EE40, Spring 2015, Pre-Lab 4 Wheatstone Bridge and Photo-Detection Circuit

Logistics

You should submit your prelab assignments on Gradescope before your lab section. This will be different than the Gradescope page of the course where you submit your homework and you should already be able to see the page of your lab in addition to the main course in your Gradescope account. If not, you can email your lab GSI to do that for you. You will complete the lab assignments during the lab sessions. You should ask your GSI to verify it and check you off the list at the end of each lab session before you leave. If you complete your lab at home before beginning of the session, please show up to your lab session

and make sure your GSI verifies that you have completed all the required tasks correctly and checks you

off.

Objectives of Lab 4

For prelab 4 you will simulate the Wheatstone Bridge circuit using Multisim. You will also build these

circuits in lab 4. Below are the tasks you should complete for lab 4.

Prelab assignment (to be completed before your lab session)

o Simulations: S.1, S.2, S.3

o Questions: Q.1, Q.2

Lab assignment (to be completed during your lab session)

o Build the Wheatstone Bridge circuits for the whisker and eye

Outline

1. Prelab Assignment

2. Lab guidelines

1. Prelab Assignment

You should complete your prelab assignment and submit it to the Gradescope account of your lab

section before beginning of your lab session.

Your prelab assignment consists of two parts. In the first part you will do a simulation using Multisim

and in the second part you should answer a few intuitive questions about the simulation.

1.1. Resistors and Ohms Law

This lab requires a basic understanding of the following topics. If you dont feel adequately up

to speed in any of these topics, please refer to section 4 of prelab 2 to refresh your knowledge.

1.1.1. Basic resistor theory

1.1.2. Ohms Law

1.1.3. KVL & KCL

1.1.4. Parallel and series connection of resistors

1.1.5. Equivalent resistance

1.2. Variable Resistors and Voltage Divider Simulation

In class, we learned about voltage and current dividers. If needed, please refer to the Edge -edx

module 2.4 section about voltage and current dividers.

We will now see what happens when one of the resistors in the divider is a variable resistor.

For information on variable resistors, please refer to the Edge-edx module 2.3.

First, lets build a voltage divider circuit:

Just for reference, I named the net (aka wire) between the two resistors Vout_Vdiv, to signify

that its the output voltage of the voltage divider. You can name nets by right clicking on the

net and selecting Properties. Dont forget to select the Show net name box after naming the

net.

Now were going to do a DC sweep. In other words, well measure the DC operating point while

sweeping a particular parameter. In our case, were interested in measuring Vout while

sweeping the bottom resistor (R5 in the diagram) of the voltage divider. Sweeping this value

will simulate turning a potentiometer, etc.

To run a DC sweep, go to Simulate -> Analyses -> Parameter Sweep. Fill out the information as

is applicable to your circuit:

S.1 Please run the DC sweep and include the resulting plot in your prelab writeup. Yo u

should plot Vout on the Y axis and the resistance value on the X axis. Make sure to

sweep with enough points such that the general curve is easily visible. We wont care

about any exact values just the general shape of the curve. On that note, please use a

linear sweep for simplicity (should be the default setting).

S.2 Run a DC sweep on the current divider circuit below and include the resulting plot in

your prelab writeup. You should sweep the resistance of the resistor on the right (R7 in

this picture) between 100 ohms and 2000 ohms. You should include a plot of the

current through this resistor.

1.3. Wheatstone Bridge Simulation

Now well try simulating a Wheatstone bridge. For information on how a Wheatstone bridge

works, please refer to Edge-edx module 2.4.

Heres our Wheatstone bridge:

As is the theme of this prelab, were going to do a DC sweep. Since were taking a differential

voltage between Vout_P and Vout_N, make sure to select Add Expression in our outputs.

S.3 Run a DC sweep on the Wheatstone Bridge above and include the resulting plot in your

prelab writeup. You should sweep the resistance of the bottom right resistor between 0

ohms and 10,000 ohms. You should include plot of Vout_P and Vout_N.

1.4. Questions

Q.1 Consider the circuit below. Please answer each part of this question with increases or

decreases.

a) How is Vout affected if R_variable is increased?

b) How is I_R9 (the current through R9) affected if R_variable is increased?

c) How is I_R4 affected if R_variable is increased?

Q.2 In section 1.2, we established that you can hook a variable resistor in series with a

normal resistor and measure the voltage across either to detect a change in the variable

resistor. In section 1.3, we now use a Wheatstone bridge to detect a change in the

variable resistor. When would you use a Wheatstone bridge instead of a simple voltage

divider?

2. Lab Guidelines

Vin = 3.3V (output from your voltage regulator)

You will want to build a Wheatstone bridge for sensing the resistance of a flex resistor in the

whisker and converting it to a voltage value. Heres some starting values for the whiskers

Wheatstone Bridge:

R1, R2, R4 = 20 kohms

R3 = 18-40 kohms (flex sensor)

You will also want to build a Wheatstone bridge for sensing the resistance of a photocell in the

eye. Heres some starting values for the eyes Wheatstone Bridge:

R1, R2, R4 = 2.7 kohms

R3 = 1-10 kohms (photocell)

Keep in mind that components and conditions vary. You can get a sense for how your variable

resistors (R3) will behave by using a multimeter to measure their resistance in their neutral

state. You may ultimately want to adjust your fixed resistance values (particularly R4)

depending on your variable resistors neutral value (R3). But dont worry about that for this

lab just follow the schematic and use the suggested values for now.