ENGR201-2024 lab2-(Part 1 of 2) - Node Voltages and Resistive Sensors

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Oregon State University, Corvallis *

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Electrical Engineering

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May 4, 2024

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Oregon State University Lab Session #2 (Part 1 of 2) ENGR 201 Electrical Fundamentals I (Ecampus) Node Voltages and Resistive Sensors Ed Rissberger 4/12/2024 V3 EXPERIMENTAL LAB #2 (Part 1 of 1) – NODE VOLTAGES AND RESISTIVE SENSORS This set of laboratory experiments is to be completed with your lab partners. While each student has a complete kit of parts it is recommended that partners work together (Virtually or Physically) as this will help the learning process. Your lab partner is the first resource to utilize to answer questions, check progress and just check if you are on the right track. Please keep this handout, and make sure to write down (and keep) all calculations and measurements you make. A brief, written lab report from each lab group is due to receive credit for the lab. The lab report should include names of the members of the lab group, lab group number, answers to all bold questions in this lab handout, as well as all drawings/tables described. In this lab you will solve the same problems by hand, lab measurement, and simulation. You MUST include all your work for all three methods for full credit. The written procedure contained in this document contains all the information necessary to conduct the lab. However, in the on-line environment it has been found that additional guidance is helpful for the students. For that reason there is an introductory video provided with each lab, which attempts to ‘show’ you some of the tricky parts of the lab. It is recommended to view it prior to starting the lab. PROCEDURE 1. Voltage Ladder 1.1.A string of series resistors can be combined to form a multi-output voltage divider, called a voltage ladder. An example of this is shown in Figure 1. (Note: You will see variations on this circuit on many exams, homework, and labs in both ENGR201 and ENGR202. The components will vary, but the fundamental property of a single source with components in series will remain the same. This is something you should invest the time to be able to recognize and solve it without pause) Page 1 of 7

220 Ω 2.2 kΩ 10 kΩ 22 kΩ 10 V A B C D E I Figure 1 - Circuit diagram of a voltage ladder Q1.1 : Calculate the node voltage you expect at each node (V A , V B , V C , V D ), relative to node E (i.e. V E = 0V) . Show all your work. Q1.2 : Calculate the expected current (I) you would expect to measure. Q1.3 : Setup up the circuit in LTSPICE. Simulate the circuit to determine the same node voltages you found in Q1.1. Q1.4: Use the LTSPICE simulation to find the current. 1.2.Construct the circuit shown in Figure 1 on your breadboard. Q1.5 : Include a photograph of your experimental setup. Q1.6 : Measure and record the voltage at each node (V A , V B , V C , V D ), relative to node E (V E = 0V). Record your measurement in Table 1. Q1.7 : Measure and record the current (I). (Remember that you will need to break the circuit to properly use the ammeter and measure current, as the ammeter only measures current passing through it.). Record your measurement in Table 1. Q1.8 : Are the voltage and current values close to what you calculated and simulated in Q1.1 – Q1.4 ? Fill out Table 1 to clearly document your results. Table 1: Resistor Ladder Results Item Hand Calculation LTSPICE Simulation Lab Measurement Va (Volts) Vb (Volts) Vc (Volts) Vd (Volts) Current (mA) Page 2 of 7

2. Ambient Light Sensor 2.1.For certain semiconductor materials, the resistivity of the material changes under certain conditions. One such material, cadmium sulfide, changes its resistivity when exposed to light. This material can be used to create a light-sensitive variable resistor called a photocell or photoresistor. Get the photocell from your lab kit, and measure its resistance using your DMM. There will be some variations in all photocell measurements due to the ambient lighting and the degree to which light is blocked during the experiment. Here are some suggestions to improve the consistency of your results (You are still going to see significant variation. This isn’t an experiment that you are going to match to the second digit).  Build the circuit on your bread board before taking any measurements. (To measure the resistance of the photocell you need to isolate from the circuit and power supply).  Try to use artificial light via a fixed lamp rather than ambient light. Don’t try to use the flashlight from your cell phone as it will vary too much. I had ambient light and it changed during my measurements. Be careful not to change the artificial light setup once you start (e.g. don’t move or bump the lamp).  Try to be quick in making the measurements to reduce ambient light variation.  If you have a significant variation, particularly in the light case, try measuring the resistance again when you are done. Q2.1 : What is the resistance of the photocell when exposed to ambient room light? Q2.2 : What is the resistance of the photocell when you block the light with your hand? 2.2.Consider the circuit shown in Figure 2, which uses a photocell to construct a voltage divider. Vo 10V 2.2 kΩ Photocell Vout I Page 3 of 7

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