Table of Contents
Goals
In this exercise, you will become acquainted with a few standard tools that are used for building and testing electronic circuits. These include:
- An adjustable DC power supply
- An Electronics breadboard (or protoboard)
- A handheld multimeter
In addition, you will be using a variety of test leads to make connections between test devices and the circuit you are working on.
Electronics vs Physics Labs
In most of the instructional lab courses on physics, the focus has (broadly) been using evidence to make conclusions about physical systems. Learning about electronics typically has a different goal: for you to learn enough about circuit design and functionality to start to be able to understand and design circuits in service of doing experimental physics. As such, the structure of these tutorials will be markedly different that what you've seen before.
- Uncertainty analysis has no place here While it is certainly possible to perform uncertainty analysis on circuit behavior, it is unlikely to be helpful for this course. The issues you encounter are usually either enormous (improper connections or incorrect components for example) or very subtle (capacitive behavior in diodes or bandwidth limitations on test instruments) that they are unlikely to be explained by analyzing uncertainties.
- Failure is always an option It will not be uncommon to observe a circuit malfunction because you haven't built it correctly, or because you've made mistakes (or unwarranted assumptions) involving the test & measurement equipment. This still happens to experienced designers, and as you get more experienced you'll be able to troubleshoot problems more quickly.
Pros at work
- Troubleshooting is an integral part of design Since things may not work the first time, you'll have to figure out both why they aren't behaving as expected and how to fix them. Some general guidelines can be found here.
- Before you ask your TA for help, you should be able to provide some more specific information than “It isn't working.”
- Predictions are important As a corollary to the previous point, you need to know what a circuit is supposed to do to figure out if it isn't doing it. Sometimes you can make very specific predictions (e.g. “The amplitude should be cut in half when we increase the frequency to 39 kHz”) and other times you might have broader predictions (e.g. “I should see the voltage change when I turn on the function generator”), and when you're really struggling to figure out things you might fall back to extremely basic things (e.g. “It should behave differently when I turn the power off” or “touching a resistor shouldn't do anything”).
On Wiki Formatting
We've set up a system to help differentiate information from instructions from rhetorical questions. Information and general background has no special formatting
Rounded grey boxes are used to denote instructions: What you need to physically do as you proceed through the exercises.
Text that's offset like this is used to indicate points to test or consider. Not all such instructions need to be recorded in your report, but you should be able to answer them as you go through the exercise. They will help you gauge how much you're absorbing the content.
Sometimes there are clickable notes in bold text
These notes typically:
- Instructions on how to do something specific that not everyone will need,
- Longer derivations of mathematical terms for the curious, or
- Bits of supplementary information/trivia/history that aren't strictly necessary.
Some electronics jargon will have clickable or hoverable links to explain new terms more fully
Exploring the basic setup
To start out with, we will be wiring circuits on a standard electronics breadboard or protoboard. In addition, we will typically be using the regulated power supply to provide a DC voltage for our circuits. In this section, you will use the digital multimeter (or DMM) to investigate the breadboard and the power supply.
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| Your digital multimeter, with minigrabber leads. You will need to turn the knob to the yellow $\Omega$ symbol to measure resistance. |
Exploring the breadboard.
A breadboard like the one we'll use in this course is shown below. Explore the interconnections between various points on the breadboard using your DMM in resistance mode (Ω). A low reading shows an electrical connection while a very large reading or overload (OL) message indicates that there is no connection. Note that the leads on the DMM do not fit into the breadboard holes; you will need to use jumper wires from your kit to connect the meter to the breadboard.
Using your multimeter, explore the connectivity of the breadboard. You'll often use them for a first round of prototyping, so taking some time now to really understand its layout can help save you time later when you're building circuits.
Make sure that you understand how the contact points are connected, paying particular attention to the vertical running holes on the sides of each board segment and to the horizontal running holes at the top of the board. These points are often used as apower bus
Bus
A bus is something used to transmit signals or powering voltages over long distances. In this class we'll use the later definition more frequently, as often we'll want to connect multiple components to the same voltage (commonly +5V or 0V) without turning everything into a mess of wires.
to bring power and ground close to your components.
