Lab: Electronics – ITP Physical Computing

Introduction

This lab will introduce you to a few basic electronic principles by trying them in action. You’ll learn how to measure voltage, amperage, and resistance using a multimeter. You will also learn about components in series vs. parallel and be introduced to Ohm’s Law in practice. For more information on the theory behind this lab, please check out these notes.

When you’re building electronics, you run into problems all the time. Diagnosing those problems relies not only on a good theoretical knowledge of how circuits work, but also on practical knowledge of how to test them. The most common tool for testing circuits is the multimeter, a device that can measure current, voltage, resistance, and electrical continuity. More expensive multimeters can also measure other electrical properties, but if you can measure these four, you can diagnose most common circuit problems.

What You’ll Need to Know

To get the most out of this lab, you should be familiar with the following concepts beforehand. If you’re not, review the links below:

Safety Warnings! Check below when measuring Amperage in order to avoid damaging your meter!

Things You’ll Need

Photo of a solderless breadboard — Figure 1. A solderless breadboard with two rows of holes along each side.

Photo of flexible jumper wires — Figure 2. Flexible jumper wires. These wires are quick for breadboard prototyping, but can get messy when you have lots of them on a board.

5-volt regulator, model 7805. This component has three legs and a tab at the top with a hole in it. If you hold the component with the tab at the top and the bulging side of the component facing you, the legs will be arranged, from left to right, voltage input, ground, and voltage output. — Figure 3. 5-volt voltage regulator, model 7805

A DC power jack. It pairs with a plug with a 2.1mm inside diameter, 5.5mm outside diameter plug, and has screw terminals on the back so that you can attach wires to it. — Figure 4. A DC Power Jack

Wire strippers. The jaws of this wire stripper have multiple hole sizes so that it can strip wires of variable sizes. — Figure 5. Wire stripper tool

Photo of a handful of LEDs — Figure 6. LEDs. The long leg goes to voltage and the short leg goes to ground

Photo of two potentiometers — Figure 7. Potentiometer

Photo of a handful of 220-ohm resistors. — Figure 8. 220-ohm resistors. These ones are 4-band resistors

Photo of four breadboard-mounted pushbuttons — Figure 9. Pushbuttons

Multimeter tool. This tool has a dial to set the function, and three holes into which to plug the testing leads. The leads are currently plugged into the center hole and the right hand hole. — Figure 10. Multimeter tool

Testing The Meter

by Deqing Sun

Before you get started, you will want to make sure your meter is working. This is a particularly good idea if you’re using a meter that lots of other people use, such as the ones at ITP. Here is how to test that:

Insert the two probes. Insert the Black probe in the “COM” jack. This is the COMmon, or ground, connection. The Red probe should be in the “V” jack (Figure 11). This connection is for measuring voltage. It can also be used to measure resistance in Ohms, or frequency in megaHertz, on the meter shown here.

Photo of the bottom half of a multimeter, showing the holes for the test leads. The left hole, labeled A, is empty. The center hole, labeled COM, has the black lead in it. The right hole, labeled VΩmA, has the red lead in it. — Figure 11. Multimeter detail, showing the holes for the probes.

Turn the function knob to the Diode/Continuity Function and switch the meter on. If the word “Hold” appears on the screen, press the hold button once to disable the hold function (not all meters have a clearly labeled Hold function; check your meter’s manual to be sure – see Figure 12). This function is used to hold a value onscreen after you remove the probes from a circuit. The “1.” on left picture means the value is out of range now.

Photo of a multimeter's top half, showing the screen and the power button on the left, and the hold button button on the right. Not all meters are arranged in the same way, so check your manual for the location of the buttons on yours. — Figure 12. This multimeter’s hold button is on the right side below the screen.

Touch the tips of the probes together. The meter will beep and the display value should be less than 0.01 (Figure 13). If it works, congratulations! you have a usable meter. If not, try to push the plug of the probes to improve the contacts (Figure 14). In many cases the failure is caused by loose contact of the jacks. In other cases, you might have a weak or dead battery.

Photo detail of .a meter measuring continuity. The two probes are touching each other. The meter reads .009. There is a diode symbol and a musical note on the left side of the screen. — Figure 13. Detail of .a meter measuring continuity. The meter is should be making a beeping sound in this case.

