Electricity and ElectronicsLast modified July 14, 2015

Abstract:

In this module, we will be learning about the basics of electricity and electronics. We will start by understanding the basic components of every electronics device: resistors, capacitors, and inductors. From there, we will make electrical filters, much like the ones you find in speaker systems, and characterize their properties. Finally, we will learn the basics of soldering and make our own radio and radio transmitter from recycled electronics parts. By the end of this module, you should be comfortable identifying working and broken components in broken electronics, and then using the working components to construct a new circuit.

Please read the entire lab manual before coming to the first lab session. Having an overview of what you will be doing for the module will help you make more effective use of your time and identify key concepts that you should be focusing on.

Preparatory Problems:

Please complete these problems before coming to the first lab session. If you do not know how to answer these questions, feel free to ask the instructors or look it up on the Internet. Make sure to do this before the start of the first lab session.

1. Suppose you are given the circuit below. Calculate the voltage drop across resistors 1 and 2. What would happen to those voltage drops if you closed the switch?

2. Suppose the error in a measured value of R is 10% and the error in the corresponding measured value of C is 5%. What is the error in the calculated quantity RC?

Safety:

Any time you are dealing with voltages >10V, please be cautious. What you don't want to happen is that the voltage discharges very quickly through your heart. This can happen if you have one hand touching the (+) wire and the other hand touching the (-) wire. This will cause the voltage to run from one hand to the other, which will go straight through your heart, which can result in death. To avoid this, when dealing with high voltages, always have one hand on your pocket, so that any voltage that passes through you will pass travel towards your feet and will miss your heart.

Some electronics components can be easily broken if the wrong voltage (too large or wrong polarity) is connected; these components include operational amplifiers, capacitors, and resistors. Of course, we always encourage you to play around with the lab equipment, but to prevent them from breaking too quickly, please always read the specifications sheet before using them!

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Resistance, Capacitance, and Inductance:

What is resistance? What is capacitance? What is inductance? You may have heard of all of these three things, and may have even done some short experiments to show them, but do you actually know what they mean? The best way to understand these three quantities is to build a resistor, capacitor, and inductor all from scratch, and observe how these quantities change when you change the devices that you built.

Resistance

Every material is resistive, meaning that it resists charge going through it. Even a conductor, like a copper wire, has some resistance. A commonly available and easily shapeable resistive material is graphite, which you can draw into any size or shape. Take a clean sheet of paper and draw some shapes on it with pencil. Now put the multimeter on the "resistance" setting and measure this quantity called resistance. Try to find a pattern in the resistance of the shapes. Are bigger shapes more resistive? Smaller? Longer? Shorter? Thicker? Thinner?

Try to think of some ways that you can test your pattern. For instance, do you notice that lines of the same width, but different lengths have different resistances? Try to test this observation systematically. Once you've played with the patterns a little bit, derive a law that predicts the resistance of a graphite rectangle given its length and width. Your law must have experimental data supporting it. Remember that all experimental data has uncertainties, and the size of the uncertainties will either make the law credible or not credible.

CapacitanceYou may know capacitance as the ability to store charge, but what exactly does it mean to store charge? Now we're going to make a capacitor in 2 different ways to clear up this question. For both capacitors, we will have one conductive materials and one insulating material.

The first capacitor will have 2 sheets of tin foil as the conductor and 1 sheet of paper as the insulator. Start with 3 large sheets. Sandwich the paper between the 2 pieces of tin foil, making sure that the tin foils are not touching. If that happens, current will flow between the two pieces and no charge will be stored. Use the multimeter on the "capacitance" setting to measure this quantity called capacitance. Is there capacitance between the paper and the tin foil? How about between the two pieces of tin foil? Why? Make diagram xx.

Now cut the pieces smaller and measure the capacitance again. Continue to do this so that you have a set of data relating piece size to capacitance. Derive a law

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between these two quantities. Look up the capacitance of tin foil per area and compare your answer. Does the theoretical value fall within your uncertainties?

