Electronics

Overview

This lab will introduce you to a few basic electronic principles by trying them in action. Before you do this lab, you should familiarize yourself with the solderless breadboard and make yourself a power connector.

To do this, you'll need:

1. Measuring Voltage

The first thing you should do when working with electronic circuits is to get comfortable checking voltages in the circuit. Wire a 7805 5-volt voltage regulator on a breadboard as shown in the breadboard lab and connect it to power.

Voltage is a measure of the difference in electrical 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. This is known as measuring the voltage "across" the component. When you're measuring voltage across a component, you're putting the meter in parallel with the component. In that case, the voltage across both should be the same.

Set your multimeter to measure DC volts. 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 about 15 volts. If your meter has a variety of ranges for DC volts, choose a range that matches this. For example, many meters have a setting for 20 volts, meaning that they can read up to 20V DC at that setting. Measure for voltage between the power and ground bus rows on the breadboard. You should have 5 volts, or very close to that.

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

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2. A Basic LED Circuit

Now you're going to make your first working circuit. Disconnect the board from power and add an LED, a switch, and a resistor in series like so:

Connect the board to power and turn the switch on and off until you get bored. While you're doing so, measure the voltage across the switch as you did in the last step, both in the on position and the off position. Measure the voltage across the LED and the resistor as well. Does the total resistance across all the components add up to the voltage between power and ground on your board? Remember, in any circuit, all of the voltage must be used up. 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 some loss.

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3. Components in Series

Connect two LEDs in series from power to ground like so:

Measure the voltage across each LED. You should get the same voltage across both, and the total should add up to approximately the total between power and ground.

Even though there is no resistor in this circuit, the LEDs don't appear to be burning out. Why not?

Add a third LED in the series. Do they light? Why or why not?

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4. Components in parallel; measuring amperage

Connect three LEDs in parallel like so:

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 lead to the hole for measuring amperage. On many meters, there are three holes, one marked "Volts/Ohms/Hz", another marked "mA", and another marked "20A". The middle one can be used for measuring amperage when the expected amperage is less than 1A. The latter is for measuring high amperage, up to 20A. If you're not sure, it's best to use the hole for 20A. Then set your meter to measure DC amperage.

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.

To measure the amperage through any one of the leds in this circuit, you'll need to disconnect one of its ends from the circuit (disconnect power first!) and use the meter to complete the circuit, like so:

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 leads are connected to when you're using a meter. Measuring amperage with the red lead in the voltage hole, or measuring voltage with it in the amperage holes is a good way to damage the meter.

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5. 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 leads are the ends of a fixed value resistor. The center lead connects to a wiper which slides along the fixed resistor. The resistance between the center lead and either of the outside leads changes as the pot's knob is moved.

Solder hook-up wires to the pot leads as shown here. Then connect the pot to an LED and a 220-ohm resistor using the following circuit:

As you turn the potentiometer from one end to the other, measure the voltage at the center position. 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 overvoltage when the resistance between the pot's 5V lead and its center lead is 0 ohms.