Photo detail of the bottom of a multimeter, showing the red lead not plugged in all the way. — Figure 14. If the meter is not working correctly, check to see if the probes are plugged in properly.

The Controls on a Meter

Different meters have different controls, but most meters will have the following:

Voltage: This setting is generally broken up into Volts DC, indicating that the polarity of the voltage will not change, and Volts AC, indicating that the polarity will alternate.
Amperage: This setting measures the current in a circuit. Again, it’s usually broken up into AC and DC. There are commonly two holes for the positive probe to measure current, one that’s low amperage and the other that’s high amperage. Don’t try to measure high amperages with the positive probe in the low amperage hole or you will damage the meter.
Resistance: Resistance is measured in ohms. This function is sometimes grouped with the continuity check.
Continuity: Continuity measures for a connection, generally very low or no resistance.
Diode Check: Diode check measures for a voltage drop across a diode, typically 2.7V or less. If you hold the positive probe on the anode of the diode and the common probe on the cathode, you’ll see a voltage drop. If you reverse the probes, you’ll see no reading.

Some meters are auto-ranging, meaning that they will choose the right order of magnitude for a reading automatically. These meters will only have one setting for a given property (volts, ohms, amps, etc). Other meters are not auto-ranging. These meters will have multiple settings for a given property. For example, the meter in the next section below has multiple settings for the resistance (or ohms) property: 2M (megohms), 200k (kilohms), 20k, 2k, and 200 ohms.

Many meters will have additional functions, like temperature, capacitance, transistor checks, and more. The functions listed above are the minimum that you can expect, however.

Figures 15 and 16 show two multimeters. Figure 16 is autoranging and Figure 15 is not. Notice how the autoranging meter has only one setting per function, while the non-autoranging meter has several per function. For example, Voltage on the meter in Figure 15 is divided into AC (marked by a ~) and DC (marked by a ⎓). Within those two areas, there are multiple possible ranges: DC voltage can be set to 200mV, 2V, 20V, 200V, or 6ooV. Each range setting represents the maximum voltage you can read on that setting.

Photo of a non-autoranging multimeter. — Figure 15. A non-autoranging multimeter. Each function has multiple possible range settings.

Photo of an autoranging multimeter. — Figure 16. An autoranging multimeter. It has only one setting for each function and the meter automatically detects the range.

The Symbols on a Meter

Here are a few of the common symbols on electrical meters:

Volts: V
Ohms (resistance): Ω
Amps (current): A
AC: ~, ⏦ (tilde, sine wave)
DC: ⎓
Continuity: diode, speaker
Diode Check:diode
Non-Contact Voltage: NCV
Ground: ⏚ (vertical line with three horizontal lines below it)

Measuring Continuity

Continuity is simply whether or not there is a connection between two points. You just used this function to test your meter (Figure 17). Now you’ll use it to test a conductor.

Photo of a meter set to check continuity. The dial indicator is pointed to a symbol that has a dot next to three concentric arcs radiating out from the dot. This is a symbol for a speaker. The continuity check will beep. — Figure 17. This meter is ready to do a continuity check. The screen reads 1 on the right hand side. For this meter, this indicates that there is no continuity at the moment.

You can use the continuity check to find connections on a switch or if the pushbutton is connected when you press the button. You can also use it to measure whether there’s a break in a wire, or whether a given material conducts electricity. Set your meter as shown here, and try touching the probes together. The meter should beep.

Touching Two Ends of a Wire

Now try touching the probes to two ends of a wire. Again, the meter should beep. The wire conducts electricity. There is a continuous flow of electricity from one end of the wire to another.

Touching Two Points on a Switch

If you touch two points on a switch, what happens when you switch the switch? Beep or no beep? When you close the switch, the meter should beep, indicating that there is continuity between the two probes of the meter. If the meter beeps whether the switch is open or closed, then those two points are always connected.

Probing Points on a Breadboard

Put a wire in one hole of a breadboard that has no circuit on it. Then put another wire in another hole, chosen at random. Measure continuity between the two wires. Did you get what you expected? If the two holes were in the same row (or in the same column, on the side of the board) then you would get continuity and the meter would beep. If they were in different rows, then it would not beep.

Measuring Continuity Across Your Hand

Try measuring the continuity across your hand. Do you get anything? Why or why not? You probably don’t because the resistance across your skin is so high that it doesn’t register as a continuous conductor. It can conduct small amounts of current though. You don’t want your body to carry high amounts of current or voltage though, because it can damage or kill you.