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| A photo of the breadboard you have for this exercise. Note that this has six identical modules on the lower portion. | A schematic of the breadboard. Click on the image for a larger version to annotate or download. |
Exploring the power supply
The triple DC power supplies we use (shown below) can each produce two independent variable DC voltages (with maximum voltage and maximum current settings of 30V and 3A, respectively) as well as a constant 5 V output. When using the variable supplies, they will typically be used as constant voltage sources and the currents drawn will usually be on the order of mA. Given that the supplies are capable of supplying 3 A of current, it is a good idea to set the current limit to approximately 0.5 A (1/6 of the total current range) or less; you can do this by turning the current knob fully counter-clockwise and then adjusting it clockwise by about 3 or 4 small gradations.
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| Our benchtop power supplies. Note that the knobs and indicator lights are mirrored from left to right channels. |
What if I want more precision?
To set a precise limit, you can short the terminals with a wire while adjusting the current limit knob. Note that this is a terrible idea if you are working with an unknown device! Always check the manual unless you are keen on voiding warranties or starting fires.
It is important to note that the power supply is floating supply; a setting of 5 V only tells us that the potential difference between the two supply terminals is 5 V. It says nothing about how either potential compares to ground (0 V). In order to set one of the terminals to ground, you must connect that terminal (and that terminal alone) to ground (green GND label) using a banana cable or some other type of connector.
Configure your power supply such that there is a 3 V difference between the red and black terminals, and set up a multimeter to measure this. Leaving the first multimeter in place, connect the black terminal to the green one (ground) and borrow a second multimeter to measure the differences between ground and the other two terminals. Then, connect the red terminal to ground and repeat.
Did the connection to ground change the potential difference between the terminals?
The reading between terminals (the 1st meter) shouldn't change here unless something's gone horribly wrong.
Selectively grounding the negative or positive terminal is how we can select if we want a positive or negative voltage from the supply. It didn't matter much in intro courses, but semiconductor devices can be permanently damaged or destroyed by flipping the sign of voltages. Thankfully, all the parts we're working with are cheap.
Configure your power supply such that the + terminal is set to +3.0 V and the – terminal is grounded (0.0 V). Connect it to the terminals at the top of your breadboard, and wire those to two different horizontal bus strips (the long top rows, used to ‘bus’ signals long distances) on the breadboard.
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| To connect the power supply to the breadboard, unscrew one of the banana jack connectors at the top of the board. | When re-tightening the connector, make sure that only exposed metal is in the hole. | Don't tighten the connector on the insulation; even if it works some of the time it will cause intermittent problems. |
When in doubt, measure! You can always use a multimeter to test if their connections are being made properly. In this instance, detaching the power supply and measuring the resistance from the wire to socket will tell if you've made a good connection or not. Wiggling wires while testing can help identify poor connections that can otherwise be hard to spot, such as in the case on the right.
Your first circuit
The basis of quite a few surprisingly useful circuits is called a voltage divider. The name (and purpose) should become evident as you work to build and test your first circuit. Consider the circuit shown in the figure below.
Predict whether the absolute value of $V_{out}$ will be greater than, less than, or equal to the voltage of the power supply, +3 V.
On Predictions
Making predictions is more important when working with electronics than typical apparatus. You can't see what's happening in circuits directly, so if you don't have some expectations for how something should behave it is hard to tell if it is doing what it is supposed to or if there is a problem. Sometimes predictions can be as simple as “If I disconnect the input I should see 0V at the output,” or “If I wiggle a cable nothing should change.” These basic low-level predictions are at the heart of troubleshooting circuits, which you will inevitably need to do.
Shouldn't I be learning about stuff in series and parallel first?
Yes and no. In order to model more complex systems in detail that knowledge is critical. On the other hand, many modern designs have a lot of the complexity stuffed into the integrated circuit chips and only need a handful of external parts, or they have networks that can't be simplified into series and parallel components that behave nicely. We can do a lot with just putting two resistive things in series and expecting the voltage to be divided across them proportionally.
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| Figure 1: A basic voltage divider circuit. Why 22k instead of 20k? | |
| The circuit shown in a traditional format from PHYS 132/142 courses. Note that the open white circles indicate points that are being measured across. | The same circuit depicted in a compact notation. $V_{out}$ is still measured with respect to ground, and the more negative terminal of the power supply is also connected to ground. |
Building nice circuits
While circuit diagrams only care about the topology of connections, there are better and worse ways to make connections. Consider the four instances shown below before you build your own circuit.
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| Four examples of the same circuit. All are topologically identical, but the building style on the right (utilizing the power rails, laying out parts along clear & straight lines, etc.) is preferable for working on larger projects. |
You'll see far messier layouts than this as you work with more complex circuits. For the messier ones, ask yourself how you can correlate the paths in on your breadboard to paths the circuit diagram (e.g., asking “Where is the connection to +3 V? What does that connect to?”).
