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Measuring Stuff with a Multimeter

A multimeter (see Figure 4-15) is a testing device that, um, tests multiple things, including resistance, voltage, and current. Using certain multimeter models, you can test to be sure that components — such as diodes, capacitors, and transistors — function properly. You can also troubleshoot your circuit to see where current is failing and pinpoint the problem spot.

figure 4-15

You don’t have to break into your piggy bank to buy a multimeter. You can find them for about $10; if you want fancy features, you can spend over $100. Find a model whose price you like and then splurge on the next higher-priced model. You will use a multimeter all the time. Trust us: It’s worth a few extra bucks for a better model. See "Discrete components" section  for more information about multimeter features.

How a multimeter works

A multimeter has a set of leads: a black one and a red one. You attach these leads to the component or portion of the circuit that you’re testing, and a digital readout provides the results. You adjust a knob to set the test you wish to perform such as resistance, voltage, or current as well as the range to test.

Note: Some multimeters have an auto-ranging feature that saves you the trouble of setting the range. Test leads that typically come with multimeters use simple cone-shaped tips. You can buy test clips that slip onto the cone-shaped tips to make it easier to clip them onto the leads of a component. This makes testing much easier, trust us.

The two things we test most often with our multimeter are resistance and voltage.

Reading resistance

The problem with resistors is that manufacturers seem to expect you to memorize the color code that identifies the resistance rating. Here is an easier way:

1. Clip your test leads onto the resistor leads.

2. Dial your multimeter to the resistance range you think the resistor fits in.

3. Read the value.

If your multimeter reads 1, you guessed too low of a value. Move the dial to the next range up until you get a valid reading. If your multimeter reads at close to 0 (zero), you guessed too high of a value. Dial to the next range down until you get a valid reading; if you get to the lowest range and the value is still 0, whatever you’re testing has zero resistance.

Testing switches or relays is another common use of the resistance-testing feature of your multimeter. You can clip your test leads onto the lugs of an SPST switch to verify that it’s working. (Hint: Occasionally, they don’t work.) When the switch is open, you should get a value of 1, meaning that the resistance is higher than your meter can measure. When the switch is closed, you should get a low resistance — close to 0 (zero) ohms. You can also test SPDT or DPDT switches or relays, like those we use in "Sensitive Sam Walks the Line" project, to make sure which lugs are connected in which switch position.

Measuring voltage

To run a test to measure voltage, you connect the red multimeter lead to the positive side of the battery or circuit that you’re testing and the black lead to the negative or ground side and set the dial to the voltage range you expect. We often check the voltage at the contacts of a battery pack. To do this, touch the red lead to one of the battery pack outputs and the black test lead to the other. With a 4-battery pack loaded with fresh batteries, you should get a reading of about 6 volts. (If you get a reading of –6 volts, don’t worry: Just reverse which lead you are touching to which battery pack output.) When batteries get old, the voltage drops. If you get less than 5 volts from a 4-battery pack, it’s time to get new batteries.

When a circuit doesn’t work, one of the first things to check is the voltage between the +V bus and the ground bus of the breadboard. Here’s how:

1. Strip both ends of a 3" piece of 22 gauge wire.

2. Clip one end of each wire to one of your test leads.

3. Slip the free end of the wire attached to your red test lead into any contact on the +V bus.

4. Slip the free end of the wire attached to your black test lead into any contact on the ground bus.

Although you might not get a reading of the full 6 volts because of drain on the battery from the circuit, you should get a reading above 3.5 volts.

If you get a reading close to 0 (zero) volts, check to make sure that your battery pack and the wires from the battery pack terminal block are connected properly.