Setup the Breadboard

For the rest of this lab, you’ll need a breadboard connected to a +5 Volt or +3.3 volt power supply. You can use an Arduino as your power supply (Using Arduino Uno shown in Figure 18, using Arduino Nano shown in Figure 20), if it’s connected to a USB power supply or a DC power supply, or you can solder together a DC power jack as shown in the Soldering lab, and use a 9-12V DC power supply and a 7805 voltage regulator (Figure 19). The voltage regulator will take the DC power supply’s Voltage and regulate it down to 5 Volts DC.

If you are using a Nano for the first time, you might want to check out these videos before you get started, to prep your board and care for your microcontroller.

Note: Schematics

The diagrams to the left of some of the breadboard images in this exercise are called schematics.They show the electrical layout (as opposed to the physical layout) of the components. For a good rundown on reading and understanding schematics, see this page and some very useful videos on this topic in the videos section of this site.

Breadboard drawing of an Arduino Uno connected to a breadboard. The Arduino's 5V hole has a red wire connecting it to the outer left side row of the board. This is the voltage bus on the left side. The Arduino's ground hole has a black wire connecting it to the inner left side row of the board. This is the ground bus on the left side. At the bottom of the breadboard, a red wire connects the left side voltage bus to the inner row on the right side. This is the right side voltage bus. Similarly, a black wire connects the left side ground bus to the outer row on the right side. This is the right side ground bus. — Figure 18. An Arduino Uno connected to a breadboard. The Arduino’s 5V and ground holes are supplying voltage to the breadboard.

Breadboard drawing of a DC power jack and a voltage regulator on a breadboard. At the top of the drawing, there is a DC power jack. Red and black wires from the jack connect to a 7805 5-volt voltage regulator mounted in the top right three rows of the breadboard with its tab facing to the right. input. The power supply's red wire is connected to the regulator's top pin row, the input pin. The power supply's black wire is connected to the regulator's middle pin, or ground. Another black wire connects the regulator's middle pin, ground, to the inner left side row of the board. This is the ground bus on the left side. A red wire connects the regulator's bottom pin, the output pin, to the outer left side row of the board. This is the voltage bus on the left side. At the bottom of the breadboard, a red wire connects the left side voltage bus to the inner row on the right side. This is the right side voltage bus. Similarly, a black wire connects the left side ground bus to the outer row on the right side. This is the right side ground bus. — Figure 19. DC voltage jack and 7805 voltage regulator on a breadboard. The regulator is supplying 5V and ground holes are supplying voltage to the rest of breadboard.

Arduino Nano on a breadboard. — Figure 20. Breadboard view of an Arduino Nano connected to a breadboard.When plugged into a USB port, this board will supply 3.3V across the voltage and ground buses. The +3.3 volts and ground pins of the Arduino are connected by red and black wires, respectively, to the left side rows of the breadboard. +3.3 volts is connected to the left outer side row (the voltage bus) and ground is connected to the left inner side row (the ground bus). The side rows on the left are connected to the side rows on the right using red and black wires, respectively, creating a voltage bus and a ground bus on both sides of the board.

From here on out, diagrams will show the DC power supply and voltage regulator version, but feel free to use the Arduino version instead if you prefer.

Measuring Resistance of a Component

Resistance is a material property of any component, like a resistor or a wire. To measure the resistance of a component, you have to remove the component from the circuit. To measure resistance, turn your meter to the setting marked with the Greek letter Omega (Ω), as shown in Figure 21.

Photo of a multimeter set to measure resistance. The dial is pointing to a setting marked 20k in a section of the dial marked with the Greek letter omega. — Figure 21. Multimeter set to measure resistance.

If your meter is not an auto-ranging meter, you want the meter set to the approximate range, and slightly higher than, of the component’s resistance. For example, to measure a 10-Kilohm resistance, you’d choose 20K, because 10K and 20K are in the same order of magnitude. For a 50K resistance, or anything over 20K, you’d have to step up to 200K. If you don’t know the component’s resistance, start with the meter set to a high reading, like 2M (2 Megohms). If you get a reading of zero, turn the meter one step lower, and keep doing so until you get a good reading. A reading of 1 on the left side of the meter, or of 0L, indicates you’re set to too low a range.