Connecting to the voltage divider circuit.
On the bottom part of your breadboard, build the circuit and check your prediction.
Adjust the voltage of your power supply throughout the range of values available to you, and observe the effect on the circuit's output $V_{out}$
Does the voltage divider behave the same way as the voltage varies?
These power supplies go up to around 32V or so (it varies), but the ratio of $V_{out}$ to $V_{in}$ should be about $\dfrac{10k\Omega}{10k\Omega + 22k\Omega} = \dfrac{1}{3.2} \approx .313$. This is based on the nominal (stated) resistor values, which can be off from the actual values by 5% for our resistors. In that case, the worst case scenarios are $\dfrac{V_{out}}{V_{in}} = \dfrac{10.5}{10.5+20.9} \approx .334$ or $\dfrac{V_{out}}{V_{in}} = \dfrac{9.5}{9.5+23.1} \approx .291$ If things are further off than that, then I'd suggest you measure the actual resistor values to make sure that previous students didn't mix the values into the wrong place.
Using Variable resistance
Now that you've seen how a fixed voltage divider work, let's go a step further to make one we can change. An example of a common type of variable resistor – called a potentiometer – is shown below. Potentiometers contain some resistive material that has fixed connections at either end and a movable contact point. This is shown in the schematic diagram as a resistor with an arrow pointed partway along its length:
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| A photograph of your potentiometer, with connections annotated. The 1, 2, and 3 refer to the three pin connections. This one has been soldered to a tiny bit of PCB to make it easier to use. | The schematic symbol for a potentiometer |
Take the provided potentiometer and place it in your breadboard (make sure the pins are vertical and not horizontal!). Connect three wires to the three terminals, and then use your multimeter to test its behavior.
Does the resistance between terminal 1 and 2 increase, decrease, or stay the same when the screw is turned?
The resistance between 1 and 2 should decrease when the screw is turned clockwise, and increase when turned counter-clockwise.
Does the resistance between terminal 1 and 3 increase, decrease, or stay the same when the screw is turned?
Here the resistance shouldn't substantially change; it should be measured across the entirety of the resistive material in the potentiometer.
There may be small fluctuations while turning due to poor contact, but after everything settles it should be the same.
Creating a variable divider
Okay, we can adjust a resistor. Let's try putting it into a voltage divider circuit now.
Connect the potentiometer to the power supply as shown below
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| A schematic of a circuit using a potentiometer as a voltage divider. |
What range of voltages can you measure at the middle terminal, $V_{out}$?
When the knob is turned fully clockwise, there would be 1k resistance between points 2 and 3, and no resistance between points 1 and 2.
The resulting output would then be 3V, as there's essentially just a wire between $V_{in}$ and $V_{out}$
On the other hand, with the screw turned fully counterclockwise, the resistances would be switched, which would result in there being no resistance between ground (0V) and $V_{out}$
In between, the voltage should vary linearly with the knob position
Creating practical sensors
Now that you've seen how a fixed voltage divider and an adjustable voltage divider work, it isn't too large a leap to translate these concepts into a practical circuit. If one of the resistors is fixed and the other one is allowed to vary (either from a mechanical response – like turning a knob – or from a physical response – like reacting to light or pressure), then the output voltage of the divider circuit will change in response.
For example, we can create a dimmer (for a light) or a volume control (for a speaker) by making one resistor into a variable resistor that the user controls. As the resistance changes, the voltage to the light or speaker changes as well.
Or, conversely, we can use a variable resistor that reacts to an external stimulus and then monitor the output voltage of the divider. Suppose, for example, that we had an object whose resistance changed as the light intensity changed; as the voltage output increased or decreased, we could use these values as a sort of light intensity meter. Or, suppose we had a resistor that responded to pressure; in this way, we could create a weight scale.
Light sensor
We can purchase components whose resistance depends on the intensity of the light shining on the surface, called photoresistors. They are typically oblong with a red substrate and a squiggle between two metal terminals, shown below.
Grab the ends of the photoresistor with your multimeter and test how its resistance changes as you cover/uncover it.
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| A CdS based photoresistor. You may have seen these in night lights. | A light-detection circuit, using a photoresistor. |
Build the voltage divider circuit using one $680 \Omega$ resistor and one photoresistor, as shown above.
As you cover/uncover the resistor or as you shine light directly on it (either from the provided lamp or from the flashlight feature on your cell phone), how does your output voltage respond?