To make the second capacitor, you will use salt water and tin foil (conductors) and a plastic water bottle (insulator).i Fill the bottle with water, add a spoonful of salt, and shake to mix. Then wrap the outside of the bottle in tin foil. Try to make sure there are no wrinkles in the foil; an easy way to do this is to flatten the tin foil, and gently roll the water bottle in it, taking care to smooth out any wrinkles. Screw on the top of the water bottle, and poke a nail through, so that the bottom of the nail touches the water. Now use the multimeter to measure the "capacitance" between the nail and the foil. What happens to the capacitance if you add more salt? Less tin foil? Try to derive a law between each of these parameters and capacitance. Make diagram xx

Now to really illustrate the quantity of capacitance, bring a metal object close to the nail and watch the spark. Be very careful not to put your finger between the metal object and the nail as you do this, as we are dealing with lethal voltages. Estimate the maximum distance of the object from the nail before a spark appears. Now change the amount of salt and try again. Derive a relation between the amount of salt and the maximum distance of sparking. How does this relate to capacitance?

Inductance

Make a circuit with a lightbulb. Look in pretest whether students already know circuits.Make a resonant inductor-capacitor circuit and determine its resonant frequency.

Electronic filters:

We will use smaller versions of the resistor and capacitor you just made to make circuits that will filter out noise, much like what's used in an audio speaker system. To test the response of these circuits to noise, we will need to use and measure signals with changing voltages, such as a voltage that changes rapidly between two values. A battery cannot produce these voltages and a multimeter cannot give us enough information about them, so we will use something called a function generator to create these signals and an oscilloscope to read these signals.

A function generator is a device that can create voltages in the shape of sine waves, triangle waves, square waves, and more, as shown in Figure 1. An oscilloscope in turn can read these voltages at high speeds. Conventional oscilloscopes and function generators look like ones shown in Figure 2, but we can actually get many similar functions by just using the headphone and microphone port of a computer.

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Figure 1: Waveforms that can be created by a function generator. A function generator creates an regularly alternating pattern of voltage (y axis) as a function of time (x axis).

Figure 2: (left) A digital oscilloscope that measures voltages over time and displays it on a screen. The knobs are for adjusting the display properties and for saving the displayed data. Taken from cibomahto.com. (right) A digital function generator that creates regularly alternating voltages in different patterns (sine wave, triangle wave, square wave, irregular wave). The knobs are for adjusting the timing and amplitude of the voltage patterns. Taken from multimeterwarehouse.com.

Intro to computer function generator and oscilloscope

Computers use voltages to output sound through a headphone jack and read sound through a microphone jack. Since sound is made of waves, we can create and decompose any wave into perfect tones, which are sine waves. You will be using a program called xx to create voltages and a program called xx to read voltages.

Safety: it's important that you DO NOT exceed the voltage input limit of the computer. Exceeding the voltage limit will break the computer. Audio input: -500mV to 500 mVAudio output: -3.5V to 3.5V

Sometimes, you will want to measure and create signals that exceed these limits. When this happens, you want to use attenuators, which make the signal smaller, and amplifiers, which make the signal bigger.

Write more about attenuators and amplifiers.

Making RC circuits

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Cite and use David Marasco's lab guide. ii

Your goal is to create a circuit that filters out noise that is above xx Hz. We will test different configurations of resistors and capacitors, as well as different places to connect the input and output signals. First, try putting the resistor and capacitor in series. Then insert a 100 Hz square wave between the resistor and capacitor and measure the signal across the capacitor (see Figure xx). Show both the input and output signals on the computer oscilloscope, and zoom into a "step" in the square wave. What do you notice? Try changing the values of the resistance and capacitance that you use. Now what do you notice? Make a law that predicts the lag time of the output signal given the values of the resistances and capacitances.

Does this law hold for different configurations of resistor and capacitor? Reverse the position of the resistor and capacitor. Again observe what happens when you change the values of the resistance and capacitance. How has the law changed?

Now use a range of frequencies from 1 Hz to 10 kHz. What do you notice about the size of the output signal for the two configurations? This is why one configuration is called a "high-pass filter" and the other configuration is called the "low-pass filter". This is how speaker filters work to make the sound you hear much more pleasing. (talk more about filtering xx)

Using the low-pass filter configuration, adjust the values of the resistances and capacitances until it only allows sound below xx Hz through. To test your sound filter, record yourself talking (or singing) into the microphone of the computer. Now, play it back by connecting the output of the first computer into the input of the second computer. Note how it sounds. Connect your sound filter between the two computers and enjoy the difference in sound quality!

Making LC-circuits

Soldering tutorial

Soldering is a technique to make conducting contact between two metal surfaces. We can do this by melting metal that wants to stick to other metals. See the below tutorial on how to solder. We'll be trying it out in class for 1 day.

