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

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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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Figure 2 - Circuit diagram for a photocell-based variable voltage divider. Q2.3 : Calculate and record the expected output voltage (V out ) and expected current (I) under ambient light conditions. Q2.4 : Simulate the circuit for ambient light conditions with LTSPICE finding the output voltage (V out ) and the expected current (I). Q2.5 : Calculate and record the expected output voltage (V out ) and expected current (I) under dark conditions. Q2.6 : Simulate the circuit for ambient dark conditions with LTSPICE finding the output voltage (V out ) and the expected current (I). Q2.7 : As light shining on the photocell increases, do you expect the output voltage (V out ) to increase or decrease? Q2.8 : Calculate and record how much power this circuit uses both in the light and in the dark? 2.3.Build the circuit shown in Figure 2 on your breadboard. Q2.9: Include a photograph of your experimental setup. Q2.10 : Measure and record the output voltage (V out ) and current (I) under ambient light conditions. Q2.11 : Measure and record the output voltage (V out ) and current (I) under dark conditions. Q2.12 : Are the voltage and current values close to what you calculated and simulated in Q.2.3 – Q2.6 ? Fill out Table 2 to clearly document your results. Table 2: PhotoCell Results Item Hand Calculation LTSPICE Simulation Lab Measurement Light Vout (V) Light Current (A) Dark Vout (V) Dark (V) Q2.13 : Exam the results in table 2. How do the various methods compare in determining the voltage and current? Try to explain the variation? Q2.14: Figure 1 is the condensed form of the data sheet for the photocell that is part of your kit. LUX is a measure of total ‘light power’ as perceived by the human eye. Thus, it adds up all the wavelengths within the visible spectrum and applies a weighting functional that attempts to replicate the human eye. It is a funky unit. To provide a qualitative feel for the LUX I have included a table from Wikipedia. Study the datasheet in Figure 1 for a little while and try to figure out what each parameter means (This is a really simple datasheet, and it is even Page 4 of 7

confusing at first). Imagine for a minute you are going to use this component in a practical design where the change in resistance is going to stimulate some action by a circuit. Comment and explain on parameters from the data sheet and the LUX table that would be important considerations in a design and why (Yes, this is an open-ended question. I figure by now you are bored of just answering questions. Think about it and think about your experience when you used the device. What worked well, what caused problems.) Table 3: Common Lux Values Illuminance (lux) Surfaces illuminated by 0.0001 Moonless, overcast night sky ( starlight ) [4] 0.002 Moonless clear night sky with airglow [4] 0.05–0.3 Full moon on a clear night [5] 3.4 Dark limit of civil twilight under a clear sky [6] 20–50 Public areas with dark surroundings [7] 50 Family living room lights (Australia, 1998) [8] 80 Office building hallway/ toilet lighting [9] [10] 100 Very dark overcast day [4] 150 Train station platforms [11] 320–500 Office lighting [8] [12] [13] [14] 400 Sunrise or sunset on a clear day. 1000 Overcast day; [4] typical TV studio lighting 10,000– 25,000 Full daylight (not direct sun) [4] 32,000– 100,000 Direct sunlight Page 5 of 7

Figure 1: Condensed Photocell Datasheet. Page 6 of 7

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