Breadboard drawing of measuring resistance. A breadboard circuit similar to the one above is shown, with a 5-volt regulator and DC power jack. In this circuit, a red wire connects the left side power bus to row twelve. An LED is connected from row 15 to row 16, and a black wire connects row 16 to the left hand side ground bus. A 220-ohm resistor is off to the left side of the breadboard. It has been removed from the board to measure resistance. Two multimeter probes are attached to either end of the resistor. — Figure 22. Measuring resistance. Note that this circuit is not complete. To measure a component’s resistance, you have to take it out of the circuit.

Not all components will register resistance. For example, a wire will ideally register 0 Ohms, because you want wires to have as little resistance as possible so they don’t affect the circuit. The longer the wire, the greater the resistance, however. Likewise, switches have ideally zero resistance.

The circuit shown in Figure 22 is not complete. The resistor connecting the LED to voltage has been removed to measure its resistance. The resistor would normally connect in the row connecting to the red wire (row 12) to the anode of the LED (row 15). To measure resistance of a component, you must remove it from the circuit.

Resistance and Diodes

If you measure the resistance of a diode (such as the LED shown in Figure 20), you may see a number flash briefly on the meter, then disappear. This is because diodes ideally have little or no resistance once voltage is flowing through them, but have what’s called a forward bias, which is a minimum voltage needed to get current flowing. Before you reach the forward bias voltage, the diode’s resistance is ideally infinite. After you reach it, the resistance is ideally zero. There are no ideals in electronics, however, which is why you see a resistance value flash briefly as the meter meets the diode’s forward bias. Most meters have a diode check setting, marked with a diode symbol, that will allow you to check the forward bias of the diode.

Measuring Resistance Across Your Hand

Try measuring the resistance across your hand. Set the meter really high, perhaps 20 Megohms. Do you get anything? You should get a resistance in the 2-20 Megohm range. Make your palm sweaty, or lick it, and try again. You should get a lower resistance, perhaps 0.2 Megohms or so.

Measuring Voltage

Once a circuit is complete and powered, you should learn to read voltages between different points in the circuit. Wire a 7805 5-Volt voltage regulator on a breadboard as shown above and connect it to power. If you don’t have one, you can use the 5V or 3.3V output from an Arduino instead, as explained above. Now add an LED and a 220-ohm resistor to the breadboard as shown in Figure 23.

Breadboard drawing of a 220-ohm resistor and an LED powered by a 5-volt regulator . The breadboard is wired above, with a 5-volt voltage regulator and DC power jack. One side of the resistor is connected to the left side voltage bus of the breadboard. The other side is connected to row 15. The anode of an LED is connected to row 15 as well. The cathode of the LED is connected to row 16. A black wire connects row 16 to the left side ground bus. — Figure 23. Breadboard view of a 220-ohm resistor and an LED powered by a 5-volt regulator.

Note how the long leg, or anode, of the LED goes to voltage through the resistor, and the short leg, or cathode, goes to ground. Next you’re going to measure voltage in this circuit.

Voltage is a measure of the difference in electrical potential energy between two points in a circuit. It’s always measured between two points in a circuit. Measuring the voltage between the two sides of a component like an LED tells you how much voltage that component uses. When you’re measuring voltage between one side of a component and another, for example, it’s called measuring the voltage drop “across” the component.

Set your multimeter to measure DC volts (Figure 24). The voltage regulator you’re using can take an input voltage range of about 8 to 15 volts, and it outputs 5 volts, so you know that no voltage you’ll read in this circuit is over 15 volts. If your meter has a variety of ranges for DC volts, choose a range that ‘s closest to, and greater than, 15 volts. For example, many meters have a setting for 20 volts, meaning that they can read up to 20V DC at that setting.

Photo of a multimeter set to measure DC voltage. The dial is pointing to a setting marked 20V in a section of the dial marked with the letter V and a horizontal line with a dashed line under it. — Figure 24. Multimeter set to measure voltage. Note the horizontal and dashed lines indicating DC.

Measure for voltage between the power and ground bus rows on the breadboard. You should have 5 volts, or very close to that.

Now measure the voltage drop across the LED (Figure 25). When you’re measuring the voltage drop across a component, you put the meter probes in parallel with the component. In this case, the voltage across both the component and the meter will be the same.

