volts. It can measure either resistance or voltage. Plug the red lead into this socket.

The voltage/ohms socket may also be used for measuring small currents in mA (milliamps) … or you may see a separate socket for this, which will require you to move the red lead sometimes.

An additional socket may be labeled 2A, 5A, 10A, 20A, or something similar, to indicate a maximum number of amps. This is used for measuring high currents.

Fundamentals: Ohms

You’re going to evaluate the resistance of your tongue, in ohms. But what is an ohm?

We measure distance in miles or kilometers, mass in pounds or kilograms, and temperature in Fahrenheit or Centigrade. We measure electrical resistance in ohms, which is an international unit named after Georg Simon Ohm, who was an electrical pioneer.

The Greek omega symbol indicates ohms, but for resistances above 999 ohms the uppercase letter K is used, which means kilohm, equivalent to a thousand ohms. For example, a resistance of 1,500 ohms is equal to 1.5K.

Above 999,999 ohms, the uppercase letter M is used, meaning megohm, which is a million ohms. In everyday speech, a megohm is often referred to as a “meg.” If someone is using a “two-point-two meg resistor,” its value is 2.2M.

A conversion table for ohms, kilohms, and megohms is shown in Figure 4.

Figure 4

Figure 4

In Europe, the letter R, K, or M is substituted for a decimal point, to reduce the risk of errors. Thus, 5K6 in a European circuit diagram means 5.6K, 6M8 means 6.8M, and 6R8 means 6.8 ohms. I won’t be using the European style here, but you may find it in some circuit diagrams elsewhere.

A material that has very high resistance to electricity is called an insulator. Most plastics, including the colored sheaths around wires, are insulators.

A material with very low resistance is a conductor. Metals such as copper, aluminum, silver, and gold are excellent conductors.

Measuring Your Tongue

Inspect the dial on the front of your multimeter, and you’ll find at least one position identified with the ohm symbol. On an autoranging meter, turn the dial to point to the ohm symbol as shown in Figure 5, touch the probes gently to your tongue, and wait for the meter to choose a range automatically. Watch for the letter K in the numeric display. Never stick the probes into your tongue!

Figure 5

Figure 5. On an autoranging meter, just turn the dial to the ohm (omega) symbol.

On a manual meter, you must choose a range of values. For a tongue measurement, probably 200K (200,000 ohms) would be about right. Note that the numbers beside the dial are maximums, so 200K means “no more than 200,000 ohms” while 20K means “no more than 20,000 ohms.” See the close-ups of the manual meters in Figure 6.

Figure 6

Figure 6. A manual meter requires you to select the range.

Touch the probes to your tongue about one inch apart. Note the meter reading, which should be around 50K. Put aside the probes, stick out your tongue, and use a tissue to dry it carefully and thoroughly, as you did before. Without allowing your tongue to become moist again, repeat the test, and the reading should be higher. Using a manual ranging meter, you may have to select a higher range to see a resistance value.

When your skin is moist (for instance, if you perspire), its electrical resistance decreases. This principle is used in lie detectors, because someone who knowingly tells a lie, under conditions of stress, may tend to perspire.

Here’s the conclusion that your test may suggest. A lower resistance allows more electric current to flow, and in your initial test, more current created a bigger tingle.

Fundamentals: Inside a Battery

When you used a battery for the original tongue test, I didn’t bother to mention how a battery works. Now is the time to rectify that omission.

A 9-volt battery contains chemicals that liberate electrons (particles of electricity), which want to flow from one terminal to the other as a result of a chemical reaction. Think of the cells inside a battery as being like two water tanks — one of them full, the other empty. If the tanks are connected with each other by a pipe and a valve, and you open the valve, water will flow between them until their levels are equal. Figure 7 may help you to visualize this. Similarly, when you open up an electrical pathway between the two sides of a battery, electrons flow between them, even if the pathway consists only of the moisture on your tongue.

You can think of a battery as being like a pair of interconnected water reservoirs.

Figure 7. You can think of a battery as being like a pair of interconnected water reservoirs.

Electrons flow more easily through some substances (such as a moist tongue) than others (such as a dry tongue).

Further Investigation

The tongue test was not a very well-controlled experiment, because the distance between the probes might have varied a little between one trial and the next. Do you think that may be significant? Let’s find out.

Hold the multimeter probes so that their tips are only ¼” apart. Touch them to your moist tongue. Now separate the probes by 1″ and try again. What readings do you get?

When electricity travels through a shorter distance, it encounters less resistance. As a result, the current will increase.

Try a similar experiment on your arm, as shown in Figure 8. You can vary the distance between the probes in fixed steps, such as ¼”, and note the resistance shown by your meter. Do you think that doubling the distance between the probes doubles the resistance shown by the meter? How can you prove or disprove this?

Vary the distance between the probes, and note the reading on your meter.

Figure 8. Vary the distance between the probes, and note the reading on your meter.

If the resistance is too high for your multimeter to measure, you will see an error message, such as L, instead of some numbers. Try moistening your skin, then repeat the test, and you should get a result. The only problem is, as the moisture on your skin evaporates, the resistance will change. You see how difficult it is to control all the factors in an experiment. The random factors are properly known as uncontrolled variables.

There is still one more variable that I haven’t discussed, which is the amount of pressure between each probe and the skin. If you press harder, I suspect that the resistance will diminish. Can you prove this? How could you design an experiment to eliminate this variable?

If you’re tired of measuring skin resistance, you can try dunking the probes into a glass of water. Then dissolve some salt in the water, and test it again. No doubt you’ve heard that water conducts electricity, but the full story is not so simple. Impurities in water play an important part.

What do you think will happen if you try to measure the resistance of water that contains no impurities at all? Your first step would be to obtain some pure water. So-called purified water usually has minerals added after it was purified, so, that’s not what you want. Similarly, spring water is not totally pure. What you need is distilled water, also known as deionized water. This is often sold in supermarkets. I think you’ll find that its resistance, per inch between the meter probes, is higher than the resistance of your tongue. Try it to find out.

Those are all the experiments relating to resistance that I can think of, right now. But I still have a little background information for you.

Background: The Man Who Discovered Resistance

Georg Simon Ohm, pictured in Figure 9, was born in Bavaria in 1787 and worked in obscurity for much of his life, studying the nature of electricity using metal wire that he had to make for himself (you couldn’t truck on down to Home Depot for a spool of hookup wire back in the early 1800s).

Georg Simon Ohm, after being honored for his pioneering work, most of which he pursued in relative obscurity.

Figure 9. Georg Simon Ohm, after being honored for his pioneering work, most of which he pursued in relative obscurity.

Despite his limited resources and inadequate mathematical abilities, Ohm was able to demonstrate in 1827 that the electrical resistance of a conductor such as copper varied in inverse proportion with its area of cross-section, and the current flowing through it is proportional to the voltage applied to it, so long as temperature is held constant. Fourteen years later, the Royal Society in London finally recognized the significance of his contribution and awarded him the Copley Medal. Today, his discovery is known as Ohm’s Law.

Cleanup and Recycling

Your battery should not have been damaged or significantly discharged by this experiment. You can use it again.

Remember to switch off your multimeter before putting it away. Many multimeters will beep to remind you to switch them off if you don’t use them for a while, but some don’t. A meter consumes a very small amount of electricity while it is switched on, even when you are not using it to measure anything.