Soldering tutorial: Draw from PEN soldering tutorial (http://eburn.scripts.mit.edu/pened/doku.php/background/how.to.guides/soldering)

Electronics recycling:Identifying electronics componentsLabel chart (possibly make another one)

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(instructables.com)

Reusing electronics components:

We will be using old electronics parts to build a radio to receive broadcasts and a radio transmitter to give broadcasts. You will build one and your partner will build the other--the goal is to transmit and receive broadcasts between you an your partner without interfering with the other partner pairs' broadcasts! In order to do this, you will first need to determine what frequency you want to transmit at, estimate the possible uncertainty in the frequency from imperfections in the components, and talk to your classmates to decide how far away the next radio frequency should be.

1. Radio iiiiv

The way that radio stations broadcast their signals is by converting an audio signal, like music, into a wave of electromagnetic radiation. (reading to review electromagnetic radiation xx) We will call this the signal wave. The frequency of the wave determines the pitch that is heard and the amplitude determines the loudness.

However, if all radio stations transmitted waves like these into the air, there would be no way of distinguishing them, and we would not be able to pick out one particular radio station's signal. Thus, each radio station mixes the signal wave with another wave of a specific frequency, which we will call the reference wave. This

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mixed electromagnetic wave is what is then sent into the air. The frequency of the reference wave is the number that you dial to when you tune a radio set. (diagram for wave mixing xx)

A radio set picks out a specific reference frequency to amplify, and then converts the signal into something that can be processed by our ears as sound. The basic components of a radio include an inductor-capacitor circuit, a diode, an antenna, and an earpiece. The inductor-capacitor circuit amplifies a specific frequency of the electromagnetic waves in the air, which depends on the value of the inductance and capacitance (see LC circuits on page 5). It then converts it into a voltage signal, and this produces the mixed signal that is described in the last paragraph.

However, if we listen to this mixed voltage signal now, we would not be able to hear anything, because our ears average out the sine waves to zero. We use a diode in order to convert the mixed signal into an audio signal. The diode only lets positive voltages through, and the resulting signal is no longer averaged to zero. (show diagram xx)

Finally, the signal passed into an earpiece, which converts the voltage signal into pressure waves that enter our ears as sound. The antenna can help to amplify the signal by xx. Often the signal is so weak that even an antenna would not produce a signal that can be heard by our ears. Thus, we will also learn how to build a signal amplifier in section 3.

Below is a basic schematic for the radio that you will be building.

2. Radio transmitter v

We can make a radio transmitter to mimic the signal transmitted by a radio station by using a signal filter, inductor-capacitor circuit, and transistor. The signal filter is composed of a capacitor and a resistor. It takes in an audio voltage signal and filters out the high frequency noise that may make the audio sound fuzzy. The inductor-capacitor circuit creates a voltage signal with a specific frequency, which is mixed with the filtered signal. Finally, the transistor amplifies the signal, which is then transmitted as an electromagnetic wave into the air.

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A basic schematic of a radio transmitter is shown in Figure xx.

3. Amplifiers and attenuators

4. Challenge

The challenge is for you and your partner to make a radio and radio transmitter set that can communicate with each other. In other words, when you play music with the radio transmitter, your partner should hear the music in the radio set.

However, as the rest of the class will be doing the same thing, your signals will interfere with each other. Thus, in order to pick up your partner's signal clearly, you must talk with your classmates to determine which range of frequencies you will use for each of your radio sets. From the chosen frequency, you can calculate the value of the inductors and capacitors that give the desired frequency.

You should also keep in mind that your radio and radio transmitter will not be perfect, and there will be an uncertainty in the frequencies that you will be able to transmit and receive. Thus, you and your classmates should pick a range of frequencies that is large enough to account for error, but not too large that not everyone in the class can have a frequency range. We recommend using a range of frequencies between xx and xx.

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iReferences (possible optional reading)

David Marasco, Homemade Capacitor, Foothill College Undergraduate Physics Labs, http://www.foothill.edu/~marasco/5labs/lab5b8.htmlii David Marasco, Resonance in a Driven RLC Circuit, Foothill College Undergraduate Physics Labs, http://www.foothill.edu/~marasco/labnine/iii Radios: http://sci-toys.com/scitoys/scitoys/radio/radio.html, http://makezine.com/Project/Bottle-Radio/2077/1/#.UAgwsGme7Yoiv Radio with adjustable volume using homemade capacitor:https://www.youtube.com/watch?annotation_id=annotation_141846&feature=iv&src_vid=sArNxGnYhNU&v=VqdcU9ULAlAv Sean Ragan, Super Simple iPod FM Transmitter, http://makezine.com/projects/super-simple-fm-transmitter/