Breadboard drawing of measuring voltage across an LED. The LED circuit is the same as shown above. The red probe of a multimeter is touching the anode of the LED. The black probe of the meter is touching the cathode of the LED. — Figure 25. Breadboard view of measuring voltage across an LED. The red lead is on the anode and the black lead is on the cathode of the LED.

Getting a Negative Voltage

Did you get a negative voltage? Why would that happen? That means you placed the red probe on the point of lower voltage, and the black probe on the point of higher voltage. In other words, you reversed the polarity.

A Switched LED Circuit

Now you’re going to make a more complex circuit. Disconnect the board from power and add a switch in series with the LED and resistor as shown in Figure 26 and 27. Remember, long leg (anode) goes to voltage, short leg (cathode) goes to ground).

Schematic image of a pushbutton, a 220-ohm resistor and an LED connected to a 7805 5-volt regulator. At left, there is a power plug. The positive terminal of the power plug is connected to the voltage input of a 7805 voltage regulator. The negative terminal of the power plug is connected to the ground terminal of the regulator. The voltage output of the regulator is connected to one side of a pushbutton. The other side of the pushbutton is connected to a 220-ohm resistor. The other side of the resistor is connected to the anode of an LED. The cathode of the LED is connected to the ground terminal of the regulator. — Figure 26. Schematic view of a pushbutton controlling an LED. A DC power supply of 8-12V is connected to the input and ground of a 7805 5V voltage regulator. The output of the regulator is connected to a pushbutton. The other side of the pushbutton is connected to one side of a 220 ohm resistor. The other side of the resistor is connected to the anode of an LED. The cathode of the LED is connected to the voltage regulator’s ground connection.

Breadboard drawing of a solderless breadboard with a 7805 voltage regulator mounted on it as shown in the drawings above. The regulator is connected to a DC power plug, and the ground and voltage output of the regulator is connected to the voltage and ground bus rows on the right side of the breadboard. The ground and voltage bus rows on the right are connected to the ground and voltage bus rows on the left with wires at the bottom of the board. A pushbutton is mounted across the center of the breadboard, connected to rows ten and twelve. A red wire connects row ten to the voltage bus on the left side of the board. A 220-ohm resistor is connected to row twelve on the left side of the center section of the board. Its other end is connected to row sixteen in the center area. An LED is connected to another hole in row sixteen. The other side of the LED is connected to a hole in row seventeen. Another hole in row seventeen connects to the ground bus on the right side of the board. — Figure 27. Breadboard view of a pushbutton controlling an LED. The components are connected as described in Figure 26.

Connect the board to your power supply and press the switch. It will illuminate the LED. Let go of the switch and it will turn the LED off again. By pressing the switch you are completing a circuit and allowing the resistor and LED to begin consuming electricity. The resistor is very important in this circuit as it protects the LED from being over-powered, which will eventually burn it out. A typical LED operates at a voltage of 2.0-2.6 volts (V) and consumes approximately 20 milliamps (mA). The resistor limits the current to a level that is safe for the LED. The higher the resistor value, the less voltage that will reach the LED. The lower the resistor value (with 0 ohms being no resistor at all), the more the voltage that will reach the LED.

Adding Up Voltage

Now, while playing with the pushbutton, measure the voltage across the pushbutton as you did in the last step, both in the on position and the off position. Is there a voltage drop across the pushbutton? What voltage do you read when the pushbutton is not pressed?

Measure the voltage across the LED and the resistor as well. Does the total voltage across all the components add up to the voltage between voltage and ground on your board? Remember, in any circuit, all of the voltage must be used up. Why? If the voltage across all the components doesn’t add up, that indicates to you that some of the electrical energy is getting converted to light, heat, and other forms of energy. No component is 100% efficient, so there’s always the possibility for some loss.

Components in Series

Change your circuit to add another LED in series with the first one, as shown in Figures 28 and 29.

Schematic image of a pushbutton, a 220-ohm resistor and two LEDs connected to a 7805 5-volt regulator. At left, there is a power plug. The positive terminal of the power plug is connected to the voltage input of a 7805 voltage regulator. The negative terminal of the power plug is connected to the ground terminal of the regulator. The voltage output of the regulator is connected to one side of a pushbutton. The other side of the pushbutton is connected to a 220-ohm resistor. The other side of the resistor is connected to the anode of an LED. The cathode of the LED is connected to another LED's anode. The cathode of the second LED is connected to the ground terminal of the regulator. — Figure 28. Schematic view of a pushbutton controlling two LEDs. This circuit is similar to the one in Figure 24, but there are two LEDs in series after the resistor. The cathode of the first LED connects to the anode of the second, and the cathode of the second LED connects to the regulator’s ground.

Breadboard drawing of a solderless breadboard as shown in the drawing above, with a 5-volt voltage regulator, a pushbutton, a 220-ohm resistor and 2 LEDs. The red probe of a multimeter is touching the side of the resistor that's connected to the pushbutton. The black probe of the meter is touching the other side of the resistor. — Figure 29. Breadboard view of a pushbutton controlling two LEDs. The components are wired as described in Figure 28.

Adding Up Voltage

Measure the voltage across the resistor, as shown in Figure 30. Then measure the voltage across each LED, as shown in Figures 31 and 32. Does the total add up to the voltage from power to ground? If not, where does the missing voltage go? The remaining energy is lost as heat generated from the components.

Did you use two different color LEDs and get a different voltage drop across each one? That’s normal. Different color LEDs are made with different elements, and have slightly different voltage drops. Did you get no reading when you measured? Did you remember to push the button before you took your reading?

Adding a Third LED in Series

Add a third LED in series with the other two. Do the LEDs light? Why or why not? They most likely will not light up. Each LED needs about 2V to reach its forward bias and turn on. If you have three in series, and a 5-volt supply, each is getting less than the 2V it needs to turn on.

Components in Parallel/Measuring Amperage

Connect three LEDs in parallel as shown in Figure 33 and 34 (remember, long leg (anode) goes to voltage, short leg (cathode) goes to ground):

Schematic image of a pushbutton, a 220-ohm resistor and three LEDs in parallel connected to a 7805 5-volt regulator. At left, there is a power plug. The positive terminal of the power plug is connected to the voltage input of a 7805 voltage regulator. The negative terminal of the power plug is connected to the ground terminal of the regulator. The voltage output of the regulator is connected to one side of a pushbutton. The other side of the pushbutton is connected to a 220-ohm resistor. The other side of the resistor is connected to the anodes of three LEDs. The cathodes of the three LEDs are connected to the ground terminal of the regulator. — Figure 33. Schematic view of a pushbutton controlling three LEDs wired in parallel. The circuit is similar to Figure 26, but the two LEDs have been replaced by three LEDs which are all in parallel. The anodes of all three LEDs are connected to the resistor, and the cathodes are all connected to the regulator’s ground connection.

A pushbutton is mounted across the center of the breadboard, connected to rows ten and twelve. A red wire connects the switch to the left side voltage bus. A 220-ohm resistor is connected to row twelve on the left side of the center section of the board. Its other end is connected to row sixteen in the center area. Three LEDs' anodes are connected to another hole in row sixteen. The cathodes of the LEDs are connected to holes in row seventeen. A black wire connects row seventeen to the ground bus on the left side of the board. — Figure 34. Breadboard view of a pushbutton controlling three LEDs wired in parallel. This circuit is wired as described in Figure 31.

Measure the voltage across each LED. It should be the same across each one.

Now you’re going to read the amperage at various points in the circuit. Move your meter’s red probe to the hole for measuring high amperage. On many meters, there are three holes, one marked “Volts/Ohms/mA”, and another marked “10A”. The right one can be used for measuring amperage when the expected amperage is less than 1A. The left is for measuring high amperage, up to 10A (Figure 35). If you’re not sure, it’s best to use the hole for 10A. Then set your meter to measure DC amperage.

Photo of a multimeter set to measure High DC Amperage. The dial is pointing to a setting marked 10A in a section of the dial marked with the letter A and a horizontal line with a dashed line under it. The red lead is in the hole marked 10A. — Figure 35. Multimeter set to measure amperage up to 10A.

To measure the amperage through a given component, you need to place your meter in series with the component. When two components are in series, the amperage flowing through both of them is the same. For this, use the circuit with the three parallel LEDs that’s shown in Figures 36 and 37. To measure the amperage through any one of the LEDs in this circuit, you’ll need to disconnect one of the LED’s ends from the circuit (disconnect power first!) and use the meter to complete the circuit, as shown in Figures 36 and 37:

Schematic image of how to measure amperage of three LEDs in parallel. The circuit is similar to the parallel circuit shown above. At left, there is a power plug. The positive terminal of the power plug is connected to the voltage input of a 7805 voltage regulator. The negative terminal of the power plug is connected to the ground terminal of the regulator. The voltage output of the regulator is connected to one side of a pushbutton. The other side of the pushbutton is connected to a 220-ohm resistor. The other side of the resistor is connected to the anodes of three LEDs. A meter set to measure amperage is inserted between the resistor and one of the LEDs. The cathodes of the three LEDs are connected to the ground terminal of the regulator. — Figure 36. Schematic view of measuring amperage of one LED of three LEDs wired in parallel. This is the same circuit as shown in Figure 31, but the anode of one LED has been disconnected and then connected to one lead of a meter measuring amps. The other lead of the meter is connected to the junction between the resistor and the anodes of the other two LEDs.

You’ll find that the amperage drawn by the LEDs is tiny, on the order of 10 or 20 milliamps at the most. That’s normal for LEDs. Make sure that you check which holes your meter probes are connected to when you’re using a meter.

Warning: Measuring amperage with the red probe in the voltage hole when you have no idea how big the current is, or measuring voltage with it in the amperage holes is a good way to damage the meter.

Generating a Variable Voltage with a Potentiometer

In this last step, you’ll generate a changing voltage using a potentiometer. A potentiometer is a resistor that can change its resistance. A potentiometer (or pot) has three connections. The outer connections are the ends of a fixed value resistor. The center connection connects to a wiper which slides along the fixed resistor. The resistance between the center connection and either of the outside connection changes as the pot’s knob is moved. Related video: Potentiometer schematic

If your potentiometer does not have pins that will insert into a breadboard, then solder hook-up wires to the pot connections as shown in Figures 38 and 39.

Photo of a potentiometer with metal ring contacts. — Figure 38. A potentiometer with ring contacts, ready for soldering.

Photo of a potentiometer with ring contacts. Three wires are soldered to the three contacts, from left to right: black wire, blue wire, red wire. — Figure 39. Potentiometer with wires successfully soldered.

Next, connect the pot to an LED and a 220-ohm resistor using the circuit shown in Figure 38:

Schematic image of a potentiometer controlling an LED. At left, there is a DC power plug. The positive terminal of the power plug is connected to the voltage input of a 7805 voltage regulator. The negative terminal of the power plug is connected to the ground terminal of the regulator. The voltage output of the regulator is connected to one side of a potentiometer. The other side of the potentiometer is connected ground. The wiper of the potentiometer is connected to a 220-ohm resistor. The other side of the resistor is connected to the anode of an LED. The cathode of the three LEDs are connected to ground. — Figure 40. Schematic view of a potentiometer controlling an LED. The two ends of a potentiometer are attached to the voltage output and ground of a 7805 voltage regulator. The middle contact of the potentiometer is connected to one end of a 220-ohm resistor. The other end of the resistor is connected to the anode of an LED. The cathode of the LED is connected to ground.

Breadboard drawing of a potentiometer controlling an LED. A 7805 5-volt voltage regulator powers the board as shown in the circuits above. A potentiometer's outer connections are connected to the voltage bus and ground bus on the left side of the board. The center of the potentiometer connects to row seventeen on the left side of the center section of the board. A 220-ohm resistor is connected to row seventeen on the left side as well. Its other end is connected to row twenty-one in the left center area. An LED's anode is connected to another hole in row twenty-one. The LED's cathode is connected to row twenty-two. A black wire connects row twenty-two to the ground bus on the left side of the board. — Figure 41. Breadboard view of a potentiometer controlling an LED. This circuit is wired as described in Figure 40.

As you turn the potentiometer from one end to the other, measure the voltage between the center position and ground. The pot is acting as a voltage divider, dividing the 5V into two parts. As the voltage feeding the LED goes up or down, the LED gets brighter or dimmer. The 220-ohm resistor in the circuit protects the LED from over-voltage when the resistance between the pot’s voltage lead and its center lead is 0 ohms.

Now you’ve got a basic understanding of how to use a meter to measure voltage, current, resistance, and electrical continuity. You’ll use these tests all the time.

Next, check out the lab on Switches.